perf(mcp): checkNewComments — параллелизм с капом (#490 )

checkNewComments делал O(N) последовательных REST-вызовов listComments по страницам working set — большой space линеен по round-trip'ам. Теперь per-page фетчи идут с ограниченным параллелизмом (cap 6, середина полосы 5–8): независимые чтения не ждут друг друга, но и не заваливают сервер/сокеты. mapWithConcurrency — крошечный пул без зависимости от p-limit: N воркеров тянут следующий индекс с общего курсора. Порядок результатов сохраняется (по входному порядку страниц), поэтому вывод детерминирован независимо от того, какой фетч завершился первым. Серверный batch-эндпоинт «comments updated since T по space» — опционально, отдельным заходом. Тест (mock-HTTP): 13 страниц, задержанный /api/comments — maxInFlight > 1 и <= 6 (последовательная реализация дала бы 1), порядок результатов = порядок обхода. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
perf(ai-chat): snapshotOpenPage fast-path (#490 )
2026-07-11 11:35:58 +03:00 · 2026-07-11 11:30:46 +03:00 · 2026-07-11 11:27:15 +03:00 · 2026-07-11 11:21:27 +03:00 · 2026-07-11 10:43:35 +03:00 · 2026-07-11 10:39:42 +03:00
45 changed files with 4900 additions and 2916 deletions
@@ -225,11 +225,26 @@ MCP_DOCMOST_PASSWORD=

 # Silence timeout (ms) for EXTERNAL-MCP transport ONLY (not the chat provider).
 # Tighter than AI_STREAM_TIMEOUT_MS so a byte-silent/hung MCP server is broken in
-# ~1 min instead of 15. Note it also cuts a legitimately long but byte-silent
-# single tool call (a slow crawl that emits nothing until done) and an SSE
-# transport idling >1 min BETWEEN tool calls. Default 60000 (1 min).
+# ~1 min instead of 15. It cuts a legitimately long but byte-silent single tool
+# call (a slow crawl that emits nothing until done) on the HTTP (streamable)
+# transport, which opens a fresh request per call. The SSE transport — one
+# long-lived body across many calls — is NO LONGER governed by this timeout
+# (as of #489): its idle-BETWEEN-calls window has its own, raised bodyTimeout,
+# AI_MCP_SSE_BODY_TIMEOUT_MS below. Default 60000 (1 min).
 # AI_MCP_STREAM_TIMEOUT_MS=60000

+# bodyTimeout (ms) for the EXTERNAL-MCP SSE transport ONLY (#489). The SSE
+# transport holds ONE response body open across many tool calls, so undici's
+# bodyTimeout (time between body bytes) counts the LEGITIMATE silence BETWEEN the
+# model's tool calls, not just a hung single call. At the tight 1-min silence
+# timeout above, a normal >1-min gap between calls would break the SSE socket and
+# the cache would serve a dead client until TTL — so the SSE transport gets its
+# OWN, RAISED bodyTimeout. A single stuck call is still bounded by the per-call
+# cap (AI_MCP_CALL_TIMEOUT_MS), and a socket that does break is healed by the
+# in-run transport-error retry. The HTTP (streamable) transport keeps the tight
+# timeout. Default 600000 (10 min).
+# AI_MCP_SSE_BODY_TIMEOUT_MS=600000
+
 # Total wall-clock cap (ms) for ONE external MCP tool call (app-level, not
 # transport). Aborts a tool that keeps the socket warm (SSE heartbeats / trickle)
 # but never returns a result — which the silence timeout above never breaks.
@@ -29,6 +29,10 @@ packages/mcp/build/
 # is a build artifact like build/ — never committed, always fresh.
 packages/mcp/src/registry-stamp.generated.ts

+# token-estimate compiled output (#490; built in CI/Docker via `pnpm build` /
+# the server `pretest`, never committed, so src/ and prod can never diverge).
+packages/token-estimate/dist/
+
 # Logs
 logs
 *.log
@@ -336,6 +336,23 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0

 ### Fixed

+- **A chat with one malformed message part no longer 500s on every turn, and a
+  failed send no longer duplicates the user's message.** Incoming client parts
+  are now whitelisted to `text` (a forged tool-result part can no longer reach
+  the persisted history or the model context), and the turn is converted BEFORE
+  the user row is inserted, so a mid-flight failure cannot leave a duplicate
+  user row that a retry then compounds. A single part that still fails to convert
+  degrades to a `[tool context omitted]` marker on that one row instead of
+  bricking the whole chat. (#489)
+- **A transport drop to an external MCP server now heals within the same turn.**
+  On an undici transport error, a read-only MCP tool reconnects its server and
+  retries once within the run; a write is never auto-retried (it may already have
+  applied). One flapping server no longer nulls the shared client cache, so other
+  servers' cached clients are untouched. The SSE transport also gets a raised
+  body-timeout so a legitimate >1-min idle between the model's tool calls no
+  longer breaks a long-lived SSE socket (new `AI_MCP_SSE_BODY_TIMEOUT_MS`, default
+  10 min; see `.env.example`). (#489)
+
 - **The server no longer runs out of heap during long autonomous agent runs.** A
  new pnpm patch on `ai@6.0.134` stops the SDK from building a cumulative
  snapshot of the ENTIRE turn text on every streamed text-delta when no output
@@ -22,6 +22,7 @@
    "@casl/react": "5.0.1",
    "@docmost/editor-ext": "workspace:*",
    "@docmost/prosemirror-markdown": "workspace:*",
+    "@docmost/token-estimate": "workspace:*",
    "@excalidraw/excalidraw": "0.18.0-3a5ef40",
    "@mantine/core": "8.3.18",
    "@mantine/dates": "8.3.18",
@@ -86,11 +86,19 @@ const MIN_HEIGHT = 400;
 // Margin kept between the window and the viewport edges while dragging.
 const EDGE_MARGIN = 8;

-// #184 phase 1.5 / #430 / #488: the degraded-poll fallback. The window owns only
-// a DUMB 2.5s timer, gated by an armed flag; the THREAD's run-lifecycle FSM owns
-// arm/disarm AND the inactivity cap that turns a stuck run into a `stalled` banner
-// (#488 commit 4a — the cap moved into the thread so polling->stalled is a single
-// FSM transition; the window no longer silently stops polling at the cap).
+// #184 phase 1.5 / #430: backstop for the degraded-poll fallback. The poll is
+// armed when a resume attempt could not attach to the live run and disarmed by the
+// thread on settle / local stream; this cap is the ONLY backstop against an endless
+// tick (a stuck 'streaming' row before the boot-sweep, or a user-tail 204 with no
+// run).
+//
+// #430: measured from RUN ACTIVITY, not from arm-time. A real autonomous run takes
+// 11-25 min — longer than a fixed 10-min-from-start cap, which used to cut the poll
+// off mid-run. Instead we cap on INACTIVITY: keep polling as long as the run is
+// still making progress (its persisted rows keep changing), and only give up after
+// this long with NO new activity. A genuinely stuck run produces no row changes, so
+// the idle cap still bounds it; a long-but-progressing run polls to completion.
+const DEGRADED_POLL_IDLE_MAX_MS = 10 * 60_000;

 /** Compact token formatter: 1.2M / 3.4k / 950. */
 function formatTokens(n: number): string {
@@ -251,13 +259,17 @@ export default function AiChatWindow() {
    [roles],
  );

-  // #184 phase 1.5 / #488: degraded-poll fallback. ChatThread's FSM arms this via
-  // onResumeFallback(true) when it enters a poll-bearing recovery (attach 204 /
-  // starved finish / stop) and disarms it on settle / local stream / stalled. The
-  // window owns ONLY the dumb 2.5s timer; the THREAD owns arm/disarm AND the
-  // inactivity cap (a stuck run -> the thread's `stalled` banner disarms this).
+  // #184 phase 1.5: degraded-poll fallback (replaces the F4/F5/F7 latches). When
+  // ChatThread could not attach to a still-running run it arms this via
+  // onResumeFallback(true); the thread disarms it on settle / local stream. The
+  // window only OWNS the timer (armedAtRef stamps when it was armed for the cap).
  const [degradedPoll, setDegradedPoll] = useState(false);
+  // #430: timestamp of the LAST run activity while the poll is armed — stamped on
+  // arm and re-stamped whenever the polled rows change (see the effect below). The
+  // idle cap is measured from this, so a long-but-progressing run keeps polling.
+  const lastActivityAtRef = useRef(0);
  const onResumeFallback = useCallback((active: boolean): void => {
+    if (active) lastActivityAtRef.current = Date.now();
    setDegradedPoll(active);
  }, []);
  // Reset the degraded poll whenever the open chat changes: it is scoped to the
@@ -269,17 +281,33 @@ export default function AiChatWindow() {
  const { data: messageRows, isLoading: messagesLoading } =
    useAiChatMessagesQuery(
      activeChatId ?? undefined,
-      // DELIBERATELY DUMB: poll every 2.5s WHILE ARMED, otherwise off. NO error
-      // checks (TanStack resets fetchFailureCount each fetch; the poll must survive
-      // a server restart), NO tail checks, NO cap here — the settled/stalled/idle-cap
-      // semantics all live in ChatThread's FSM, which disarms via onResumeFallback.
-      () => (degradedPoll === true ? 2500 : false),
+      // DELIBERATELY DUMB (invariant 8 / task 2.4): poll every 2.5s while armed
+      // and while the run is still active (#430: under the INACTIVITY cap, not a
+      // fixed-from-start cap); otherwise off. NO error checks (TanStack v5 resets
+      // fetchFailureCount each fetch, so consecutive errors are not expressible —
+      // and the poll must survive a server restart) and NO tail checks (the
+      // settled/local-stream semantics live in ChatThread, which disarms via
+      // onResumeFallback(false)). The idle cap is the only backstop.
+      () =>
+        degradedPoll === true &&
+        Date.now() - lastActivityAtRef.current < DEGRADED_POLL_IDLE_MAX_MS
+          ? 2500
+          : false,
      // #344: gate on windowOpen too — no message history is fetched (and no
      // degraded poll runs) while the window is closed; it loads when the window
      // opens with an active chat.
      windowOpen,
    );

+  // #430: re-stamp the activity clock whenever the polled rows change while the
+  // poll is armed. TanStack keeps the same `messageRows` reference across refetches
+  // that return deep-equal data (structural sharing), so a new reference means the
+  // run genuinely progressed — which extends the inactivity cap above. A stuck run
+  // yields no reference change, so the cap eventually fires and stops the poll.
+  useEffect(() => {
+    if (degradedPoll) lastActivityAtRef.current = Date.now();
+  }, [degradedPoll, messageRows]);
+
  // #184 reconnect-and-live-follow. Whether detached agent runs are enabled for
  // this workspace. When the feature is off no runs are ever created, so the
  // resume attempt would only ever 204; gating ChatThread's resume on it avoids a
@@ -57,25 +57,6 @@ export async function stopRun(
  return req.data;
 }

-/**
- * #488: the run-fact — "is a run active on this chat?" — first-class from the
- * server (POST /ai-chat/run). Called on mount to seed the client FSM's run-fact
- * and to VERIFY after a supersede mismatch (an observer following a superseded
- * run asks for the latest run and follows it). Returns the latest run row (with
- * its `id` and `status`) and its projected assistant message, or `run: null` when
- * the chat has never had a run. Owner-gated server-side.
- */
-export async function getRun(chatId: string): Promise<{
-  run: { id: string; status: string } | null;
-  message: IAiChatMessageRow | null;
-}> {
-  const req = await api.post<{
-    run: { id: string; status: string } | null;
-    message: IAiChatMessageRow | null;
-  }>("/ai-chat/run", { chatId });
-  return req.data;
-}
-
 /**
 * Resolve the chat bound to a document (the current user's most-recent chat
 * created on that page), or null when there is none. Drives auto-open-on-page.
@@ -1,183 +0,0 @@
-# AI-chat run-lifecycle FSM — design spec (#488)
-
-This is the written design that `run-fsm.ts` implements. It ships in the PR (issue
-#488 commit 1: "the spec is written FIRST and enters the PR"). It has four parts:
-(1) the event × state transition table, (2) the map of every `chat-thread.tsx` ref
-to {FSM state | FSM context | stays data}, (3) the run-fact protocol, (4) the
-invariants.
-
-The reducer is a **pure function** `reduce(machine, event) → machine`. The returned
-machine carries the **command effects** for that transition; a thin runtime in
-`chat-thread.tsx` dispatches events and executes effects. Because it is pure, the
-whole machine is enumerable and unit-tested directly (event × state → next state is
-the observable property) — see `run-fsm.test.ts`.
-
---
-
-## 1. Event × state transition table
-
-Phases: `idle | sending | streaming | attaching | reconnecting(attempt,failed) |
-polling(reason) | stalled | stopping | superseding | error(kind)`.
-Context (orthogonal): `epoch`, `ownership: local|observer`, `runFact: {runId}|null`,
-`liveFollow` (are we following a live run we locally streamed — the reconnect
-ladder — vs a one-shot mount-attach resume? both are `observer`, but a live-follow
-drop RE-ENTERS the ladder (#488 commit 3) while a mount-resume drop polls).
-
-Legend: **†** = command-transition (bumps `epoch`, I1). Effects in `[…]`.
-
-| Event (source) | From phase(s) | → To phase | Effects / ctx |
-|---|---|---|---|
-| `SEND_LOCAL` (user send) | idle, error, polling, stalled, reconnecting | sending **†** | `[cancelReconnect, disarmPoll]`, ownership=local |
-| `STREAM_START{runId}` (SDK `start` metadata) | sending, attaching, reconnecting, superseding | streaming | `[cancelReconnect, disarmPoll]`, runFact←runId |
-| `FINISH_CLEAN` (onFinish clean) | streaming, … | idle | `[disarmPoll, cancelReconnect]`, runFact←null |
-| `FINISH_ABORT` (onFinish isAbort) | streaming, stopping | idle | `[disarmPoll, cancelReconnect]`, runFact←null (I4 exits stopping by this DATA) |
-| `FINISH_DISCONNECT` (observer, NOT liveFollow) | streaming(observer) | polling(disconnect-visible) | `[armPoll]` (a mount-resume drop polls) |
-| `FINISH_DISCONNECT{hasVisibleContent}` (local drop OR liveFollow) | streaming | reconnecting(1) **†** *iff runFact\|liveFollow* | `[scheduleReconnect(1)]` (+`armPoll` if visible), ownership=observer, liveFollow=true (commit 3: repeatable) |
-| `FINISH_DISCONNECT` (no runFact, not liveFollow) | streaming | idle | runFact←null (plain terminal "connection lost") |
-| `STREAM_INCOMPLETE{reason}` (observer starved/torn clean finish) | streaming(observer) | polling(reason) | `[armPoll(reason)]` |
-| `FINISH_ERROR{kind}` (onFinish isError) | any | error(kind) | `[disarmPoll, cancelReconnect]`, runFact←null |
-| `STREAM_START{runId}` (first assistant frame of a local turn) | sending | streaming | runFact←runId, `[cancelReconnect, disarmPoll]` |
-| `ATTACH_START{runId}` (mount resume) | **idle only** (F2) | attaching **†** | `[resumeStream]`, ownership=observer, runFact←runId; ignored from any non-idle phase |
-| `ATTACH_LIVE` (attach GET 2xx) | attaching | streaming | — |
-| `ATTACH_NONE` (attach GET 204/err/throw) | attaching | polling(attach-none) | `[armPoll(attach-none)]` |
-| `RECONNECT_ATTEMPT{n}` (backoff timer) | reconnecting | reconnecting(n) **†** | `[resumeStream]` |
-| `RECONNECT_ATTACHED` (reconnect GET 2xx) | reconnecting | streaming | `[cancelReconnect, disarmPoll]` — **counter reset** (commit 3) |
-| `RECONNECT_NONE` (reconnect GET 204/err), attempt<MAX | reconnecting | reconnecting(n+1) **†** | `[armPoll(attach-none), scheduleReconnect(n+1)]` |
-| `RECONNECT_NONE`, attempt=MAX | reconnecting | reconnecting(MAX, failed) | `[armPoll(reconnect-exhausted)]` |
-| `RETRY` (manual, failed banner) | reconnecting(failed) | reconnecting(1) **†** | `[resumeStream]` |
-| `RETRY` (manual, stalled banner) | stalled | polling(attach-none) **†** | `[armPoll]` |
-| `POLL_TERMINAL` (settled tail merged) | polling, reconnecting, stopping | idle | `[disarmPoll, cancelReconnect]`, runFact←null (I4) |
-| `POLL_IDLE_CAP` (inactivity cap) | polling, reconnecting | stalled | `[disarmPoll, cancelReconnect]` (commit 4a — no more silent) |
-| `RUN_FACT{null}` (POST /run → null/terminal, 204) | reconnecting/attaching/polling/stopping | idle | `[cancelReconnect, disarmPoll]`, runFact←null (I3 fresh-negative gate) |
-| `RUN_FACT{runId}` | any | (same) | runFact←runId (pessimism toward an attempt) |
-| `STOP_REQUESTED` (user Stop) | streaming, reconnecting, polling | stopping **†** | `[stopRun, abortAttach, cancelReconnect, armPoll]` (poll drives the terminal — I4 exit by data) |
-| `SUPERSEDE_REQUESTED{targetRunId}` (interrupt+send) | streaming, reconnecting, polling, error | superseding **†** | `[supersede(target), cancelReconnect, disarmPoll]` |
-| `SUPERSEDE_READY{runId}` (CAS ok) | superseding | streaming | ownership=local, runFact←runId |
-| `SUPERSEDE_MISMATCH{currentRunId}` (409 SUPERSEDE_TARGET_MISMATCH) | superseding | error(supersede-mismatch) | `[postRun(verify)]`, runFact←currentRunId |
-| `SUPERSEDE_TIMEOUT` (409 SUPERSEDE_TIMEOUT) | superseding | error(supersede-timeout) | — (composer keeps text; no auto-retry) |
-| `SUPERSEDE_INVALID` (409 SUPERSEDE_INVALID) | superseding | error(supersede-invalid) | — |
-| `RUN_ALREADY_ACTIVE` (409 A_RUN_ALREADY_ACTIVE, plain POST) | sending | error(run-already-active) | — (composer offers supersede; NO auto-retry) |
-| `DISPOSE` (unmount) | any | idle **†** | `[abortAttach, cancelReconnect, disarmPoll]` (I1/I5 — epoch++ kills late callbacks) |
-
-**`stopping` honors any finish (re-review MEDIUM):** BEFORE the epoch filter, a
-stream finish (`FINISH_*`/`STREAM_INCOMPLETE`) arriving in phase `stopping` exits
-`stopping -> idle` regardless of generation. A plain Stop has no successor stream,
-so the aborted stream's finish IS the expected end (I4 exit by data) — and it
-carries the PRE-stop generation (STOP_REQUESTED bumped the epoch), so the filter
-would otherwise strand the machine in `stopping` (no idle-cap covers it). The filter
-stays in force for `superseding` (that is the F1 supersede drop).
-
-**Epoch filter (I1):** the reducer then drops any event carrying an `epoch` that
-does not equal the current `ctx.epoch`. Outcome events (`STREAM_START`, `ATTACH_*`,
-`RECONNECT_*`, `SUPERSEDE_*`, **`FINISH_*`/`STREAM_INCOMPLETE`**, `RUN_FACT`) are
-stamped with the generation the corresponding STREAM started under (the runtime
-holds a per-owned-stream `turnEpoch`); trigger events (user actions, fresh
-disconnects) carry no epoch. **F1:** this is what makes a SUPERSEDED stream's late
-`onFinish` (a dead stream A closing after the CAS started stream B) get dropped, so
-A cannot drive the live new run into a false reconnect or reset its run-fact. The
-supersede path additionally ABORTS A and starts B only from A's onFinish (a
-microtask), because ai@6 `AbstractChat.makeRequest` corrupts overlapping streams
-(A's `finally` reads then nulls the shared `activeResponse`).
-
-**Removed events (scope-cut, internal review):** `RUN_SUPERSEDED` (a ghost feature —
-never dispatched; the observer-superseded case is handled by the degraded poll,
-which follows the latest rows regardless of runId), `RECONNECT_BEGIN` (reconnect is
-entered by `FINISH_DISCONNECT`), and `POLL_ACTIVITY` (the window's activity clock was
-removed when the idle-cap moved into the thread). The reducer and this table now
-share exactly the dispatched event set.
-
-### 409-code → event map (the real #487 contract consumed here)
-
-| Server response | Event dispatched | error kind → banner |
-|---|---|---|
-| 409 `A_RUN_ALREADY_ACTIVE` (plain POST) | `RUN_ALREADY_ACTIVE` | run-already-active → "already answering / interrupt & send" |
-| 409 `SUPERSEDE_TARGET_MISMATCH` (+ body.runId) | `SUPERSEDE_MISMATCH{currentRunId}` | supersede-mismatch → verify via /run |
-| 409 `SUPERSEDE_TIMEOUT` | `SUPERSEDE_TIMEOUT` | supersede-timeout → "couldn't interrupt in time, resend" |
-| 409 `SUPERSEDE_INVALID` | `SUPERSEDE_INVALID` | supersede-invalid → "couldn't interrupt this run" |
-| 503 `A_RUN_BEGIN_FAILED` | `FINISH_ERROR{begin-failed}` | begin-failed → "could not start, temporary" |
-
---
-
-## 2. Ref-map — every `chat-thread.tsx` ref → its new home  (MIGRATION RESOLVED)
-
-The migration is COMPLETE: the 13 run-lifecycle FLAGS below are GONE from
-`chat-thread.tsx` (collapsed into FSM phase/ctx/effects, or deleted). What remains
-are identity/data mirrors, effect-owned controllers/timers, and ONE React-liveness
-bit — none of which is a run-lifecycle flag, so the post-merge "no new flags" rule
-holds. **Pending column: empty.**
-
-| # | Old ref | Resolved to | Where now |
-|---|---|---|---|
-| 1 | `reconcileTailRef` | **FSM phase** | reconcile-merge gated on `phase ∈ {polling, reconnecting, stopping}` |
-| 2 | `noStreamHandledRef` | **FSM epoch (I1)** | the attach outcome's epoch guard drops the stale/second outcome |
-| 3 | `onNoActiveStreamRef` | **FSM event** | transport → `handleAttachOutcome` dispatches `ATTACH_NONE`/`RECONNECT_NONE` |
-| 4 | `onReconnectAttachedRef` | **FSM event** | transport dispatches `ATTACH_LIVE` / `RECONNECT_ATTACHED` |
-| 5 | `resumedTurnRef` + `resumedTurn` state | **FSM ctx `ownership`** | `ownership==='observer'` ⇒ never flush; hides "Send now" |
-| 6 | `reconnectStateRef` + `reconnectState` state | **FSM phase** | `reconnecting(attempt,failed)` renders the banner |
-| 7 | `reconnectTimerRef` | **effect-owned timer** | owned by `scheduleReconnect`/`cancelReconnect` effects (not a flag) |
-| 8 | `flushOnAbortRef` | **DELETED** | the stop→flush dance is replaced by the CAS supersede (commit 5) |
-| 9 | `interruptNextSendRef` | **DELETED** | the server injects the interrupt note from the supersede itself |
-| 10 | `supersedeRetryRef` | **DELETED** (commit 5) | the client 409 retry ladder is gone; CAS supersede replaces it |
-| 11 | `stopPendingRef` | **FSM phase `stopping`** | the deferred stop fires from the chat-id adoption effect while `stopping` |
-| 12 | `mountedRef` | **retained (React liveness)** | orthogonal to run-lifecycle; gates imperative onFinish side-effects post-unmount. Epoch (I1) handles stale COMMAND-outcomes; DISPOSE bumps it |
-| 13 | `attemptResumeRef` | **FSM `ATTACH_START` + run-fact** | mount arms attach ONLY on a confirmed active run (commit 4b: streaming-tail status, or POST /run for a user tail) |
-| 14 | `stripRef` | **data** (attachStrategy) | strip+replay detail; the `resumeStream` effect reads it |
-| 15 | `strippedRowRef` | **data** (attachStrategy) | the anchor row |
-| 16 | `attachAbortRef` | **effect-owned controller** | aborted by the `abortAttach` effect in cleanup (I5) |
-| 17–25 | `chatIdRef`, `openPageRef`, `getEditorSelectionRef`, `roleIdRef`, `stableIdRef`, `queuedRef`, `sendMessageRef`, `statusRef`, `lastForwardedChatIdRef` | **data** (identity/send mirrors) | unchanged — not lifecycle flags |
-| NEW | `pendingSupersedeRef` | **data** (send-plumbing) | the runId injected into the next `POST /stream {supersede}`; the single replacement for the 3 DELETED one-shots (#8/#9/#10) — net −2 refs |
-| NEW | `idleCapTimerRef` | **effect-owned timer** | the stalled inactivity cap → `POLL_IDLE_CAP` (commit 4a); not a flag |
-
-Net: the 13 lifecycle flags (#1–#13) are eliminated: **8** → FSM phase/ctx/epoch/event
-(#1–#6, #11, #13), **3** deleted (#8/#9/#10), **`reconnectTimerRef` (#7)** becomes an
-effect-owned controller, and **`mountedRef` (#12)** is retained as React liveness
-(8 + 3 + 1 + 1 = 13). (`attachAbortRef` (#16) is outside the #1–#13 set — it was
-already an effect-owned controller.) Two effect-owned timers + one send-plumbing data
-ref are added — none is a boolean lifecycle latch.
-
---
-
-## 3. Run-fact protocol (`runFact: {runId} | null`) — I3
-
-"A run is active" is first-class from the SERVER, not inferred from an assistant
-message. Sources, in the order they update `ctx.runFact`:
-
-1. **Init (mount):** `POST /ai-chat/run { chatId }` → `{ run, message }`. A `run`
-   with a non-terminal `status` seeds `runFact = { runId: run.id }`; a null/terminal
-   run seeds `null`. This is what arms the resume attempt (`ATTACH_START`) — the
-   attempt is armed ONLY on a positive fact (commit 4b: a user-tail with no active
-   run no longer arms a pointless poll on every open).
-2. **Live update:** the `start` stream metadata carries `runId` → `STREAM_START{runId}`.
-3. **Attach outcomes:** `ATTACH_LIVE` (2xx) confirms active; a 204 on a non-stripped
-   path is an authoritative NEGATIVE fact → the runtime dispatches `RUN_FACT{null}`,
-   which cancels recovery (I3 fresh-negative gate).
-4. **Poll (future resume-stack iteration #491):** the delta will carry the run field;
-   until then the poll drives to a terminal ROW, dispatched as `POLL_TERMINAL`.
-
-Pessimism rule: a stale-but-positive fact PERMITS entering recovery (attach); the
-204 then cuts it. A fresh negative fact gates recovery OUT immediately.
-
---
-
-## 4. Invariants
-
- **I1 — Epoch (generation counter).** Every command-emitting transition bumps
-  `ctx.epoch`; every async outcome event carries its issuing epoch; the reducer
-  drops stale-epoch outcomes. Replaces the one-shot-ref zoo (`noStreamHandledRef`,
-  the flush/interrupt/supersede one-shots, the `mountedRef` late-callback gate).
- **I2 — Ownership is context, not state.** `local | observer` is orthogonal to the
-  transport phase. The queue flushes ONLY under local ownership; an observer
-  following a detached run never flushes (was `resumedTurnRef`).
- **I3 — Run-fact is first-class from the server.** Reconnect is entered by the
-  run-fact, not by an assistant message (commit 2). A fresh negative fact cancels
-  recovery.
- **I4 — Exit `stopping` by DATA.** A terminal row / negative run-fact / terminal
-  finish exits `stopping`, never the stopRun HTTP response (which returns after the
-  abort but before finalization — keying off it would unlock the composer on a 409).
- **I5 — Dispose protocol.** Command controllers (attach GET, POST /stream, POST
-  /run) are effect-owned and aborted in cleanup (`abortAttach` on `DISPOSE`), not
-  render-phase refs. A client abort of an already-sent POST does not cancel the
-  server action, so disarming on unmount is safe.
- **attachStrategy** (strip+replay today) is behind the `resumeStream` effect; the
-  resume-stack iteration (#491) swaps it to tail-only WITHOUT touching the FSM.
- **Queue** stays a data structure; flush/interrupt decisions are transitions.
@@ -1,461 +0,0 @@
-import { describe, it, expect } from "vitest";
-import {
-  reduce,
-  initialMachine,
-  reconnectDelayMs,
-  RECONNECT_MAX_ATTEMPTS,
-  type Machine,
-  type Effect,
-  type Event,
-} from "./run-fsm";
-
-// Drive a sequence of events through the reducer, returning the final machine.
-function run(m: Machine, ...events: Event[]): Machine {
-  return events.reduce(reduce, m);
-}
-function withRunFact(runId = "run-1"): Machine {
-  return {
-    ...initialMachine(),
-    ctx: { epoch: 0, ownership: "local", runFact: { runId }, liveFollow: false },
-  };
-}
-function effectTypes(m: Machine): string[] {
-  return m.effects.map((e) => e.type);
-}
-function hasEffect(m: Machine, type: Effect["type"]): boolean {
-  return m.effects.some((e) => e.type === type);
-}
-
-describe("run-fsm — epoch invariant (I1)", () => {
-  it("drops an outcome carrying a stale epoch", () => {
-    // A command bumps the epoch; an outcome stamped with the OLD epoch is dropped.
-    const m0 = reduce(initialMachine(), { type: "ATTACH_START", runId: "r" }); // epoch 0->1, attaching
-    expect(m0.ctx.epoch).toBe(1);
-    expect(m0.phase.name).toBe("attaching");
-    // A late ATTACH_LIVE from a SUPERSEDED attempt (epoch 0) must NOT drive us.
-    const stale = reduce(m0, { type: "ATTACH_LIVE", epoch: 0 });
-    expect(stale.phase.name).toBe("attaching");
-    expect(stale.effects).toEqual([]);
-  });
-
-  it("applies an outcome carrying the current epoch", () => {
-    const m0 = reduce(initialMachine(), { type: "ATTACH_START", runId: "r" });
-    const live = reduce(m0, { type: "ATTACH_LIVE", epoch: m0.ctx.epoch });
-    expect(live.phase.name).toBe("streaming");
-  });
-
-  it("an outcome with no epoch is never dropped (trigger events)", () => {
-    const m0 = reduce(initialMachine(), { type: "ATTACH_START", runId: "r" });
-    const disposed = reduce(m0, { type: "DISPOSE" });
-    expect(disposed.phase.name).toBe("idle");
-    expect(hasEffect(disposed, "abortAttach")).toBe(true);
-  });
-
-  it("every command-transition increments the epoch exactly once", () => {
-    let m = initialMachine();
-    const before = m.ctx.epoch;
-    m = reduce(m, { type: "SEND_LOCAL" });
-    expect(m.ctx.epoch).toBe(before + 1);
-    m = reduce(m, { type: "STOP_REQUESTED" });
-    expect(m.ctx.epoch).toBe(before + 2);
-  });
-});
-
-describe("run-fsm — local turn", () => {
-  it("SEND_LOCAL → sending, local ownership, cancels recovery", () => {
-    const m = reduce(withRunFact(), { type: "SEND_LOCAL" });
-    expect(m.phase.name).toBe("sending");
-    expect(m.ctx.ownership).toBe("local");
-    expect(effectTypes(m)).toEqual(
-      expect.arrayContaining(["cancelReconnect", "disarmPoll"]),
-    );
-  });
-
-  it("STREAM_START adopts the runId into the run-fact and goes streaming", () => {
-    const m = run(initialMachine(), { type: "SEND_LOCAL" });
-    const s = reduce(m, { type: "STREAM_START", runId: "run-9", epoch: m.ctx.epoch });
-    expect(s.phase.name).toBe("streaming");
-    expect(s.ctx.runFact).toEqual({ runId: "run-9" });
-  });
-
-  it("FINISH_CLEAN → idle, run-fact cleared, poll/reconnect disarmed", () => {
-    const streaming = run(initialMachine(), { type: "SEND_LOCAL" }, { type: "STREAM_START", runId: "r" });
-    const done = reduce(streaming, { type: "FINISH_CLEAN" });
-    expect(done.phase.name).toBe("idle");
-    expect(done.ctx.runFact).toBeNull();
-  });
-});
-
-// #488 commit 2 — SSE break BEFORE the first assistant frame must still recover.
-describe("run-fsm — commit 2: reconnect by run-fact, not by assistant message", () => {
-  it("FINISH_DISCONNECT with an active run-fact → reconnecting (even with no visible content)", () => {
-    // Setup-phase break: no assistant frame yet, but a run-fact exists.
-    const streaming = withRunFact("run-2");
-    const m = reduce(streaming, {
-      type: "FINISH_DISCONNECT",
-      hasVisibleContent: false,
-      epoch: streaming.ctx.epoch,
-    });
-    expect(m.phase.name).toBe("reconnecting");
-    if (m.phase.name === "reconnecting") expect(m.phase.attempt).toBe(1);
-    expect(m.ctx.ownership).toBe("observer");
-    expect(hasEffect(m, "scheduleReconnect")).toBe(true);
-    // No visible content -> no poll arm yet (the reconnect ladder rebuilds it).
-    expect(hasEffect(m, "armPoll")).toBe(false);
-  });
-
-  it("FINISH_DISCONNECT WITH visible content also arms the poll", () => {
-    const m = reduce(withRunFact("run-2"), {
-      type: "FINISH_DISCONNECT",
-      hasVisibleContent: true,
-      epoch: 0,
-    });
-    expect(m.phase.name).toBe("reconnecting");
-    expect(hasEffect(m, "armPoll")).toBe(true);
-  });
-
-  it("FINISH_DISCONNECT with NO run-fact → idle (plain connection-lost)", () => {
-    const m = reduce(initialMachine(), {
-      type: "FINISH_DISCONNECT",
-      hasVisibleContent: true,
-      epoch: 0,
-    });
-    expect(m.phase.name).toBe("idle");
-  });
-});
-
-// #488 commit 3 — a SECOND break after a successful re-attach starts a NEW ladder.
-describe("run-fsm — commit 3: repeated reconnect cycles", () => {
-  it("two breaks in a row produce two reconnect cycles (counter resets on attach)", () => {
-    let m = withRunFact("run-3");
-    // First break -> reconnecting(1).
-    m = reduce(m, { type: "FINISH_DISCONNECT", hasVisibleContent: false, epoch: m.ctx.epoch });
-    expect(m.phase.name).toBe("reconnecting");
-    // Attempt fires, re-attaches live.
-    m = reduce(m, { type: "RECONNECT_ATTEMPT", attempt: 1, epoch: m.ctx.epoch });
-    m = reduce(m, { type: "RECONNECT_ATTACHED", epoch: m.ctx.epoch });
-    expect(m.phase.name).toBe("streaming");
-    // SECOND break: the counter was reset, so a fresh ladder starts at attempt 1
-    // (the old one-shot !wasResumed gate would have sent this to silent poll).
-    m = reduce(m, { type: "FINISH_DISCONNECT", hasVisibleContent: false, epoch: m.ctx.epoch });
-    expect(m.phase.name).toBe("reconnecting");
-    if (m.phase.name === "reconnecting") expect(m.phase.attempt).toBe(1);
-    expect(hasEffect(m, "scheduleReconnect")).toBe(true);
-  });
-
-  it("a MOUNT-attach observer drop falls to POLL, not the reconnect ladder", () => {
-    // Distinguishes commit 3 from a one-shot resume: an observer that never
-    // live-followed (liveFollow false) polls on a drop.
-    let m = reduce(initialMachine(), { type: "ATTACH_START", runId: "r" });
-    m = reduce(m, { type: "ATTACH_LIVE", epoch: m.ctx.epoch });
-    expect(m.ctx.ownership).toBe("observer");
-    expect(m.ctx.liveFollow).toBe(false);
-    m = reduce(m, { type: "FINISH_DISCONNECT", hasVisibleContent: true, epoch: m.ctx.epoch });
-    expect(m.phase.name).toBe("polling");
-    expect(hasEffect(m, "armPoll")).toBe(true);
-  });
-
-  it("STREAM_INCOMPLETE (observer starved/torn finish) → polling", () => {
-    let m = reduce(initialMachine(), { type: "ATTACH_START", runId: "r" });
-    m = reduce(m, { type: "ATTACH_LIVE", epoch: m.ctx.epoch });
-    m = reduce(m, { type: "STREAM_INCOMPLETE", reason: "starved", epoch: m.ctx.epoch });
-    expect(m.phase).toEqual({ name: "polling", reason: "starved" });
-    expect(hasEffect(m, "armPoll")).toBe(true);
-  });
-
-  it("liveFollow is set on the first local drop and kept across a re-attach", () => {
-    let m = withRunFact("run-3");
-    m = reduce(m, { type: "FINISH_DISCONNECT", hasVisibleContent: false, epoch: m.ctx.epoch });
-    expect(m.ctx.liveFollow).toBe(true);
-    m = reduce(m, { type: "RECONNECT_ATTEMPT", attempt: 1, epoch: m.ctx.epoch });
-    m = reduce(m, { type: "RECONNECT_ATTACHED", epoch: m.ctx.epoch });
-    expect(m.ctx.liveFollow).toBe(true); // kept — so a second drop reconnects
-    // A clean finish clears it.
-    m = reduce(m, { type: "FINISH_CLEAN", epoch: m.ctx.epoch });
-    expect(m.ctx.liveFollow).toBe(false);
-  });
-
-  it("RECONNECT_NONE backs off through the ladder, then fails at the cap", () => {
-    let m = withRunFact("run-3");
-    m = reduce(m, { type: "FINISH_DISCONNECT", hasVisibleContent: false, epoch: m.ctx.epoch });
-    for (let n = 1; n < RECONNECT_MAX_ATTEMPTS; n++) {
-      m = reduce(m, { type: "RECONNECT_ATTEMPT", attempt: n, epoch: m.ctx.epoch });
-      m = reduce(m, { type: "RECONNECT_NONE", epoch: m.ctx.epoch });
-      expect(m.phase.name).toBe("reconnecting");
-      if (m.phase.name === "reconnecting") {
-        expect(m.phase.attempt).toBe(n + 1);
-        expect(m.phase.failed).toBe(false);
-      }
-      // The belt-and-suspenders poll is armed each failed attempt.
-      expect(hasEffect(m, "armPoll")).toBe(true);
-    }
-    // Final attempt fails -> failed banner (Retry), poll armed.
-    m = reduce(m, { type: "RECONNECT_ATTEMPT", attempt: RECONNECT_MAX_ATTEMPTS, epoch: m.ctx.epoch });
-    m = reduce(m, { type: "RECONNECT_NONE", epoch: m.ctx.epoch });
-    expect(m.phase.name).toBe("reconnecting");
-    if (m.phase.name === "reconnecting") expect(m.phase.failed).toBe(true);
-    // RETRY restarts at attempt 1.
-    m = reduce(m, { type: "RETRY" });
-    expect(m.phase.name).toBe("reconnecting");
-    if (m.phase.name === "reconnecting") {
-      expect(m.phase.attempt).toBe(1);
-      expect(m.phase.failed).toBe(false);
-    }
-    expect(hasEffect(m, "resumeStream")).toBe(true);
-  });
-
-  it("reconnectDelayMs is the exponential backoff 1s,2s,4s,8s,16s", () => {
-    expect([1, 2, 3, 4, 5].map(reconnectDelayMs)).toEqual([1000, 2000, 4000, 8000, 16000]);
-  });
-});
-
-// #488 commit 4 — polling stalled-state + user-tail gating.
-describe("run-fsm — commit 4: stalled + run-fact gating", () => {
-  it("POLL_IDLE_CAP: polling → stalled with a banner (poll disarmed), not silent", () => {
-    let m = reduce(withRunFact(), { type: "ATTACH_START", runId: "r" });
-    m = reduce(m, { type: "ATTACH_NONE", epoch: m.ctx.epoch });
-    expect(m.phase.name).toBe("polling");
-    m = reduce(m, { type: "POLL_IDLE_CAP" });
-    expect(m.phase.name).toBe("stalled");
-    expect(hasEffect(m, "disarmPoll")).toBe(true);
-  });
-
-  it("RETRY from stalled re-arms the poll", () => {
-    let m = reduce(withRunFact(), { type: "ATTACH_START", runId: "r" });
-    m = reduce(m, { type: "ATTACH_NONE", epoch: m.ctx.epoch });
-    m = reduce(m, { type: "POLL_IDLE_CAP" });
-    m = reduce(m, { type: "RETRY" });
-    expect(m.phase.name).toBe("polling");
-    expect(hasEffect(m, "armPoll")).toBe(true);
-  });
-
-  it("a fresh NEGATIVE run-fact while attaching cancels recovery (user-tail, no active run)", () => {
-    // The mount POST /run returns no active run: attaching → idle, no poll armed.
-    let m = reduce(withRunFact(), { type: "ATTACH_START", runId: "r" });
-    m = reduce(m, { type: "RUN_FACT", runFact: null, epoch: m.ctx.epoch });
-    expect(m.phase.name).toBe("idle");
-    expect(m.ctx.runFact).toBeNull();
-    expect(hasEffect(m, "disarmPoll")).toBe(true);
-  });
-
-  it("a negative run-fact while polling stops the poll", () => {
-    let m = reduce(withRunFact(), { type: "ATTACH_START", runId: "r" });
-    m = reduce(m, { type: "ATTACH_NONE", epoch: m.ctx.epoch });
-    m = reduce(m, { type: "RUN_FACT", runFact: null, epoch: m.ctx.epoch });
-    expect(m.phase.name).toBe("idle");
-  });
-
-  it("POLL_TERMINAL settles polling → idle (I4 data-driven exit)", () => {
-    let m = reduce(withRunFact(), { type: "ATTACH_START", runId: "r" });
-    m = reduce(m, { type: "ATTACH_NONE", epoch: m.ctx.epoch });
-    m = reduce(m, { type: "POLL_TERMINAL" });
-    expect(m.phase.name).toBe("idle");
-    expect(m.ctx.runFact).toBeNull();
-  });
-});
-
-// #488 commit 5 — error classification + supersede CAS transitions.
-describe("run-fsm — commit 5: supersede CAS + error classification", () => {
-  it("SUPERSEDE_REQUESTED → superseding, fires the CAS effect, bumps epoch", () => {
-    const streaming = withRunFact("run-old");
-    const m = reduce(streaming, { type: "SUPERSEDE_REQUESTED", targetRunId: "run-old" });
-    expect(m.phase.name).toBe("superseding");
-    expect(m.ctx.epoch).toBe(streaming.ctx.epoch + 1);
-    const sup = m.effects.find((e) => e.type === "supersede");
-    expect(sup).toEqual({ type: "supersede", targetRunId: "run-old" });
-  });
-
-  it("SUPERSEDE_READY → streaming as the new local owner", () => {
-    let m = reduce(withRunFact("run-old"), { type: "SUPERSEDE_REQUESTED", targetRunId: "run-old" });
-    m = reduce(m, { type: "SUPERSEDE_READY", runId: "run-new", epoch: m.ctx.epoch });
-    expect(m.phase.name).toBe("streaming");
-    expect(m.ctx.ownership).toBe("local");
-    expect(m.ctx.runFact).toEqual({ runId: "run-new" });
-  });
-
-  it("SUPERSEDE_MISMATCH → error(supersede-mismatch) + verify via /run (no blind banner)", () => {
-    let m = reduce(withRunFact("run-old"), { type: "SUPERSEDE_REQUESTED", targetRunId: "run-old" });
-    m = reduce(m, { type: "SUPERSEDE_MISMATCH", currentRunId: "run-x", epoch: m.ctx.epoch });
-    expect(m.phase).toEqual({ name: "error", kind: "supersede-mismatch" });
-    expect(hasEffect(m, "postRun")).toBe(true);
-    expect(m.ctx.runFact).toEqual({ runId: "run-x" });
-  });
-
-  it("SUPERSEDE_TIMEOUT → error(supersede-timeout), no auto-retry effect", () => {
-    let m = reduce(withRunFact("run-old"), { type: "SUPERSEDE_REQUESTED", targetRunId: "run-old" });
-    m = reduce(m, { type: "SUPERSEDE_TIMEOUT", epoch: m.ctx.epoch });
-    expect(m.phase).toEqual({ name: "error", kind: "supersede-timeout" });
-    expect(m.effects).toEqual([]);
-  });
-
-  it("SUPERSEDE_INVALID → error(supersede-invalid)", () => {
-    let m = reduce(withRunFact("run-old"), { type: "SUPERSEDE_REQUESTED", targetRunId: "run-old" });
-    m = reduce(m, { type: "SUPERSEDE_INVALID", epoch: m.ctx.epoch });
-    expect(m.phase).toEqual({ name: "error", kind: "supersede-invalid" });
-  });
-
-  it("a stale SUPERSEDE outcome from a superseded epoch is dropped", () => {
-    let m = reduce(withRunFact("run-old"), { type: "SUPERSEDE_REQUESTED", targetRunId: "run-old" });
-    const supersedingEpoch = m.ctx.epoch;
-    // The user retriggers, bumping the epoch again.
-    m = reduce(m, { type: "SUPERSEDE_REQUESTED", targetRunId: "run-old" });
-    // The first CAS's late TIMEOUT (old epoch) must NOT knock us out of superseding.
-    const late = reduce(m, { type: "SUPERSEDE_TIMEOUT", epoch: supersedingEpoch });
-    expect(late.phase.name).toBe("superseding");
-  });
-
-  it("RUN_ALREADY_ACTIVE (plain POST gate) → error(run-already-active), no retry effect", () => {
-    const m = reduce(run(initialMachine(), { type: "SEND_LOCAL" }), { type: "RUN_ALREADY_ACTIVE" });
-    expect(m.phase).toEqual({ name: "error", kind: "run-already-active" });
-    expect(m.effects).toEqual([]);
-  });
-});
-
-// #488 F2 — a late mount `getRun → ATTACH_START` must not hijack a local turn.
-describe("run-fsm — F2: ATTACH_START only from idle", () => {
-  it("ATTACH_START from a local `sending` turn is ignored (no observer hijack)", () => {
-    const sending = reduce(initialMachine(), { type: "SEND_LOCAL" }); // idle -> sending, local
-    const m = reduce(sending, { type: "ATTACH_START", runId: "r" });
-    expect(m.phase.name).toBe("sending");
-    expect(m.ctx.ownership).toBe("local"); // NOT flipped to observer
-    expect(m.effects).toEqual([]); // no resumeStream
-  });
-
-  it("ATTACH_START from idle attaches as normal", () => {
-    const m = reduce(initialMachine(), { type: "ATTACH_START", runId: "r" });
-    expect(m.phase.name).toBe("attaching");
-    expect(m.ctx.ownership).toBe("observer");
-    expect(hasEffect(m, "resumeStream")).toBe(true);
-  });
-});
-
-describe("run-fsm — stop (I4: exit by data)", () => {
-  it("STOP_REQUESTED → stopping, fires stopRun + abortAttach, no data-independent exit", () => {
-    const m = reduce(withRunFact(), { type: "STOP_REQUESTED" });
-    expect(m.phase.name).toBe("stopping");
-    expect(effectTypes(m)).toEqual(expect.arrayContaining(["stopRun", "abortAttach"]));
-  });
-
-  it("stopping exits on the aborted stream's finish carrying the PRE-STOP epoch", () => {
-    // MEDIUM (#488 re-review): STOP_REQUESTED is a command that BUMPS the epoch, but
-    // the runtime stamps the aborted stream's onFinish with the stream's START (pre-
-    // stop) generation — exactly what the component sends. `stopping` must HONOR
-    // that finish regardless of generation (no idle-cap covers `stopping`).
-    // MUTATION-VERIFY: remove the honor-in-`stopping` branch and this hangs in
-    // `stopping` (the epoch filter drops the pre-stop finish) -> red.
-    const preStopEpoch = withRunFact().ctx.epoch; // E1 (the stream's start epoch)
-    let m = reduce(withRunFact(), { type: "STOP_REQUESTED" }); // E1 -> E2, stopping
-    expect(m.ctx.epoch).toBe(preStopEpoch + 1);
-    m = reduce(m, { type: "FINISH_ABORT", epoch: preStopEpoch }); // NOT the current epoch
-    expect(m.phase.name).toBe("idle");
-    expect(m.ctx.runFact).toBeNull();
-  });
-
-  it("stopping exits on a clean finish carrying the pre-stop epoch too", () => {
-    const preStopEpoch = withRunFact().ctx.epoch;
-    let m = reduce(withRunFact(), { type: "STOP_REQUESTED" });
-    m = reduce(m, { type: "FINISH_CLEAN", epoch: preStopEpoch });
-    expect(m.phase.name).toBe("idle");
-  });
-
-  it("stopping exits on a negative run-fact (data)", () => {
-    let m = reduce(withRunFact(), { type: "STOP_REQUESTED" });
-    m = reduce(m, { type: "RUN_FACT", runFact: null, epoch: m.ctx.epoch });
-    expect(m.phase.name).toBe("idle");
-  });
-
-  // Review #4: `stopping` arms the poll but had no inactivity backstop.
-  it("review-4: POLL_IDLE_CAP in `stopping` exits to idle (bounded), NOT stalled", () => {
-    let m = reduce(withRunFact(), { type: "STOP_REQUESTED" });
-    expect(m.phase.name).toBe("stopping");
-    expect(hasEffect(m, "armPoll")).toBe(true);
-    // MUTATION-VERIFY: drop the `stopping` branch in POLL_IDLE_CAP and this hangs
-    // in `stopping` (poll forever) -> red.
-    m = reduce(m, { type: "POLL_IDLE_CAP" });
-    expect(m.phase.name).toBe("idle");
-    expect(hasEffect(m, "disarmPoll")).toBe(true);
-    expect(m.ctx.ownership).toBe("local");
-  });
-});
-
-// Review #1: positive attach outcomes must be guarded by the SOURCE phase — the
-// epoch filter alone is insufficient because POLL_TERMINAL uses to() (no epoch
-// bump) and does not abort the in-flight GET.
-describe("run-fsm — review-1: attach outcomes guarded by source phase", () => {
-  it("a late RECONNECT_ATTACHED after POLL_TERMINAL stays idle (no phantom streaming)", () => {
-    let m = withRunFact("run-1");
-    m = reduce(m, { type: "FINISH_DISCONNECT", hasVisibleContent: true, epoch: m.ctx.epoch });
-    m = reduce(m, { type: "RECONNECT_ATTEMPT", attempt: 1, epoch: m.ctx.epoch }); // attach GET
-    const epoch = m.ctx.epoch;
-    // The armed degraded poll reaches the terminal row FIRST (epoch unchanged).
-    m = reduce(m, { type: "POLL_TERMINAL" });
-    expect(m.phase.name).toBe("idle");
-    expect(m.ctx.epoch).toBe(epoch); // POLL_TERMINAL did NOT bump the epoch
-    // The slow GET returns live 2xx under the SAME epoch — must NOT resurrect.
-    m = reduce(m, { type: "RECONNECT_ATTACHED", epoch });
-    expect(m.phase.name).toBe("idle");
-  });
-
-  it("a late ATTACH_LIVE / ATTACH_NONE after leaving `attaching` is ignored", () => {
-    let m = reduce(initialMachine(), { type: "ATTACH_START", runId: "r" });
-    const epoch = m.ctx.epoch;
-    m = reduce(m, { type: "ATTACH_NONE", epoch }); // attaching -> polling
-    m = reduce(m, { type: "POLL_TERMINAL" }); // -> idle (epoch unchanged)
-    expect(m.phase.name).toBe("idle");
-    m = reduce(m, { type: "ATTACH_LIVE", epoch }); // late 2xx, same epoch
-    expect(m.phase.name).toBe("idle");
-    // And a late ATTACH_NONE (not `attaching`) is a no-op too.
-    m = reduce(m, { type: "ATTACH_NONE", epoch });
-    expect(m.phase.name).toBe("idle");
-  });
-});
-
-// Review #2: every terminal transition resets ownership to local.
-describe("run-fsm — review-2: terminal transitions reset ownership to local", () => {
-  const observer = (): Machine => {
-    let m = reduce(initialMachine(), { type: "ATTACH_START", runId: "r" });
-    m = reduce(m, { type: "ATTACH_LIVE", epoch: m.ctx.epoch });
-    expect(m.ctx.ownership).toBe("observer");
-    return m;
-  };
-  it("FINISH_CLEAN resets ownership", () => {
-    const m = reduce(observer(), { type: "FINISH_CLEAN", epoch: observer().ctx.epoch });
-    expect(m.ctx.ownership).toBe("local");
-  });
-  it("FINISH_ERROR / POLL_TERMINAL / RUN_FACT(null) reset ownership", () => {
-    let o = observer();
-    expect(reduce(o, { type: "FINISH_ERROR", kind: "stream", epoch: o.ctx.epoch }).ctx.ownership).toBe("local");
-    // POLL_TERMINAL from an observer polling phase
-    let p = reduce(observer(), { type: "STREAM_INCOMPLETE", reason: "starved", epoch: observer().ctx.epoch });
-    expect(reduce(p, { type: "POLL_TERMINAL" }).ctx.ownership).toBe("local");
-    // RUN_FACT(null) from an observer attaching phase
-    let a = reduce(initialMachine(), { type: "ATTACH_START", runId: "r" });
-    expect(reduce(a, { type: "RUN_FACT", runFact: null, epoch: a.ctx.epoch }).ctx.ownership).toBe("local");
-  });
-});
-
-describe("run-fsm — ownership (I2) is context, orthogonal to phase", () => {
-  it("attach/reconnect set observer; send/supersede-ready set local", () => {
-    let m = reduce(initialMachine(), { type: "ATTACH_START", runId: "r" });
-    expect(m.ctx.ownership).toBe("observer");
-    m = reduce(m, { type: "ATTACH_LIVE", epoch: m.ctx.epoch });
-    expect(m.phase.name).toBe("streaming");
-    expect(m.ctx.ownership).toBe("observer"); // still observing a detached run
-    // A local send flips ownership back to local.
-    m = reduce(m, { type: "SEND_LOCAL" });
-    expect(m.ctx.ownership).toBe("local");
-  });
-});
-
-describe("run-fsm — dispose (I5)", () => {
-  it("DISPOSE from any phase aborts controllers and bumps epoch", () => {
-    let m = reduce(withRunFact(), { type: "ATTACH_START", runId: "r" });
-    const before = m.ctx.epoch;
-    m = reduce(m, { type: "DISPOSE" });
-    expect(m.phase.name).toBe("idle");
-    expect(m.ctx.epoch).toBe(before + 1);
-    expect(effectTypes(m)).toEqual(
-      expect.arrayContaining(["abortAttach", "cancelReconnect", "disarmPoll"]),
-    );
-  });
-});
@@ -1,595 +0,0 @@
-/**
- * Run-lifecycle finite state machine for a single AI-chat thread (#488).
- *
- * ============================================================================
- * WHY THIS EXISTS
- * ----------------------------------------------------------------------------
- * The resume/reconnect/poll/stop/supersede lifecycle used to be spread across
- * ~26 `useRef` one-shot flags in `chat-thread.tsx`, each disarmed "on every
- * path". Ownerless flag combinations produced silent UI freezes, and every fix
- * added another ref (the #381 -> #432 -> #456 spiral). This module replaces that
- * ref-zoo with ONE pure reducer whose transitions are enumerable and unit-
- * testable in isolation (event x state -> next state is the observable property).
- *
- * The reducer is PURE: it owns no timers, no fetches, no React state. It maps
- * `(machine, event) -> machine`, where the returned machine carries the list of
- * COMMAND EFFECTS to run for that transition. A thin runtime in `chat-thread.tsx`
- * dispatches events (from SDK callbacks / HTTP outcomes) and executes the
- * effects (attach GET, POST /stream, POST /run, POST /stop, backoff timers,
- * poll arm/disarm). The runtime lives in a THREAD, not the window, so a late SDK
- * callback dies with the owner (kills the "event from a dead view" class, #161).
- *
- * ============================================================================
- * INVARIANTS (see run-fsm.spec.md for the full spec + tables)
- * ----------------------------------------------------------------------------
- *  I1  EPOCH (generation counter). Commands (`resumeStream`, `postRun`, `stop`,
- *      `supersede`, `scheduleReconnect`) are async; their outcomes arrive on the
- *      SAME SDK/HTTP callbacks. Every command-emitting transition increments
- *      `ctx.epoch`; every OUTCOME event carries the epoch it was issued under;
- *      the reducer DROPS an outcome whose epoch != the current epoch. This is
- *      what the one-shot-ref zoo used to approximate by hand.
- *  I2  OWNERSHIP is a CONTEXT FIELD (`'local' | 'observer'`), not a state —
- *      orthogonal to the transport phase. The queue is flushed ONLY by a local
- *      owner (an observer following a detached run never flushes).
- *  I3  RUN-FACT ("a run is active") is first-class from the server: `runFact`
- *      holds the server-confirmed active run id (POST /run on mount, the `start`
- *      metadata runId, attach outcomes). Reconnect is entered by the RUN-FACT,
- *      not by the presence of an assistant message (#488 commit 2). A fresh
- *      negative fact (null) cancels reconnect immediately.
- *  I4  Exit `stopping` by DATA (a terminal row / negative run-fact), NEVER by the
- *      stopRun HTTP response (which returns after abort, before finalization).
- *  I5  Command controllers are effect-owned (abort in cleanup), NOT render-phase
- *      refs — expressed here as the `abortAttach` effect on disposing transitions.
- * ============================================================================
- */
-
-// ---------------------------------------------------------------------------
-// Phases (the transport lifecycle). Ownership / runFact are CONTEXT, not here.
-// ---------------------------------------------------------------------------
-
-/** Why the degraded poll is the active recovery. */
-export type PollReason =
-  | "attach-none" // mount attach returned 204 / error — nothing live to attach
-  | "starved" // a resumed finish carried no visible content
-  | "disconnect-visible" // a live disconnect WITH on-screen content — poll to terminal
-  | "reconnect-exhausted"; // the live re-attach ladder gave up
-
-/** The classified error kind (drives the banner text + composer behavior). */
-export type ErrorKind =
-  | "stream" // a generic provider/network stream error (useChat error)
-  | "run-already-active" // 409 A_RUN_ALREADY_ACTIVE (a plain POST hit the gate)
-  | "supersede-mismatch" // 409 SUPERSEDE_TARGET_MISMATCH (CAS target moved)
-  | "supersede-timeout" // 409 SUPERSEDE_TIMEOUT (old run did not settle in W)
-  | "supersede-invalid" // 409 SUPERSEDE_INVALID (bad supersede target)
-  | "begin-failed"; // 503 A_RUN_BEGIN_FAILED (could not start the run)
-
-export type Phase =
-  | { name: "idle" }
-  | { name: "sending" } // local POST in flight, before the first frame
-  | { name: "streaming" } // receiving frames
-  | { name: "attaching" } // mount-time attach GET in flight
-  | { name: "reconnecting"; attempt: number; failed: boolean }
-  | { name: "polling"; reason: PollReason }
-  | { name: "stalled" } // poll hit the inactivity cap — banner + Retry
-  | { name: "stopping" }
-  | { name: "superseding" }
-  | { name: "error"; kind: ErrorKind };
-
-export type Ownership = "local" | "observer";
-
-/** The server-confirmed active run, or null when no run is active. */
-export type RunFact = { runId: string } | null;
-
-export interface Ctx {
-  /** I1: generation counter — every command-transition increments it. */
-  epoch: number;
-  /** I2: does THIS client own the turn's writes (local streamer) or observe? */
-  ownership: Ownership;
-  /** I3: the server-confirmed active run. */
-  runFact: RunFact;
-  /**
-   * Are we FOLLOWING a live run we were locally streaming (the reconnect ladder),
-   * as opposed to a one-shot mount-attach resume? Both are `ownership: 'observer'`,
-   * but they recover DIFFERENTLY on a drop: a live-follow drop RE-ENTERS the
-   * reconnect ladder (#488 commit 3 — the second break after a successful re-attach
-   * must reconnect again, not fall to silent poll), while a mount-resume drop falls
-   * to the degraded poll. This is the ctx bit that separates the two WITHOUT a new
-   * component ref (it is why commit 3 needs the FSM, not a surgical patch).
-   */
-  liveFollow: boolean;
-}
-
-export interface Machine {
-  phase: Phase;
-  ctx: Ctx;
-  /** Command effects to run for the transition that produced THIS machine.
-   *  The runtime executes them and does not read them again. */
-  effects: Effect[];
-}
-
-// ---------------------------------------------------------------------------
-// Command effects (the reducer's only side-channel — executed by the runtime).
-// ---------------------------------------------------------------------------
-
-export type Effect =
-  /** POST /run to (re)establish or verify the run-fact. `reason` is diagnostic. */
-  | { type: "postRun"; reason: "mount" | "verify" }
-  /** Trigger the SDK `resumeStream()` (attach GET via prepareReconnectToStream). */
-  | { type: "resumeStream" }
-  /** Schedule a reconnect attempt after a backoff, then dispatch RECONNECT_ATTEMPT. */
-  | { type: "scheduleReconnect"; attempt: number; delayMs: number }
-  /** Cancel any pending reconnect backoff timer. */
-  | { type: "cancelReconnect" }
-  /** Arm the degraded poll (the window's dumb timer follows the run in the DB). */
-  | { type: "armPoll"; reason: PollReason }
-  /** Disarm the degraded poll. */
-  | { type: "disarmPoll" }
-  /** POST /stop the chat's active run (authoritative detached-run stop). */
-  | { type: "stopRun" }
-  /** POST /stream { supersede: { runId } } — the CAS "interrupt and send now". */
-  | { type: "supersede"; targetRunId: string }
-  /** Abort the in-flight attach/reconnect GET controller (dispose / observer stop). */
-  | { type: "abortAttach" };
-
-// ---------------------------------------------------------------------------
-// Events. An OUTCOME event MAY carry `epoch`; if it does and it does not equal
-// the current epoch, the reducer drops it (I1). Trigger events (user actions,
-// fresh disconnects) carry no epoch and are never dropped.
-// ---------------------------------------------------------------------------
-
-export type Event =
-  // -- local turn --
-  | { type: "SEND_LOCAL" }
-  | { type: "STREAM_START"; runId?: string; epoch?: number }
-  /** An OBSERVER's attached stream ended WITHOUT reaching terminal (a starved
-   *  clean replay, or a torn resume) — fall to the degraded poll to drive the row
-   *  to its real terminal state. (A live-follow drop uses FINISH_DISCONNECT.) */
-  | { type: "STREAM_INCOMPLETE"; reason: PollReason; epoch?: number }
-  | { type: "FINISH_CLEAN"; epoch?: number }
-  | { type: "FINISH_ABORT"; epoch?: number }
-  | { type: "FINISH_DISCONNECT"; hasVisibleContent: boolean; epoch?: number }
-  | { type: "FINISH_ERROR"; kind: ErrorKind; epoch?: number }
-  // -- mount attach (resume) --
-  | { type: "ATTACH_START"; runId?: string }
-  | { type: "ATTACH_LIVE"; epoch?: number }
-  | { type: "ATTACH_NONE"; epoch?: number }
-  // -- reconnect after a live disconnect (entered by FINISH_DISCONNECT, #488 c2) --
-  | { type: "RECONNECT_ATTEMPT"; attempt: number; epoch?: number }
-  | { type: "RECONNECT_ATTACHED"; epoch?: number }
-  | { type: "RECONNECT_NONE"; epoch?: number }
-  | { type: "RETRY" }
-  // -- degraded poll --
-  | { type: "POLL_TERMINAL" }
-  | { type: "POLL_IDLE_CAP" }
-  // -- run-fact (server-confirmed active run) --
-  | { type: "RUN_FACT"; runFact: RunFact; epoch?: number }
-  // -- stop --
-  | { type: "STOP_REQUESTED" }
-  // -- supersede (CAS) --
-  | { type: "SUPERSEDE_REQUESTED"; targetRunId: string }
-  | { type: "SUPERSEDE_READY"; runId?: string; epoch?: number }
-  | { type: "SUPERSEDE_MISMATCH"; currentRunId?: string; epoch?: number }
-  | { type: "SUPERSEDE_TIMEOUT"; epoch?: number }
-  | { type: "SUPERSEDE_INVALID"; epoch?: number }
-  | { type: "RUN_ALREADY_ACTIVE" }
-  // -- lifecycle --
-  | { type: "DISPOSE" };
-
-export const RECONNECT_MAX_ATTEMPTS = 5;
-export const RECONNECT_BASE_DELAY_MS = 1000;
-/** Backoff before attempt N (1-based): 1s, 2s, 4s, 8s, 16s. */
-export function reconnectDelayMs(attempt: number): number {
-  return RECONNECT_BASE_DELAY_MS * 2 ** (attempt - 1);
-}
-
-// ---------------------------------------------------------------------------
-// Constructors / helpers.
-// ---------------------------------------------------------------------------
-
-export function initialMachine(overrides?: Partial<Ctx>): Machine {
-  return {
-    phase: { name: "idle" },
-    ctx: { epoch: 0, ownership: "local", runFact: null, liveFollow: false, ...overrides },
-    effects: [],
-  };
-}
-
-/** Build a machine result: a phase, optional ctx patch, and effects. Empty
- *  effects by default. Never mutates the input. */
-function to(
-  m: Machine,
-  phase: Phase,
-  opts?: { ctx?: Partial<Ctx>; effects?: Effect[] },
-): Machine {
-  return {
-    phase,
-    ctx: { ...m.ctx, ...(opts?.ctx ?? {}) },
-    effects: opts?.effects ?? [],
-  };
-}
-
-/** No transition: keep the phase, clear effects (so a re-run does not re-fire). */
-function stay(m: Machine): Machine {
-  return { phase: m.phase, ctx: m.ctx, effects: [] };
-}
-
-/** A command-transition: same as `to` but bumps the epoch (I1). Any outcome
- *  event issued under the old epoch is dropped once this lands. */
-function command(
-  m: Machine,
-  phase: Phase,
-  effects: Effect[],
-  ctx?: Partial<Ctx>,
-): Machine {
-  return {
-    phase,
-    ctx: { ...m.ctx, ...(ctx ?? {}), epoch: m.ctx.epoch + 1 },
-    effects,
-  };
-}
-
-// ---------------------------------------------------------------------------
-// The pure reducer.
-// ---------------------------------------------------------------------------
-
-/** The terminal stream-finish events (one turn's stream ended). */
-function isFinishEvent(event: Event): boolean {
-  return (
-    event.type === "FINISH_ABORT" ||
-    event.type === "FINISH_CLEAN" ||
-    event.type === "FINISH_DISCONNECT" ||
-    event.type === "FINISH_ERROR" ||
-    event.type === "STREAM_INCOMPLETE"
-  );
-}
-
-export function reduce(m: Machine, event: Event): Machine {
-  // MEDIUM (#488 re-review): honor ANY stream finish in `stopping` regardless of
-  // generation. A plain user Stop has NO successor stream — the aborted stream's
-  // finish IS the expected end of the stop, so exit `stopping -> idle` by that DATA
-  // (I4). The epoch filter below must NOT drop it: STOP_REQUESTED bumped the epoch,
-  // but the finish carries the PRE-stop generation (the runtime stamps it with the
-  // stream's start epoch), so I1 would otherwise strand the machine in `stopping`
-  // forever (no idle-cap covers `stopping`). The epoch filter stays in force for
-  // `superseding` (a successor B owns) — that is the F1 supersede drop.
-  if (m.phase.name === "stopping" && isFinishEvent(event)) {
-    return to(m, { name: "idle" }, {
-      // Reset ownership to local on this terminal transition (review #2): otherwise
-      // an observer-stop leaves ownership 'observer' and hides "Send now" forever.
-      ctx: { runFact: null, liveFollow: false, ownership: "local" },
-      effects: [{ type: "disarmPoll" }, { type: "cancelReconnect" }],
-    });
-  }
-
-  // I1: drop a stale outcome (an event issued under a superseded epoch).
-  if ("epoch" in event && event.epoch !== undefined && event.epoch !== m.ctx.epoch) {
-    return stay(m);
-  }
-
-  switch (event.type) {
-    // ---- local turn ----------------------------------------------------
-    case "SEND_LOCAL":
-      // A local send owns the view: leave any recovery, become the local
-      // streamer, disarm poll/reconnect. epoch++ so a late recovery outcome
-      // from the previous phase is dropped.
-      return command(
-        m,
-        { name: "sending" },
-        [{ type: "cancelReconnect" }, { type: "disarmPoll" }],
-        { ownership: "local", liveFollow: false },
-      );
-
-    case "STREAM_INCOMPLETE":
-      // An OBSERVER's attached stream ended incomplete (starved / torn) — follow
-      // the run to terminal via the degraded poll.
-      return to(m, { name: "polling", reason: event.reason }, {
-        effects: [{ type: "armPoll", reason: event.reason }],
-      });
-
-    case "STREAM_START": {
-      // First frame arrived. Adopt the run-fact runId if present. sending ->
-      // streaming; a reconnect/attach that just went live also lands here.
-      const runFact = event.runId ? { runId: event.runId } : m.ctx.runFact;
-      return to(m, { name: "streaming" }, {
-        ctx: { runFact },
-        effects: [{ type: "cancelReconnect" }, { type: "disarmPoll" }],
-      });
-    }
-
-    case "FINISH_CLEAN":
-      // A clean terminal outcome. The run is done — clear the run-fact and go
-      // idle. (The queue flush is a component concern gated by ownership; the
-      // FSM only models the phase.) Review #2: reset ownership to local so a
-      // just-finished observer-attach turn re-exposes "Send now" for the queue.
-      return to(m, { name: "idle" }, {
-        ctx: { runFact: null, liveFollow: false, ownership: "local" },
-        effects: [{ type: "disarmPoll" }, { type: "cancelReconnect" }],
-      });
-
-    case "FINISH_ABORT":
-      // A user Stop / intentional abort finished. If we were stopping, the
-      // terminal data has now arrived (I4) — go idle. The run-fact is cleared.
-      return to(m, { name: "idle" }, {
-        ctx: { runFact: null, liveFollow: false, ownership: "local" },
-        effects: [{ type: "disarmPoll" }, { type: "cancelReconnect" }],
-      });
-
-    case "FINISH_DISCONNECT":
-      // A LIVE SSE drop. Recovery depends on WHO we are (I2 + liveFollow):
-      //  - a mount-attach OBSERVER (a one-shot resume, NOT live-follow) that drops
-      //    -> the degraded poll drives the row to terminal from the DB.
-      if (m.ctx.ownership === "observer" && !m.ctx.liveFollow) {
-        return to(m, { name: "polling", reason: "disconnect-visible" }, {
-          effects: [{ type: "armPoll", reason: "disconnect-visible" }],
-        });
-      }
-      //  - a LOCAL live turn (first drop) OR a live-follow re-attach (a SUBSEQUENT
-      //    drop) -> (re-)enter the reconnect ladder. #488 commit 3: allowed
-      //    REPEATEDLY — `liveFollow` is kept across a successful re-attach, so the
-      //    second break reconnects again instead of falling to silent poll.
-      // #488 commit 2: gated on the RUN-FACT (or an existing live-follow), NOT on
-      // the presence of an assistant message — a setup-phase break still recovers.
-      //  - visible content already on screen -> keep it, ALSO poll to terminal
-      //    (a full replay could clobber the fuller live tail);
-      //  - no visible content -> the reconnect ladder rebuilds it.
-      if (m.ctx.runFact || m.ctx.liveFollow) {
-        const effects: Effect[] = [
-          { type: "scheduleReconnect", attempt: 1, delayMs: reconnectDelayMs(1) },
-        ];
-        if (event.hasVisibleContent) effects.push({ type: "armPoll", reason: "disconnect-visible" });
-        return command(m, { name: "reconnecting", attempt: 1, failed: false }, effects, {
-          ownership: "observer",
-          liveFollow: true,
-        });
-      }
-      // No run to recover: a plain disconnect. Surface the terminal notice.
-      return to(m, { name: "idle" }, {
-        ctx: { runFact: null, liveFollow: false, ownership: "local" },
-      });
-
-    case "FINISH_ERROR":
-      return to(m, { name: "error", kind: event.kind }, {
-        ctx: { runFact: null, liveFollow: false, ownership: "local" },
-        effects: [{ type: "disarmPoll" }, { type: "cancelReconnect" }],
-      });
-
-    // ---- mount attach (resume) ----------------------------------------
-    case "ATTACH_START":
-      // A reopened tab attaches to a still-running run: observer ownership.
-      // #488 F2: ONLY from idle. The mount `getRun` round-trip resolves async, and
-      // a local send may have started meanwhile (phase `sending`, ownership local);
-      // a late ATTACH_START must NOT hijack that local turn into an observer-attach
-      // (queue would stop flushing, "Send now" would hide). Guarding in the reducer
-      // covers every dispatch source.
-      if (m.phase.name !== "idle") return stay(m);
-      return command(m, { name: "attaching" }, [{ type: "resumeStream" }], {
-        ownership: "observer",
-        runFact: event.runId ? { runId: event.runId } : m.ctx.runFact,
-      });
-
-    case "ATTACH_LIVE":
-      // The attach GET returned a live 2xx stream — follow it as an observer.
-      // Review #1: guard by SOURCE phase. The epoch filter alone is not enough — a
-      // POLL_TERMINAL uses to() (no epoch bump) and does not abort the in-flight
-      // GET, so a slow 2xx landing after the machine already left `attaching` (e.g.
-      // the armed poll saw the terminal row -> idle) would resurrect a settled run
-      // into a phantom `streaming`. Only enter streaming FROM `attaching`.
-      if (m.phase.name !== "attaching") return stay(m);
-      return to(m, { name: "streaming" });
-
-    case "ATTACH_NONE":
-      // 204 / non-2xx / throw: nothing live to attach. Arm the degraded poll to
-      // follow the run to terminal from the DB. This is a soft-negative run-fact
-      // (204 on a non-stripped path is authoritative-negative; the runtime may
-      // pass a RUN_FACT null separately). Keep the run-fact as-is here.
-      // Review #1: guard by source phase for consistency (a late outcome after the
-      // machine already left `attaching` must not re-arm a poll).
-      if (m.phase.name !== "attaching") return stay(m);
-      return to(m, { name: "polling", reason: "attach-none" }, {
-        effects: [{ type: "armPoll", reason: "attach-none" }],
-      });
-
-    // ---- reconnect after a live disconnect ----------------------------
-    case "RECONNECT_ATTEMPT":
-      // A scheduled backoff fired — fire the attach GET. epoch++ so the previous
-      // attempt's late outcome cannot drive this one.
-      if (m.phase.name !== "reconnecting") return stay(m);
-      return command(
-        m,
-        { name: "reconnecting", attempt: event.attempt, failed: false },
-        [{ type: "resumeStream" }],
-      );
-
-    case "RECONNECT_ATTACHED":
-      // #488 commit 3: a live re-attach succeeded. Reset to streaming — the
-      // attempt counter is dropped, so a LATER disconnect can start a fresh
-      // ladder from attempt 1 (the old one-shot `!wasResumed` gate forbade a
-      // second cycle, sending the second break to silent poll).
-      // Review #1: guard by SOURCE phase. The armed degraded poll can reach the
-      // terminal row (POLL_TERMINAL -> idle, via to(), NO epoch bump, GET not
-      // aborted) BEFORE a slow reconnect GET returns 2xx; without this guard that
-      // late RECONNECT_ATTACHED (same epoch) would resurrect a settled run into a
-      // phantom `streaming`. Only re-enter streaming FROM `reconnecting`.
-      if (m.phase.name !== "reconnecting") return stay(m);
-      return to(m, { name: "streaming" }, {
-        effects: [{ type: "cancelReconnect" }, { type: "disarmPoll" }],
-      });
-
-    case "RECONNECT_NONE": {
-      // 204 / error during a reconnect attempt. Arm the degraded poll as the
-      // belt-and-suspenders fallback, then either back off to the next attempt
-      // or, at the cap, surface the manual Retry ("failed").
-      if (m.phase.name !== "reconnecting") return stay(m);
-      const attempt = m.phase.attempt;
-      if (attempt < RECONNECT_MAX_ATTEMPTS) {
-        return command(
-          m,
-          { name: "reconnecting", attempt: attempt + 1, failed: false },
-          [
-            { type: "armPoll", reason: "attach-none" },
-            { type: "scheduleReconnect", attempt: attempt + 1, delayMs: reconnectDelayMs(attempt + 1) },
-          ],
-        );
-      }
-      return to(m, { name: "reconnecting", attempt, failed: true }, {
-        effects: [{ type: "armPoll", reason: "reconnect-exhausted" }],
-      });
-    }
-
-    case "RETRY":
-      // Manual Retry from the "failed" reconnect banner OR the stalled banner.
-      if (m.phase.name === "reconnecting" && m.phase.failed) {
-        return command(
-          m,
-          { name: "reconnecting", attempt: 1, failed: false },
-          [{ type: "resumeStream" }],
-        );
-      }
-      if (m.phase.name === "stalled") {
-        // Re-arm the poll to try to catch the run up again.
-        return command(m, { name: "polling", reason: "attach-none" }, [
-          { type: "armPoll", reason: "attach-none" },
-        ]);
-      }
-      return stay(m);
-
-    // ---- degraded poll -------------------------------------------------
-    case "POLL_TERMINAL":
-      // The run reached a terminal row via the poll (or the reconcile merge). Go
-      // idle and disarm everything (I4: this is a DATA-driven exit, incl. exit
-      // from `stopping`). Review #2: reset ownership to local.
-      return to(m, { name: "idle" }, {
-        ctx: { runFact: null, liveFollow: false, ownership: "local" },
-        effects: [{ type: "disarmPoll" }, { type: "cancelReconnect" }],
-      });
-
-    case "POLL_IDLE_CAP":
-      // Review #4: `stopping` also arms the poll (STOP_REQUESTED) but has NO other
-      // backstop — an observer-stop with no SDK stream to fire onFinish, whose
-      // server stop never drives the run terminal, would poll the DB forever. Give
-      // it a bounded exit: cap -> idle + disarm (NOT `stalled`; Stop was already
-      // pressed, so there is nothing for the user to retry).
-      if (m.phase.name === "stopping") {
-        return to(m, { name: "idle" }, {
-          ctx: { runFact: null, liveFollow: false, ownership: "local" },
-          effects: [{ type: "disarmPoll" }, { type: "cancelReconnect" }],
-        });
-      }
-      // #488 commit 4a: the poll hit the inactivity cap. Instead of going SILENT
-      // (the old "forever half-done answer"), surface a stalled banner + Retry.
-      if (m.phase.name !== "polling" && m.phase.name !== "reconnecting") return stay(m);
-      return to(m, { name: "stalled" }, {
-        effects: [{ type: "disarmPoll" }, { type: "cancelReconnect" }],
-      });
-
-    // ---- run-fact ------------------------------------------------------
-    case "RUN_FACT": {
-      const runFact = event.runFact;
-      // A fresh NEGATIVE fact (no active run) cancels recovery immediately (I3):
-      // there is nothing to reconnect to / poll for.
-      if (!runFact) {
-        if (
-          m.phase.name === "reconnecting" ||
-          m.phase.name === "attaching" ||
-          m.phase.name === "polling" ||
-          m.phase.name === "stopping"
-        ) {
-          return to(m, { name: "idle" }, {
-            // Review #2: reset ownership to local on this terminal transition.
-            ctx: { runFact: null, liveFollow: false, ownership: "local" },
-            effects: [{ type: "cancelReconnect" }, { type: "disarmPoll" }],
-          });
-        }
-        return to(m, m.phase, { ctx: { runFact: null } });
-      }
-      // A positive fact just updates the context (pessimism toward an attempt: a
-      // stale-but-positive fact permits entering recovery; a 204 will cut it).
-      return to(m, m.phase, { ctx: { runFact } });
-    }
-
-    // ---- stop ----------------------------------------------------------
-    case "STOP_REQUESTED":
-      // Authoritative stop of a detached run. Enter `stopping` and fire stopRun +
-      // abort the local/attach reader. ALSO arm the poll so the terminal row is
-      // observed — the exit is by DATA (I4: a terminal row / negative run-fact),
-      // never by the stopRun HTTP response (which returns after abort, before
-      // finalization). For a local turn the aborted stream's onFinish (ANY finish)
-      // is HONORED in `stopping` at the top of reduce() — regardless of generation
-      // — and exits to idle; the armed poll is the fallback for an observer stop
-      // with no local onFinish.
-      return command(
-        m,
-        { name: "stopping" },
-        [
-          { type: "stopRun" },
-          { type: "abortAttach" },
-          { type: "cancelReconnect" },
-          { type: "armPoll", reason: "attach-none" },
-        ],
-      );
-
-    // ---- supersede (CAS) ----------------------------------------------
-    case "SUPERSEDE_REQUESTED":
-      // "Interrupt and send now": CAS POST /stream { supersede }. epoch++ so a
-      // late outcome of the interrupted run is dropped.
-      return command(
-        m,
-        { name: "superseding" },
-        [{ type: "supersede", targetRunId: event.targetRunId }, { type: "cancelReconnect" }, { type: "disarmPoll" }],
-      );
-
-    case "SUPERSEDE_READY": {
-      // CAS succeeded (old run stopped/settled, slot taken, new run begun). We
-      // are now the local streamer of the NEW run. Adopt its runId if provided.
-      const runFact = event.runId ? { runId: event.runId } : m.ctx.runFact;
-      return to(m, { name: "streaming" }, {
-        ctx: { ownership: "local", runFact, liveFollow: false },
-      });
-    }
-
-    case "SUPERSEDE_MISMATCH":
-      // The active run moved between the click and the CAS. Per the spec: verify
-      // via /run rather than blindly banner — the mismatch may be our own already-
-      // superseded run. Surface a classified error AND fire a run-fact verify.
-      return to(m, { name: "error", kind: "supersede-mismatch" }, {
-        ctx: { runFact: event.currentRunId ? { runId: event.currentRunId } : m.ctx.runFact },
-        effects: [{ type: "postRun", reason: "verify" }],
-      });
-
-    case "SUPERSEDE_TIMEOUT":
-      // The old run did not settle within W. Nothing persisted; the composer keeps
-      // its text. Classified error, NO auto-retry (the old client retry ladder is
-      // removed in #488 commit 5).
-      return to(m, { name: "error", kind: "supersede-timeout" });
-
-    case "SUPERSEDE_INVALID":
-      return to(m, { name: "error", kind: "supersede-invalid" });
-
-    case "RUN_ALREADY_ACTIVE":
-      // A plain POST hit the one-active-run gate. NO auto-retry — the composer
-      // offers "interrupt and send" (supersede) instead.
-      return to(m, { name: "error", kind: "run-already-active" });
-
-    // ---- lifecycle -----------------------------------------------------
-    case "DISPOSE":
-      // Unmount: abort in-flight controllers, drop timers, and bump the epoch so
-      // NO late callback can drive this (now dead) machine (I5).
-      return command(
-        m,
-        { name: "idle" },
-        [
-          { type: "abortAttach" },
-          { type: "cancelReconnect" },
-          { type: "disarmPoll" },
-        ],
-        { liveFollow: false },
-      );
-
-    default: {
-      // Exhaustiveness guard.
-      const _never: never = event;
-      void _never;
-      return stay(m);
-    }
-  }
-}
@@ -3,7 +3,6 @@ import {
  resolveAdoptedChatId,
  newlyAddedChatIds,
  extractServerChatId,
-  extractRunId,
 } from "./adopt-chat-id";

 describe("resolveAdoptedChatId", () => {
@@ -71,17 +70,3 @@ describe("extractServerChatId", () => {
    expect(extractServerChatId(undefined)).toBeUndefined();
  });
 });
-
-describe("extractRunId", () => {
-  it("reads a string runId from the start metadata", () => {
-    expect(extractRunId({ metadata: { runId: "run-1" } })).toBe("run-1");
-  });
-  it("returns undefined when runId is absent", () => {
-    expect(extractRunId({ metadata: { chatId: "c" } })).toBeUndefined();
-    expect(extractRunId({})).toBeUndefined();
-    expect(extractRunId(undefined)).toBeUndefined();
-  });
-  it("returns undefined for a non-string runId", () => {
-    expect(extractRunId({ metadata: { runId: 7 } })).toBeUndefined();
-  });
-});
@@ -56,20 +56,6 @@ export function extractServerChatId(
  return typeof m?.chatId === "string" ? m.chatId : undefined;
 }

-/**
- * #488: read the authoritative RUN id off a streaming assistant message. The
- * server attaches it as `message.metadata.runId` on the `start` part when a run
- * wraps the turn (see server `chatStreamMetadata`, #184/#487). This is the live
- * run-fact update the client FSM adopts (mirrors `extractServerChatId`). Returns
- * it only when it is a string; undefined otherwise.
- */
-export function extractRunId(
-  message: { metadata?: unknown } | undefined,
-): string | undefined {
-  const m = message?.metadata as { runId?: string } | undefined;
-  return typeof m?.runId === "string" ? m.runId : undefined;
-}
-
 /**
 * The deduped set of ids present in `afterIds` but not in `beforeIds`. A
 * paginated/flatMapped list can repeat the same id, so dedupe: one genuinely-new
@@ -6,10 +6,13 @@ describe("estimateTokens", () => {
    expect(estimateTokens("")).toBe(0);
  });

-  it("ceils chars/4 so any non-empty text is at least 1 token", () => {
+  // #490: migrated onto the shared @docmost/token-estimate module (chars/2.5, up
+  // from the old client-only chars/4) so the client counter and the server replay
+  // budgeter can never diverge.
+  it("ceils chars/2.5 so any non-empty text is at least 1 token", () => {
    expect(estimateTokens("a")).toBe(1);
-    expect(estimateTokens("abcd")).toBe(1);
-    expect(estimateTokens("abcde")).toBe(2);
-    expect(estimateTokens("12345678")).toBe(2);
+    expect(estimateTokens("ab")).toBe(1);
+    expect(estimateTokens("abcde")).toBe(2); // 5 / 2.5 = 2
+    expect(estimateTokens("x".repeat(10))).toBe(4); // 10 / 2.5 = 4
  });
 });
@@ -2,18 +2,10 @@
 * Rough client-side token estimation for AI-chat UI affordances.
 *
 * No provider streams exact per-token usage mid-stream, so any in-flight figure
- * is a CLIENT ESTIMATE (chars/≈4 heuristic). Pure + unit-testable: it never runs
- * a real BPE tokenizer (that would be O(n²) on the hot path, bloat the bundle,
- * and be wrong for Gemini/Ollama anyway). Used by the in-body reasoning counter
- * ("Thinking · N tokens").
+ * is a CLIENT ESTIMATE. This re-exports the SHARED estimator from
+ * `@docmost/token-estimate` (chars/2.5) so the in-body counter and the server's
+ * replay budgeter use the SAME heuristic — two divergent estimators would mean
+ * "the badge shows 60%" while "the budgeter already trimmed" (#490). Used by the
+ * in-body reasoning counter ("Thinking · N tokens").
 */
-
-/**
- * Rough token estimate for a piece of text using the standard chars/≈4 heuristic.
- * Returns 0 for empty/whitespace-free-of-content input, and ceils so any
- * non-empty text counts as at least one token.
- */
-export function estimateTokens(text: string): number {
-  if (!text) return 0;
-  return Math.ceil(text.length / 4);
-}
+export { estimateTokens } from "@docmost/token-estimate";
@@ -42,51 +42,6 @@ describe("describeChatError", () => {
    );
  });

-  // #488 commit 5: the #487 concurrency-gate / supersede 409s. FULL real bodies:
-  // a ConflictException(object) whose response is serialized verbatim, carrying a
-  // `code` and statusCode 409. Each must classify to a human text, not raw JSON.
-  it("classifies A_RUN_ALREADY_ACTIVE (409) as already-answering, not raw JSON", () => {
-    const body =
-      '{"message":"A run is already active for this chat","code":"A_RUN_ALREADY_ACTIVE","statusCode":409}';
-    expect(describeChatError(body, t).title).toBe(
-      "The agent is already answering",
-    );
-    // Never leaks the raw code as the detail.
-    expect(describeChatError(body, t).detail).not.toContain("A_RUN_ALREADY_ACTIVE");
-  });
-
-  it("classifies SUPERSEDE_TARGET_MISMATCH (409) as run-changed", () => {
-    const body =
-      '{"message":"active run does not match the supersede target","code":"SUPERSEDE_TARGET_MISMATCH","runId":"run-x","statusCode":409}';
-    expect(describeChatError(body, t).title).toBe(
-      "Couldn't interrupt — the run changed",
-    );
-  });
-
-  it("classifies SUPERSEDE_TIMEOUT (409) as couldn't-interrupt-in-time", () => {
-    const body =
-      '{"message":"the run did not settle within the supersede window","code":"SUPERSEDE_TIMEOUT","statusCode":409}';
-    expect(describeChatError(body, t).title).toBe("Couldn't interrupt in time");
-  });
-
-  it("classifies SUPERSEDE_INVALID (409) as couldn't-interrupt-that-run", () => {
-    const body =
-      '{"message":"supervise requires chatId","code":"SUPERSEDE_INVALID","statusCode":409}';
-    expect(describeChatError(body, t).title).toBe(
-      "Couldn't interrupt that run",
-    );
-  });
-
-  it("ORDER GUARD: A_RUN_ALREADY_ACTIVE wins over any generic status branch", () => {
-    // Even though the body could superficially look 4xx-ish, the code branch runs
-    // first, so it is never mislabeled by a generic status heading.
-    const body =
-      '{"message":"conflict","code":"A_RUN_ALREADY_ACTIVE","statusCode":409}';
-    const view = describeChatError(body, t);
-    expect(view.title).not.toBe("Something went wrong");
-    expect(view.title).not.toBe("AI provider not configured");
-  });
-
  it("classifies a dropped connection (ECONNRESET) as a lost-connection error", () => {
    expect(
      describeChatError("Cannot connect to API: read ECONNRESET", t).title,
@@ -39,44 +39,6 @@ export function describeChatError(
    };
  }

-  // #488 commit 5: the #487 concurrency-gate / supersede 409s. These arrive as a
-  // ConflictException(object) body carrying a `code` (and statusCode 409). They
-  // MUST be classified by `code` STRICTLY BEFORE any generic status branch, or the
-  // user sees the raw JSON `{"code":"A_RUN_ALREADY_ACTIVE",…}`. The code strings
-  // are the real #487 server contract (ai-chat.controller.ts) — do not invent.
-  if (/"code"\s*:\s*"A_RUN_ALREADY_ACTIVE"/.test(msg)) {
-    return {
-      title: t("The agent is already answering"),
-      detail: t(
-        "This chat already has a run in progress. Wait for it to finish, or interrupt it and send now.",
-      ),
-    };
-  }
-  if (/"code"\s*:\s*"SUPERSEDE_TARGET_MISMATCH"/.test(msg)) {
-    return {
-      title: t("Couldn't interrupt — the run changed"),
-      detail: t(
-        "The run you tried to interrupt is no longer the active one. Check the latest answer and try again.",
-      ),
-    };
-  }
-  if (/"code"\s*:\s*"SUPERSEDE_TIMEOUT"/.test(msg)) {
-    return {
-      title: t("Couldn't interrupt in time"),
-      detail: t(
-        "The previous run didn't stop in time. Nothing was sent — try sending again.",
-      ),
-    };
-  }
-  if (/"code"\s*:\s*"SUPERSEDE_INVALID"/.test(msg)) {
-    return {
-      title: t("Couldn't interrupt that run"),
-      detail: t(
-        "The run to interrupt doesn't belong to this chat. Reload and try again.",
-      ),
-    };
-  }
-
  if (/"statusCode"\s*:\s*403\b/.test(msg)) {
    return {
      title: t("AI chat is disabled"),
@@ -23,7 +23,7 @@
    "migration:reset": "tsx src/database/migrate.ts down-to NO_MIGRATIONS",
    "migration:codegen": "kysely-codegen --dialect=postgres --camel-case --env-file=../../.env --out-file=./src/database/types/db.d.ts",
    "lint": "eslint \"{src,apps,libs,test}/**/*.ts\" --fix",
-    "pretest": "pnpm --filter @docmost/editor-ext build && pnpm --filter @docmost/prosemirror-markdown build",
+    "pretest": "pnpm --filter @docmost/editor-ext build && pnpm --filter @docmost/prosemirror-markdown build && pnpm --filter @docmost/token-estimate build",
    "test": "jest",
    "test:int": "jest --config test/jest-integration.json",
    "test:watch": "jest --watch",
@@ -44,6 +44,7 @@
    "@docmost/mcp": "workspace:*",
    "@docmost/pdf-inspector": "1.9.6",
    "@docmost/prosemirror-markdown": "workspace:*",
+    "@docmost/token-estimate": "workspace:*",
    "@fastify/compress": "^9.0.0",
    "@fastify/cookie": "^11.0.2",
    "@fastify/multipart": "^10.0.0",
@@ -206,6 +207,7 @@
      "^@docmost/db/(.*)$": "<rootDir>/database/$1",
      "^@docmost/transactional/(.*)$": "<rootDir>/integrations/transactional/$1",
      "^@docmost/ee/(.*)$": "<rootDir>/ee/$1",
+      "^@docmost/token-estimate$": "<rootDir>/../../../packages/token-estimate/src/index.ts",
      "^src/(.*)$": "<rootDir>/$1",
      "^@tiptap/react$": "<rootDir>/../test/stubs/tiptap-react.js"
    }
@@ -0,0 +1,142 @@
+// #489 — client-parts validation + resilient history conversion.
+//
+// These unit tests exercise the two exported helpers against the REAL
+// `convertToModelMessages` from `ai` (NOT a mock): a genuinely malformed part
+// (a `null` element inside a parts array) makes the real converter throw
+// ("Cannot read properties of null"), which is the actual production
+// "bricked chat" mechanism this fix defends against. Asserting against the real
+// converter (rather than a mock-shaped error) is the whole point — a mock would
+// hide a version change in the converter's throw behaviour.
+import { convertToModelMessages, type UIMessage } from 'ai';
+import {
+  sanitizeUserParts,
+  convertHistoryResilient,
+  TOOL_CONTEXT_OMITTED_MARKER,
+} from './ai-chat.service';
+
+type Row = Omit<UIMessage, 'id'> & { id: string };
+
+describe('sanitizeUserParts (#489, branch: validation on receipt)', () => {
+  it('keeps whitelisted text parts unchanged', () => {
+    const drops: string[] = [];
+    const out = sanitizeUserParts(
+      [
+        { type: 'text', text: 'a' },
+        { type: 'text', text: 'b' },
+      ] as UIMessage['parts'],
+      (t) => drops.push(t),
+    );
+    expect(out).toEqual([
+      { type: 'text', text: 'a' },
+      { type: 'text', text: 'b' },
+    ]);
+    expect(drops).toEqual([]);
+  });
+
+  it('drops a non-text part (a tool-part in input-available) and reports its type', () => {
+    const drops: string[] = [];
+    const out = sanitizeUserParts(
+      [
+        { type: 'text', text: 'hi' },
+        {
+          type: 'tool-getPage',
+          toolCallId: 't1',
+          state: 'input-available',
+          input: { pageId: 'p' },
+        },
+      ] as unknown as UIMessage['parts'],
+      (t) => drops.push(t),
+    );
+    expect(out).toEqual([{ type: 'text', text: 'hi' }]);
+    expect(drops).toEqual(['tool-getPage']);
+  });
+
+  it('drops a null part (the shape that would poison convertToModelMessages)', () => {
+    const drops: string[] = [];
+    const out = sanitizeUserParts(
+      [{ type: 'text', text: 'hi' }, null] as unknown as UIMessage['parts'],
+      (t) => drops.push(t),
+    );
+    expect(out).toEqual([{ type: 'text', text: 'hi' }]);
+    expect(drops).toEqual(['(unknown)']);
+  });
+
+  it('returns undefined when nothing survives (so a null metadata is persisted)', () => {
+    const out = sanitizeUserParts(
+      [
+        { type: 'tool-x', toolCallId: 't', state: 'input-available' },
+      ] as unknown as UIMessage['parts'],
+      () => undefined,
+    );
+    expect(out).toBeUndefined();
+  });
+
+  it('returns undefined for a non-array input', () => {
+    expect(
+      sanitizeUserParts(undefined as unknown as UIMessage['parts'], () => undefined),
+    ).toBeUndefined();
+  });
+});
+
+describe('convertHistoryResilient (#489, branches: happy + per-row degradation)', () => {
+  it('happy path: healthy history converts identically to convertToModelMessages, no degrade', async () => {
+    const history: Row[] = [
+      { id: 'u1', role: 'user', parts: [{ type: 'text', text: 'hi' }] },
+      { id: 'a1', role: 'assistant', parts: [{ type: 'text', text: 'hello' }] },
+    ];
+    const degrades: number[] = [];
+    const out = await convertHistoryResilient(history, (i) => degrades.push(i));
+    const expected = await convertToModelMessages(history as UIMessage[]);
+    expect(out).toEqual(expected);
+    expect(degrades).toEqual([]);
+  });
+
+  it('REAL poison: a null part throws in the batch converter but is isolated and degraded to a marker', async () => {
+    // Sanity: the real converter genuinely throws on this shape.
+    const poisoned: Row = {
+      id: 'a1',
+      role: 'assistant',
+      parts: [
+        { type: 'text', text: 'earlier answer' },
+        null,
+      ] as unknown as UIMessage['parts'],
+    };
+    await expect(
+      convertToModelMessages([poisoned as UIMessage]),
+    ).rejects.toThrow();
+
+    const history: Row[] = [
+      { id: 'u1', role: 'user', parts: [{ type: 'text', text: 'first' }] },
+      poisoned,
+      { id: 'u2', role: 'user', parts: [{ type: 'text', text: 'second' }] },
+    ];
+    const degrades: number[] = [];
+    const out = await convertHistoryResilient(history, (i) => degrades.push(i));
+
+    // Only the poisoned row (index 1) is degraded.
+    expect(degrades).toEqual([1]);
+    // Healthy rows survive verbatim.
+    const flat = JSON.stringify(out);
+    expect(flat).toContain('first');
+    expect(flat).toContain('second');
+    // The degraded row carries its readable text AND the truncation marker so the
+    // model sees that tool context was omitted (never a silent loss).
+    expect(flat).toContain('earlier answer');
+    expect(flat).toContain(TOOL_CONTEXT_OMITTED_MARKER);
+    // The whole batch converted (3 model messages, none dropped).
+    expect(out).toHaveLength(3);
+  });
+
+  it('a fully-poisoned row (no readable text) still degrades to just the marker', async () => {
+    const history: Row[] = [
+      {
+        id: 'a1',
+        role: 'assistant',
+        parts: [null] as unknown as UIMessage['parts'],
+      },
+    ];
+    const out = await convertHistoryResilient(history, () => undefined);
+    expect(out).toHaveLength(1);
+    expect(JSON.stringify(out)).toContain(TOOL_CONTEXT_OMITTED_MARKER);
+  });
+});
@@ -97,8 +97,14 @@ describe('AiChatService.stream run-lifecycle safety net (#184)', () => {
    };
    const runService = new AiChatRunService(runRepo as never, { isCloud: () => false } as never);

-    // The user-message insert (the first bare await after beginRun) throws.
+    // The user-message insert throws. #489 runs the history load + convert BEFORE
+    // the insert (convert-before-insert, so a retry cannot duplicate the user row),
+    // so `findAllByChat` (a real repo method) is now called first — stub it to an
+    // empty history so the flow reaches the insert. Both awaits are AFTER beginRun,
+    // so the "exception after beginRun -> settled to error" invariant is unchanged;
+    // the throw point simply moved from insert to a later insert after a no-op load.
    const aiChatMessageRepo = {
+      findAllByChat: jest.fn().mockResolvedValue([]),
      insert: jest.fn().mockRejectedValue(new Error('insert boom')),
    };
    const aiChatRepo = {
@@ -181,7 +181,7 @@ describe('AiChatService.stream — abortSignal wiring (#184 F3)', () => {
      {} as never, // pageAccess
      { isAiChatDeferredToolsEnabled: () => false, isAiChatFinalStepLockdownEnabled: () => false } as never, // environment
    );
-    return { svc };
+    return { svc, aiChatMessageRepo };
  }

  const body = {
@@ -287,7 +287,7 @@ describe('AiChatService.stream — abortSignal wiring (#184 F3)', () => {
  // Drive stream() to the point streamText is called, capturing the options object
  // (which carries onStepFinish/onFinish/onError/onAbort) and the run hooks.
  async function captureStreamCallbacks() {
-    const { svc } = makeService();
+    const { svc, aiChatMessageRepo } = makeService();
    let capturedOpts: any;
    streamTextMock.mockImplementation((opts: any) => {
      capturedOpts = opts;
@@ -314,7 +314,7 @@ describe('AiChatService.stream — abortSignal wiring (#184 F3)', () => {
      runHooks: runHooks as never,
    });
    expect(capturedOpts).toBeDefined();
-    return { capturedOpts, runHooks };
+    return { capturedOpts, runHooks, aiChatMessageRepo };
  }

  it('F9: onStepFinish bumps the run step count, onFinish settles the run "completed" (the dominant autonomous-run path)', async () => {
@@ -369,6 +369,51 @@ describe('AiChatService.stream — abortSignal wiring (#184 F3)', () => {
      expect.stringContaining('provider exploded'),
    );
  });
+
+  // #490 reactive branch: a provider CONTEXT-OVERFLOW 400 in onError is classified,
+  // records a distinguishable cause, and stamps metadata.replayOverflow so the NEXT
+  // turn's budgeter trims aggressively (the recovery that un-bricks the chat).
+  it('#490: a context-overflow 400 stamps replayOverflow on the finalized row', async () => {
+    jest
+      .spyOn(Logger.prototype, 'error')
+      .mockImplementation(() => undefined as never);
+    jest
+      .spyOn(Logger.prototype, 'warn')
+      .mockImplementation(() => undefined as never);
+    const { capturedOpts, aiChatMessageRepo } = await captureStreamCallbacks();
+
+    const overflow = Object.assign(new Error('too large'), {
+      statusCode: 400,
+      message:
+        "This model's maximum context length is 128000 tokens. However, your messages resulted in 214000 tokens. Please reduce the length.",
+    });
+    await capturedOpts.onError({ error: overflow });
+
+    // The seed row exists (finalizeOwner is the owner-write path).
+    expect(aiChatMessageRepo.finalizeOwner).toHaveBeenCalled();
+    const calls = aiChatMessageRepo.finalizeOwner.mock.calls as any[][];
+    const patch = calls[calls.length - 1][2] as {
+      status: string;
+      metadata: Record<string, unknown>;
+    };
+    expect(patch.status).toBe('error');
+    expect(patch.metadata.replayOverflow).toBe(true);
+    expect(patch.metadata.error).toContain('контекстное окно');
+  });
+
+  it('#490: a non-overflow error does NOT stamp replayOverflow', async () => {
+    jest
+      .spyOn(Logger.prototype, 'error')
+      .mockImplementation(() => undefined as never);
+    const { capturedOpts, aiChatMessageRepo } = await captureStreamCallbacks();
+    await capturedOpts.onError({ error: new Error('network reset') });
+    const calls = aiChatMessageRepo.finalizeOwner.mock.calls as any[][];
+    const patch = calls[calls.length - 1][2] as {
+      status: string;
+      metadata: Record<string, unknown>;
+    };
+    expect('replayOverflow' in patch.metadata).toBe(false);
+  });
 });

 /**
@@ -13,6 +13,7 @@ import {
  compactToolOutput,
  assistantParts,
  serializeSteps,
+  type StepPartsCache,
  rowToUiMessage,
  prepareAgentStep,
  stepBudgetWarning,
@@ -28,6 +29,9 @@ import {
  FINAL_STEP_NUDGE,
  STEP_LIMIT_NO_ANSWER_MARKER,
  OUTPUT_DEGENERATION_ERROR,
+  lastAssistantContextTokens,
+  lastAssistantReplayOverflow,
+  seedActivatedTools,
 } from './ai-chat.service';
 import type { AiChatMessage, Workspace } from '@docmost/db/types/entity.types';
 import { buildSystemPrompt } from './ai-chat.prompt';
@@ -114,6 +118,54 @@ describe('compactToolOutput', () => {
 describe('assistantParts', () => {
  type AnyPart = Record<string, unknown>;

+  // #490 memoization: assistantParts builds each step's parts once and caches
+  // them by the step OBJECT's identity, so a mid-stream flush does not
+  // re-stringify every prior step's (large) output. Observable property: with a
+  // shared cache, the second call over the SAME step object returns the cached
+  // (identical) part array even if the step's underlying output was swapped —
+  // proving the work was memoized, not redone.
+  it('memoizes a step by identity (shared cache => one build per step)', () => {
+    const cache: StepPartsCache = new WeakMap();
+    const step = {
+      text: 'x',
+      toolCalls: [{ toolCallId: 'c1', toolName: 'getPage', input: {} }],
+      toolResults: [{ toolCallId: 'c1', toolName: 'getPage', output: { v: 1 } }],
+    };
+    const first = assistantParts([step], '', cache) as AnyPart[];
+    expect((first.find((p) => p.type === 'tool-getPage')!.output as any).v).toBe(
+      1,
+    );
+    // Swap the output for a NEW value; a re-build would pick it up, a cache hit
+    // keeps the first result.
+    step.toolResults[0] = {
+      toolCallId: 'c1',
+      toolName: 'getPage',
+      output: { v: 2 },
+    };
+    const second = assistantParts([step], '', cache) as AnyPart[];
+    expect((second.find((p) => p.type === 'tool-getPage')!.output as any).v).toBe(
+      1,
+    );
+    // Same cached part objects are reused.
+    expect(second.find((p) => p.type === 'tool-getPage')).toBe(
+      first.find((p) => p.type === 'tool-getPage'),
+    );
+  });
+
+  it('without a cache, each call rebuilds (no stale memo)', () => {
+    const step = {
+      text: 'x',
+      toolCalls: [{ toolCallId: 'c1', toolName: 'getPage', input: {} }],
+      toolResults: [{ toolCallId: 'c1', toolName: 'getPage', output: { v: 1 } }],
+    };
+    const first = assistantParts([step], '') as AnyPart[];
+    step.toolResults[0].output = { v: 2 };
+    const second = assistantParts([step], '') as AnyPart[];
+    expect((second.find((p) => p.type === 'tool-getPage')!.output as any).v).toBe(
+      2,
+    );
+  });
+
  it('emits output-available for a tool-call WITH a paired result', () => {
    const steps = [
      {
@@ -231,61 +283,299 @@ describe('assistantParts', () => {
  });
 });

-describe('serializeSteps', () => {
+// #490 trace format v2: per call the trace stores { input } for the call and an
+// OUTCOME element — { ok: true } on success, { error, kind: 'thrown' } on a
+// thrown tool-error, { error, kind: 'interrupted' } on a mid-step abort. The tool
+// OUTPUT is no longer duplicated here (it lives once in metadata.parts).
+describe('serializeSteps (trace v2)', () => {
  it('returns null when there are no calls or results', () => {
    expect(serializeSteps([])).toBeNull();
  });

-  it('flattens calls and results into a compact trace', () => {
+  it('pairs a successful call with an { ok: true } outcome and NO output', () => {
    const trace = serializeSteps([
      {
-        toolCalls: [{ toolName: 'getPage', input: { id: 'p1' } }],
-        toolResults: [{ toolName: 'getPage', output: { title: 'T' } }],
+        toolCalls: [{ toolCallId: 'c1', toolName: 'getPage', input: { id: 'p1' } }],
+        toolResults: [{ toolCallId: 'c1', toolName: 'getPage' }],
      },
    ]) as Array<Record<string, unknown>>;
    expect(trace).toHaveLength(2);
    expect(trace[0]).toEqual({ toolName: 'getPage', input: { id: 'p1' } });
-    expect(trace[1]).toEqual({ toolName: 'getPage', output: { title: 'T' } });
+    expect(trace[1]).toEqual({ toolName: 'getPage', ok: true });
+    // The output is NOT stored in the trace any more (dedup: it lives in parts).
+    expect(trace.some((e) => 'output' in e)).toBe(false);
  });

-  it('records a THROWN tool failure (tool-error part) with its error message', () => {
+  it('records a THROWN failure with { error, kind: "thrown" }', () => {
    const trace = serializeSteps([
      {
-        toolCalls: [{ toolName: 'editPageText', input: { id: 'p1' } }],
+        toolCalls: [
+          { toolCallId: 'c1', toolName: 'editPageText', input: { id: 'p1' } },
+        ],
        toolResults: [],
        content: [
          {
            type: 'tool-error',
+            toolCallId: 'c1',
            toolName: 'editPageText',
            error: new Error('page is locked'),
          },
        ],
      },
    ]) as Array<Record<string, unknown>>;
-    // The call element is followed by a paired error element (mirroring how a
-    // successful result is appended), so the failure survives in the trace.
    expect(trace).toHaveLength(2);
    expect(trace[0]).toEqual({ toolName: 'editPageText', input: { id: 'p1' } });
    expect(trace[1]).toEqual({
      toolName: 'editPageText',
      error: 'page is locked',
+      kind: 'thrown',
    });
  });

-  it('truncates a very long tool-error message to the tool-output limit', () => {
+  it('marks an interrupted call (no result, no throw) with kind "interrupted"', () => {
+    const trace = serializeSteps([
+      {
+        toolCalls: [
+          { toolCallId: 'c1', toolName: 'createComment', input: { x: 1 } },
+        ],
+        toolResults: [],
+        content: [],
+      },
+    ]) as Array<Record<string, unknown>>;
+    expect(trace).toHaveLength(2);
+    expect(trace[1]).toEqual({
+      toolName: 'createComment',
+      error: 'Tool call did not complete.',
+      kind: 'interrupted',
+    });
+    // Structurally distinct from a thrown hard-fail so it never inflates an
+    // error-rate scan.
+    expect((trace[1] as { kind: string }).kind).not.toBe('thrown');
+  });
+
+  it('truncates a very long thrown-error message to the tool-output limit', () => {
    const long = 'x'.repeat(5000);
    const trace = serializeSteps([
      {
-        toolCalls: [{ toolName: 'editPageText', input: {} }],
+        toolCalls: [{ toolCallId: 'c1', toolName: 'editPageText', input: {} }],
        toolResults: [],
-        content: [{ type: 'tool-error', toolName: 'editPageText', error: long }],
+        content: [
+          {
+            type: 'tool-error',
+            toolCallId: 'c1',
+            toolName: 'editPageText',
+            error: long,
+          },
+        ],
      },
    ]) as Array<Record<string, unknown>>;
    const errorText = trace[1].error as string;
-    // Truncated (not the full 5000 chars) and carries the omission marker.
    expect(errorText.length).toBeLessThan(long.length);
    expect(errorText).toContain('chars omitted');
  });
+
+  it('pairs parallel calls in one step with their outcomes by id', () => {
+    const trace = serializeSteps([
+      {
+        toolCalls: [
+          { toolCallId: 'a', toolName: 'getPage', input: {} },
+          { toolCallId: 'b', toolName: 'searchPages', input: {} },
+        ],
+        toolResults: [{ toolCallId: 'b', toolName: 'searchPages' }],
+        content: [
+          { type: 'tool-error', toolCallId: 'a', toolName: 'getPage', error: 'nope' },
+        ],
+      },
+    ]) as Array<Record<string, unknown>>;
+    // call a, outcome a (thrown), call b, outcome b (ok)
+    expect(trace).toHaveLength(4);
+    expect(trace[1]).toEqual({ toolName: 'getPage', error: 'nope', kind: 'thrown' });
+    expect(trace[3]).toEqual({ toolName: 'searchPages', ok: true });
+  });
+});
+
+// #490: every assistant row flushAssistant writes carries the v2 era marker so a
+// dual-shape diagnostic query can branch on the trace shape without inspecting it.
+describe('toolTraceVersion era marker (#490)', () => {
+  it('stamps metadata.toolTraceVersion = 2 on every flushed row', () => {
+    const seed = flushAssistant([], '', 'streaming');
+    expect(seed.metadata.toolTraceVersion).toBe(2);
+    const done = flushAssistant(
+      [
+        {
+          text: 'ok',
+          toolCalls: [{ toolCallId: 'c1', toolName: 'getPage', input: {} }],
+          toolResults: [{ toolCallId: 'c1', toolName: 'getPage' }],
+        },
+      ],
+      '',
+      'completed',
+      { finishReason: 'stop' },
+    );
+    expect(done.metadata.toolTraceVersion).toBe(2);
+  });
+});
+
+// #490 replay-budget signal helpers over persisted history.
+describe('lastAssistantContextTokens', () => {
+  const row = (
+    role: string,
+    metadata: Record<string, unknown> | null,
+  ): AiChatMessage => ({ role, metadata }) as unknown as AiChatMessage;
+
+  it('reads the most recent assistant turn contextTokens (provider fact)', () => {
+    const hist = [
+      row('user', null),
+      row('assistant', { contextTokens: 12000 }),
+      row('user', null),
+      row('assistant', { contextTokens: 41000 }),
+    ];
+    expect(lastAssistantContextTokens(hist)).toBe(41000);
+  });
+
+  it('returns undefined when the last assistant turn recorded no usage', () => {
+    const hist = [row('assistant', { error: 'boom' }), row('user', null)];
+    expect(lastAssistantContextTokens(hist)).toBeUndefined();
+    expect(lastAssistantContextTokens([])).toBeUndefined();
+  });
+});
+
+// #490 snapshotOpenPage fast-path: skip the full Markdown export + upsert when a
+// snapshot already exists at the page's CURRENT version (same updated_at instant).
+describe('snapshotOpenPage fast-path (#490)', () => {
+  function makeSvc(existingSnapshot: unknown, pageUpdatedAt: Date) {
+    const exportPageMarkdown = jest.fn(async () => '# md');
+    const upsert = jest.fn(async () => undefined);
+    const findByChatPage = jest.fn(async () => existingSnapshot);
+    const pageRepo = {
+      findById: jest.fn(async () => ({
+        id: 'p1',
+        workspaceId: 'ws1',
+        updatedAt: pageUpdatedAt,
+      })),
+    };
+    const svc = new AiChatService(
+      {} as never, // ai
+      {} as never, // aiChatRepo
+      {} as never, // aiChatMessageRepo
+      { findByChatPage, upsert } as never, // aiChatPageSnapshotRepo
+      {} as never, // aiSettings
+      { exportPageMarkdown } as never, // tools
+      {} as never, // mcpClients
+      {} as never, // aiAgentRoleRepo
+      pageRepo as never, // pageRepo
+      {} as never, // pageAccess
+      {} as never, // environment
+    );
+    return { svc, exportPageMarkdown, upsert, findByChatPage };
+  }
+
+  const args = () =>
+    [
+      'chat1',
+      'p1',
+      { id: 'ws1' } as never,
+      { id: 'u1' } as never,
+      'sess',
+    ] as const;
+
+  it('skips export + upsert when the snapshot is already at this page version', async () => {
+    const t = new Date('2026-07-07T10:00:00Z');
+    const { svc, exportPageMarkdown, upsert } = makeSvc(
+      { pageUpdatedAt: t, contentMd: '# md' },
+      t,
+    );
+    await (svc as unknown as { snapshotOpenPage: (...a: unknown[]) => Promise<void> })
+      .snapshotOpenPage(...args());
+    expect(exportPageMarkdown).not.toHaveBeenCalled();
+    expect(upsert).not.toHaveBeenCalled();
+  });
+
+  it('exports + upserts when the page advanced since the snapshot', async () => {
+    const { svc, exportPageMarkdown, upsert } = makeSvc(
+      { pageUpdatedAt: new Date('2026-07-07T10:00:00Z'), contentMd: 'old' },
+      new Date('2026-07-07T11:00:00Z'),
+    );
+    await (svc as unknown as { snapshotOpenPage: (...a: unknown[]) => Promise<void> })
+      .snapshotOpenPage(...args());
+    expect(exportPageMarkdown).toHaveBeenCalledTimes(1);
+    expect(upsert).toHaveBeenCalledTimes(1);
+  });
+
+  it('seeds (exports + upserts) on the first turn (no snapshot yet)', async () => {
+    const { svc, exportPageMarkdown, upsert } = makeSvc(
+      undefined,
+      new Date('2026-07-07T10:00:00Z'),
+    );
+    await (svc as unknown as { snapshotOpenPage: (...a: unknown[]) => Promise<void> })
+      .snapshotOpenPage(...args());
+    expect(exportPageMarkdown).toHaveBeenCalledTimes(1);
+    expect(upsert).toHaveBeenCalledTimes(1);
+  });
+});
+
+// #490 deferred-tool activation persisted across turns.
+describe('seedActivatedTools', () => {
+  const valid = new Set(['Search_web', 'getPageJson', 'diffPageVersions']);
+
+  it('seeds from persisted metadata, intersected with current valid names', () => {
+    expect(
+      seedActivatedTools(
+        { activatedTools: ['Search_web', 'getPageJson'] },
+        valid,
+      ),
+    ).toEqual(['Search_web', 'getPageJson']);
+  });
+
+  it('drops a stored tool that is no longer valid (allowlist/role changed)', () => {
+    // 'Habr_publish' was activated before but is not in the current allowlist.
+    expect(
+      seedActivatedTools({ activatedTools: ['Search_web', 'Habr_publish'] }, valid),
+    ).toEqual(['Search_web']);
+  });
+
+  it('is empty/robust for missing, non-array, or unknown-shaped metadata', () => {
+    expect(seedActivatedTools(undefined, valid)).toEqual([]);
+    expect(seedActivatedTools({}, valid)).toEqual([]);
+    expect(seedActivatedTools({ activatedTools: 'nope' }, valid)).toEqual([]);
+    expect(
+      seedActivatedTools({ activatedTools: [1, 'getPageJson', null] }, valid),
+    ).toEqual(['getPageJson']);
+  });
+
+  it('de-duplicates stored names', () => {
+    expect(
+      seedActivatedTools(
+        { activatedTools: ['getPageJson', 'getPageJson'] },
+        valid,
+      ),
+    ).toEqual(['getPageJson']);
+  });
+});
+
+describe('lastAssistantReplayOverflow', () => {
+  const row = (
+    role: string,
+    metadata: Record<string, unknown> | null,
+  ): AiChatMessage => ({ role, metadata }) as unknown as AiChatMessage;
+
+  it('is true only when the LAST assistant turn overflowed', () => {
+    expect(
+      lastAssistantReplayOverflow([
+        row('assistant', { replayOverflow: true }),
+        row('user', null),
+      ]),
+    ).toBe(true);
+    // A recovered (later, non-overflow) assistant turn clears it.
+    expect(
+      lastAssistantReplayOverflow([
+        row('assistant', { replayOverflow: true }),
+        row('user', null),
+        row('assistant', { contextTokens: 5 }),
+      ]),
+    ).toBe(false);
+    expect(lastAssistantReplayOverflow([])).toBe(false);
+  });
 });

 describe('rowToUiMessage', () => {
@@ -618,6 +908,23 @@ describe('flushAssistant', () => {
    expect(flushed.metadata.error).toBe('boom');
  });

+  // #490 observability: the replay budgeter's decision is stamped on the turn.
+  it('records replayTrimmedToTokens + replayOverflow when provided', () => {
+    const f = flushAssistant([], '', 'error', {
+      error: 'ctx',
+      replayTrimmedToTokens: 42_000,
+      replayOverflow: true,
+    });
+    expect(f.metadata.replayTrimmedToTokens).toBe(42_000);
+    expect(f.metadata.replayOverflow).toBe(true);
+  });
+
+  it('omits the replay metadata when not provided', () => {
+    const f = flushAssistant([], '', 'completed', { finishReason: 'stop' });
+    expect('replayTrimmedToTokens' in f.metadata).toBe(false);
+    expect('replayOverflow' in f.metadata).toBe(false);
+  });
+
  // #274 observability: the page-change diff the agent saw this turn is persisted
  // to metadata.pageChanged when a non-empty diff was injected, and omitted when
  // the diff is empty/whitespace or the arg is not supplied.
@@ -1440,7 +1747,7 @@ describe('AiChatService.stream — resumable pipe options (#184 phase 1.5)', ()
  }

  // Wire only the deps reached on the way to the pipe call, plus a spy registry.
-  function makeService(opts: { resumable: boolean }) {
+  function makeService(opts: { resumable: boolean; history?: unknown[] }) {
    const aiChatRepo = {
      findById: jest.fn(async () => ({ id: 'chat-1', workspaceId: 'ws-1' })),
      insert: jest.fn(),
@@ -1448,7 +1755,7 @@ describe('AiChatService.stream — resumable pipe options (#184 phase 1.5)', ()
    const aiChatMessageRepo = {
      // Both the user insert and the assistant seed return the same row id.
      insert: jest.fn(async () => ({ id: 'msg-1' })),
-      findAllByChat: jest.fn(async () => []),
+      findAllByChat: jest.fn(async () => opts.history ?? []),
      update: jest.fn(async () => ({ id: 'msg-1' })),
      // #487: the terminal owner-write + the opportunistic reconcile query.
      finalizeOwner: jest.fn(async () => ({ id: 'msg-1' })),
@@ -1487,7 +1794,7 @@ describe('AiChatService.stream — resumable pipe options (#184 phase 1.5)', ()
      } as never,
      streamRegistry as never,
    );
-    return { svc, streamRegistry };
+    return { svc, streamRegistry, aiChatMessageRepo };
  }

  const body = {
@@ -1570,6 +1877,86 @@ describe('AiChatService.stream — resumable pipe options (#184 phase 1.5)', ()
    await expect(drive(svc, makeRunHooks())).rejects.toThrow('boom');
    expect(streamRegistry.abortEntry).toHaveBeenCalledWith('chat-1', 'run-1');
  });
+
+  // #489 REGRESSION (against the REAL convertToModelMessages — not mocked here):
+  // a persisted history row whose parts contain a `null` element makes the real
+  // convertToModelMessages THROW ("Cannot read properties of null"). Pre-fix that
+  // 500-ed every turn forever and each retry appended a duplicate user row. The
+  // fix converts BEFORE the insert and isolates the poisoned row per-row, degrading
+  // it to text with a "[tool context omitted]" marker. Assert the turn still runs,
+  // the marker reaches the model, and exactly ONE user row is inserted.
+  it('#489: a poisoned OLD-history row keeps the chat working; the marker reaches the model; one user insert', async () => {
+    const { svc, aiChatMessageRepo } = makeService({
+      resumable: false,
+      history: [
+        {
+          id: 'old-1',
+          role: 'assistant',
+          content: 'earlier answer',
+          // A null part is the poison: rowToUiMessage keeps it (the array is
+          // non-empty) and the real convertToModelMessages throws on it.
+          metadata: { parts: [{ type: 'text', text: 'earlier answer' }, null] },
+          status: 'completed',
+        },
+      ],
+    });
+    // Must NOT throw — the poisoned row is degraded, not fatal.
+    await drive(svc, makeRunHooks());
+    expect(streamTextMock).toHaveBeenCalledTimes(1);
+    const passedMessages = streamTextMock.mock.calls[0][0].messages;
+    const serialized = JSON.stringify(passedMessages);
+    // The model sees the truncation marker (silent tool-context loss is not ok)
+    // AND the row's readable text is preserved alongside it.
+    expect(serialized).toContain('[tool context omitted]');
+    expect(serialized).toContain('earlier answer');
+    // Exactly ONE user row inserted (no duplicate), inserted AFTER conversion.
+    const userInserts = aiChatMessageRepo.insert.mock.calls
+      .map((c: unknown[]) => c[0] as { role?: string })
+      .filter((r) => r.role === 'user');
+    expect(userInserts).toHaveLength(1);
+  });
+
+  // #489: client-supplied non-text parts (a tool-part in `input-available`, the
+  // exact "bricking" payload) are dropped ON RECEIPT — never persisted — so they
+  // can never poison future turns. Only the text survives into metadata.parts.
+  it('#489: a non-text client part is stripped before persist (only text survives)', async () => {
+    const { svc, aiChatMessageRepo } = makeService({ resumable: false });
+    await svc.stream({
+      user: { id: 'u1' } as never,
+      workspace: { id: 'ws-1' } as never,
+      sessionId: 's1',
+      body: {
+        chatId: 'chat-1',
+        messages: [
+          {
+            id: 'm1',
+            role: 'user',
+            parts: [
+              { type: 'text', text: 'hello' },
+              // untrusted tool-part — must be dropped, never persisted
+              {
+                type: 'tool-getPage',
+                toolCallId: 't1',
+                state: 'input-available',
+                input: { pageId: 'p' },
+              },
+            ],
+          },
+        ],
+      } as never,
+      res: makeRes() as never,
+      signal: new AbortController().signal,
+      model: {} as never,
+      role: null,
+      runHooks: makeRunHooks() as never,
+    });
+    const userInsert = aiChatMessageRepo.insert.mock.calls
+      .map((c: unknown[]) => c[0] as { role?: string; metadata?: unknown })
+      .find((r) => r.role === 'user');
+    const parts = (userInsert?.metadata as { parts?: Array<{ type: string }> })
+      ?.parts;
+    expect(parts).toEqual([{ type: 'text', text: 'hello' }]);
+  });
 });

 /**
@@ -14,6 +14,7 @@ import {
  convertToModelMessages,
  stepCountIs,
  type UIMessage,
+  type ModelMessage,
  type LanguageModel,
 } from 'ai';
 import { AiService } from '../../integrations/ai/ai.service';
@@ -54,6 +55,12 @@ import {
  type SelectionContext,
 } from './tools/current-page.util';
 import { roleModelOverride } from './roles/role-model-config';
+import {
+  resolveReplayBudget,
+  isContextOverflowError,
+  trimHistoryForReplay,
+  REPLAY_AGGRESSIVE_FRACTION,
+} from './history-budget';
 import {
  startSseHeartbeat,
  stripStreamingHopByHopHeaders,
@@ -126,6 +133,15 @@ const STEP_LIMIT_NO_ANSWER_MARKER =
 const OUTPUT_DEGENERATION_ERROR =
  'Output degeneration detected (repeated token loop)';

+// Prefix recorded on the assistant row when the provider rejected the turn for
+// CONTEXT OVERFLOW (#490): the replayed history exceeded the model's window. The
+// row is ALSO stamped `metadata.replayOverflow` so the NEXT turn's budgeter trims
+// aggressively (the reactive recovery — the overflowing turn had no usage signal
+// to trigger preventive trimming, so the classified 400 is what un-bricks it).
+export const CONTEXT_OVERFLOW_ERROR_PREFIX =
+  'Диалог превысил контекстное окно модели; история будет агрессивно ' +
+  'сокращена на следующем ходу.';
+
 /**
 * Compute the step-budget warning text (#444), or '' when this step is outside
 * the warning band. The warning fires on steps
@@ -881,6 +897,21 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
      const freshPage = await this.pageRepo.findById(pageId);
      // Page deleted during the turn (or somehow foreign) => don't write.
      if (!freshPage || freshPage.workspaceId !== workspace.id) return;
+      // Fast-path (#490): if a snapshot already exists at THIS page version
+      // (same updated_at instant), its content is already current — skip the full
+      // Markdown export + upsert entirely. A turn that did NOT touch the open page
+      // (the common case) thus does no snapshot work. This mirrors the read-side
+      // fast path in detectPageChange (sameInstant): both trust that a page edit
+      // bumps updated_at. When the agent (or a human) DID edit the page this turn,
+      // updated_at advanced, so this does not match and we re-export as before.
+      const existing = await this.aiChatPageSnapshotRepo.findByChatPage(
+        chatId,
+        pageId,
+        workspace.id,
+      );
+      if (existing && sameInstant(existing.pageUpdatedAt, freshPage.updatedAt)) {
+        return;
+      }
      const currentMd = await this.tools.exportPageMarkdown(
        user,
        sessionId,
@@ -920,10 +951,17 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
    // supplied or the supplied one does not belong to this workspace.
    let isNewChat = false;
    let chatId = body.chatId;
+    // Persisted chat-level metadata bag (#490): read once here so the deferred-tool
+    // activation set can be seeded from the previous turn. Undefined for a new chat.
+    let chatMetadata: Record<string, unknown> | undefined;
    if (chatId) {
      const existing = await this.aiChatRepo.findById(chatId, workspace.id);
      if (!existing) {
        chatId = undefined;
+      } else {
+        chatMetadata = (existing.metadata ?? undefined) as
+          | Record<string, unknown>
+          | undefined;
      }
    }
    // The open page the client sent is attacker-controllable — BOTH its id and
@@ -1042,7 +1080,58 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
      const incoming = lastUserMessage(body.messages);
      const incomingText = uiMessageText(incoming);

-      // Persist the user message before contacting the model.
+      // #489: sanitize client-supplied parts ON RECEIPT. The client only ever
+      // sends `sendMessage({ text })` (a single text part); there is no
+      // file/attachment path. Any other part — most dangerously a tool-part in
+      // `input-available` state — is untrusted data that, once persisted to
+      // `metadata.parts` verbatim, is REPLAYED through convertToModelMessages on
+      // every later turn. A malformed tool-part makes that conversion throw,
+      // 500-ing every future turn of the chat forever ("bricked"). Drop any
+      // non-whitelisted part with a warn.
+      const sanitizedParts = sanitizeUserParts(incoming?.parts, (type) =>
+        this.logger.warn(
+          `Dropping unsupported user message part '${type}' on chat ${chatId}`,
+        ),
+      );
+
+      // #489: rebuild the conversation from persisted history (not the client
+      // payload) and CONVERT it to model messages BEFORE persisting the user row.
+      // Load the OLD history (WITHOUT the new row) and append the incoming turn in
+      // memory for the conversion. This makes the insert happen only after a
+      // successful conversion, so a conversion failure cannot leave a DUPLICATE
+      // user row behind on the client's retry (the "bricked chat" that accreted a
+      // dup on every 500). `findAllByChat` returns chronological order (oldest ->
+      // newest) and keeps a 5000-row memory-safety backstop (on overflow it keeps
+      // the NEWEST rows and logs a warning); that is a safety net far above any
+      // realistic chat, not a conversational limit.
+      const oldHistory = await this.aiChatMessageRepo.findAllByChat(
+        chatId,
+        workspace.id,
+      );
+      const uiMessages: Array<Omit<UIMessage, 'id'> & { id: string }> = [
+        ...oldHistory.map(rowToUiMessage),
+        {
+          id: 'pending-user',
+          role: 'user',
+          parts: (sanitizedParts && sanitizedParts.length > 0
+            ? sanitizedParts
+            : textPart(incomingText)) as UIMessage['parts'],
+        },
+      ];
+      // convertToModelMessages is async in ai@6.0.134 (returns Promise<ModelMessage[]>).
+      // Resilient (#489): a single poisoned row in the OLD history is isolated via
+      // per-row conversion and degraded to plain text with a "[tool context
+      // omitted]" marker rather than 500-ing the whole turn (silent loss of tool
+      // context is not acceptable — the model must see the truncation).
+      let messages = await convertHistoryResilient(uiMessages, (index, err) =>
+        this.logger.warn(
+          `Degraded unconvertible history row ${index} on chat ${chatId} to text: ${
+            err instanceof Error ? err.message : 'unknown error'
+          }`,
+        ),
+      );
+
+      // Persist the user message only AFTER a successful conversion (#489).
      await this.aiChatMessageRepo.insert({
        chatId,
        workspaceId: workspace.id,
@@ -1050,31 +1139,21 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
        role: 'user',
        content: incomingText,
        // jsonb column: UIMessage parts are JSON-serializable at runtime but not
-        // structurally `JsonValue`, so cast through unknown.
-        metadata: (incoming?.parts ? { parts: incoming.parts } : null) as never,
+        // structurally `JsonValue`, so cast through unknown. Persist the SANITIZED
+        // parts (never the raw client parts) so the row is always convertible.
+        metadata: (sanitizedParts ? { parts: sanitizedParts } : null) as never,
      });

-      // Rebuild the conversation from persisted history (not the client payload),
-      // so the model always sees the authoritative server-side transcript. Load
-      // the FULL history in chronological order (oldest -> newest, incl. the user
-      // message just inserted above) so NO turns are dropped — there is no
-      // recent-tail window anymore. `findAllByChat` keeps a 5000-row memory-safety
-      // backstop (on overflow it keeps the NEWEST rows and logs a warning); that
-      // is a safety net far above any realistic chat, not a conversational limit.
-      const history = await this.aiChatMessageRepo.findAllByChat(
-        chatId,
-        workspace.id,
-      );
-      const uiMessages = history.map(rowToUiMessage);
-      // convertToModelMessages is async in ai@6.0.134 (returns Promise<ModelMessage[]>).
-      const messages = await convertToModelMessages(uiMessages);
-
      // Interrupt-resume detection (#198): the client "send now" flag is only a
      // hint — confirm it against the persisted history (the preceding assistant
      // turn must really be aborted/streaming) so a spoofed flag cannot inject the
      // interrupt note onto an ordinary turn. The partial output the model needs is
      // already in `messages` (the aborted assistant row replays via findRecent).
-      const interrupted = isInterruptResume(history, body.interrupted);
+      // Append the new user turn (shape-only) so index -2 is the prior assistant.
+      const interrupted = isInterruptResume(
+        [...oldHistory, { role: 'user', status: null, metadata: null }],
+        body.interrupted,
+      );

      // Per-turn page-change detection (#274): if the open page was hand-edited by
      // the user since the agent's last turn ended, compute the unified diff so the
@@ -1093,6 +1172,58 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
      // Here we only need the admin-configured system prompt.
      const resolved = await this.aiSettings.resolve(workspace.id);

+      // History-replay token budget (#490). The full conversation is replayed to
+      // the provider every turn, so a long chat eventually 400s on the context
+      // window — forever. Bound the REPLAYED history (never the persisted rows).
+      // PRIMARY signal is the provider's own fact: the last turn's contextTokens.
+      const replayBudget = resolveReplayBudget(resolved?.chatContextWindowRaw);
+      if (replayBudget.usedDefault) {
+        // The default fires precisely for installs with NO configured window —
+        // the ones that hit terminal overflow. Warn so it is observable.
+        this.logger.warn(
+          `AI chat (chat ${chatId}): no chatContextWindow configured; ` +
+            `applying the default replay budget (${replayBudget.thresholdTokens} tokens).`,
+        );
+      }
+      // Last turn's provider-reported context size (authoritative when present).
+      const priorContextTokens = lastAssistantContextTokens(oldHistory);
+      // Reactive recovery (#490): if the LAST turn was rejected for context
+      // overflow (stamped by onError), trim AGGRESSIVELY this turn — the
+      // overflowing turn produced no usage signal, so a normal-threshold trim may
+      // not shrink enough to fit. This is what un-bricks a chat that just 400'd.
+      const priorOverflowed = lastAssistantReplayOverflow(oldHistory);
+      const effectiveThreshold =
+        priorOverflowed && replayBudget.thresholdTokens != null
+          ? Math.floor(
+              replayBudget.thresholdTokens * REPLAY_AGGRESSIVE_FRACTION,
+            )
+          : replayBudget.thresholdTokens;
+      if (priorOverflowed) {
+        this.logger.warn(
+          `AI chat (chat ${chatId}): previous turn hit context overflow; ` +
+            `applying aggressive replay budget (${effectiveThreshold} tokens).`,
+        );
+      }
+      const preTrim = trimHistoryForReplay(
+        messages,
+        effectiveThreshold,
+        // A prior OVERFLOW means the provider count is stale/absent — force the
+        // char-estimate path by ignoring priorContextTokens on recovery.
+        priorOverflowed ? undefined : priorContextTokens,
+      );
+      messages = preTrim.messages;
+      // Observability (#490): record the budgeter's decision on the turn so the UI
+      // can surface "replay truncated at N tokens". Threaded into flushAssistant.
+      let replayTrimmedToTokens: number | undefined = preTrim.trimmed
+        ? preTrim.estimatedTokens
+        : undefined;
+      if (preTrim.trimmed) {
+        this.logger.log(
+          `AI chat (chat ${chatId}): replay history trimmed to ~${preTrim.estimatedTokens} ` +
+            `tokens (budget ${replayBudget.thresholdTokens}).`,
+        );
+      }
+
      // Build the external MCP toolset FIRST so the system prompt can carry each
      // connected server's admin-authored guidance (#180). Merge in admin-
      // configured external MCP tools (web search, etc.; §6.8). A down/slow
@@ -1284,10 +1415,19 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
      //    tools + ALL external MCP tools), computed from the ACTUAL toolset so an
      //    external tool is loadable by its namespaced name. loadTools rejects any
      //    name outside this set.
-      const activatedTools = new Set<string>();
      const validDeferredNames = new Set<string>(
        Object.keys(baseTools).filter((k) => !CORE_TOOL_SET.has(k)),
      );
+      // #490: seed the activation set from the chat's PERSISTED set so the model
+      // does not re-run loadTools every turn to re-activate the same tools. Only
+      // when deferred loading is enabled, and ALWAYS intersected with the CURRENT
+      // valid deferred names — an allowlist/role change must never resurrect a tool
+      // that no longer exists (prepareAgentStep would get a phantom active name).
+      const activatedTools = new Set<string>(
+        deferredEnabled
+          ? seedActivatedTools(chatMetadata, validDeferredNames)
+          : [],
+      );
      // Add the loadTools meta-tool ONLY when the feature is enabled; when off the
      // toolset and behavior are exactly as before.
      const tools = deferredEnabled
@@ -1297,6 +1437,39 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
          }
        : baseTools;

+      // #490: persist the (deterministically ordered) activation set back onto the
+      // chat metadata at turn end, so the NEXT turn seeds from it. Once-guarded and
+      // skipped when nothing new was activated (the set equals its seed) so an
+      // ordinary turn adds no extra write. Preserves other metadata keys.
+      let activatedToolsPersisted = false;
+      const persistActivatedTools = async (): Promise<void> => {
+        if (!deferredEnabled || activatedToolsPersisted || !chatId) return;
+        activatedToolsPersisted = true;
+        const current = [...activatedTools].sort();
+        const seeded = seedActivatedTools(chatMetadata, validDeferredNames).sort();
+        if (current.length === 0 || current.join('') === seeded.join('')) {
+          return; // nothing new activated -> no write
+        }
+        try {
+          await this.aiChatRepo.update(
+            chatId,
+            {
+              metadata: {
+                ...(chatMetadata ?? {}),
+                activatedTools: current,
+              },
+            } as never,
+            workspace.id,
+          );
+        } catch (err) {
+          this.logger.warn(
+            `Failed to persist activated tools (chat ${chatId}): ${
+              err instanceof Error ? err.message : 'unknown error'
+            }`,
+          );
+        }
+      };
+
      // Accumulate the turn's streamed output so a provider error / disconnect can
      // persist the PARTIAL answer the user already saw — the SDK's onError/onAbort
      // callbacks don't hand us the in-progress text. `capturedSteps` holds finished
@@ -1305,6 +1478,11 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
      const capturedSteps: StepLike[] = [];
      let inProgressText = '';

+      // Per-turn step->parts memo (#490): shared across every flushAssistant call
+      // this turn so each finished step's (large) output is JSON-stringified ONCE,
+      // not re-stringified on every subsequent onStepFinish flush (was O(N²)).
+      const partsCache: StepPartsCache = new WeakMap();
+
      // Token-degeneration guard (#444). When the final-step lockdown is OFF, a
      // runaway repetition loop (the 255KB "loadTools." incident) is aborted via
      // this internal controller, unioned with the run/socket signal below. The
@@ -1374,7 +1552,10 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
          await this.aiChatMessageRepo.update(
            assistantId,
            workspace.id,
-            flushAssistant(capturedSteps, '', 'streaming', { pageChanged }),
+            flushAssistant(capturedSteps, '', 'streaming', {
+              pageChanged,
+              partsCache,
+            }),
            { onlyIfStreaming: true },
          );
        } catch (err) {
@@ -1607,6 +1788,8 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
                // closure scope here). Omitted/0 = no limit.
                maxContextTokens: resolved?.chatContextWindow,
                pageChanged,
+                partsCache,
+                replayTrimmedToTokens,
              }),
            );
            // #184/#487: the RUN is finalized ALWAYS (never gated on the message).
@@ -1631,6 +1814,8 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
            // own edits are baked in — and this also SEEDS the snapshot on the first
            // turn. Runs once across every terminal path (see snapshotTurnEnd).
            await snapshotTurnEnd();
+            // #490: persist the deferred-tool activation set for the next turn.
+            await persistActivatedTools();

            // Generate the chat title for a freshly created chat AFTER the stream's
            // provider call has completed — NOT concurrently with it. The z.ai coding
@@ -1654,7 +1839,16 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
            // object, so the actual provider cause is clearly logged. Reuse the
            // shared formatter so provider error formatting stays unified.
            const e = error as { stack?: string };
-            const errorText = describeProviderError(error, String(error));
+            // #490 reactive branch: classify a CONTEXT-OVERFLOW rejection (the
+            // replayed history exceeded the model window). The overflowing turn had
+            // no prior usage to trigger preventive trimming, so we record a clear,
+            // distinguishable cause AND stamp the row so the NEXT turn's budgeter
+            // trims aggressively — the reactive recovery that un-bricks the chat.
+            const overflow = isContextOverflowError(error);
+            const providerError = describeProviderError(error, String(error));
+            const errorText = overflow
+              ? `${CONTEXT_OVERFLOW_ERROR_PREFIX} (${providerError})`
+              : providerError;
            this.logger.error(`AI chat stream error: ${errorText}`, e?.stack);
            // DIAGNOSTIC (Safari stream-drop investigation) — temporary: timing of
            // an error-terminated stream.
@@ -1672,6 +1866,9 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
              flushAssistant(capturedSteps, inProgressText, 'error', {
                error: errorText,
                pageChanged,
+                partsCache,
+                replayTrimmedToTokens,
+                replayOverflow: overflow || undefined,
              }),
            );
            // #184: settle the RUN as failed, carrying the provider/transport cause.
@@ -1681,6 +1878,8 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
            // committed before the error must be baked into the snapshot, or the
            // next turn would mis-report it as a user edit.
            await snapshotTurnEnd();
+            // #490: persist the deferred-tool activation set for the next turn.
+            await persistActivatedTools();
          },
          onAbort: async ({ steps }) => {
            // #444: distinguish a degeneration abort (our internal controller) from
@@ -1695,6 +1894,7 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
                flushAssistant(capturedSteps, truncated, 'error', {
                  error: OUTPUT_DEGENERATION_ERROR,
                  pageChanged,
+                  partsCache,
                }),
              );
              if (runId)
@@ -1705,6 +1905,8 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
                );
              await closeExternalClients();
              await snapshotTurnEnd();
+              // #490: persist the deferred-tool activation set for the next turn.
+              await persistActivatedTools();
              return;
            }
            const partialChars =
@@ -1729,6 +1931,7 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
            await finalizeAssistant(
              flushAssistant(capturedSteps, inProgressText, 'aborted', {
                pageChanged,
+                partsCache,
              }),
            );
            // #184: settle the RUN as aborted (an explicit user stop reached the
@@ -1739,6 +1942,8 @@ export class AiChatService implements OnModuleInit, OnModuleDestroy {
            // committed before the client disconnect / stop() must be baked into the
            // snapshot, or the next turn would mis-report it as a user edit.
            await snapshotTurnEnd();
+            // #490: persist the deferred-tool activation set for the next turn.
+            await persistActivatedTools();
          },
        });

@@ -2049,6 +2254,70 @@ export function chatStreamMetadata(
  return undefined;
 }

+/**
+ * The provider-reported context size of the most recent assistant turn, read from
+ * its persisted `metadata.contextTokens` (#490 replay budgeter's PRIMARY signal —
+ * the provider's own fact, not an estimate). Returns undefined for a chat with no
+ * assistant turn yet, or one whose last turn recorded no usage (e.g. it errored),
+ * in which case the budgeter falls back to the char-estimate.
+ */
+export function lastAssistantContextTokens(
+  history: ReadonlyArray<AiChatMessage>,
+): number | undefined {
+  for (let i = history.length - 1; i >= 0; i--) {
+    const row = history[i];
+    if (row.role !== 'assistant') continue;
+    const meta = (row.metadata ?? {}) as { contextTokens?: unknown };
+    const n = meta.contextTokens;
+    return typeof n === 'number' && Number.isFinite(n) && n > 0 ? n : undefined;
+  }
+  return undefined;
+}
+
+/**
+ * Seed the per-turn deferred-tool activation set from a chat's persisted metadata
+ * (#490), INTERSECTED with the current valid deferred names. Persisting the set
+ * across turns saves the model re-running loadTools every turn to re-activate the
+ * same tools; intersecting on load means a changed allowlist / role can never
+ * resurrect a tool that no longer exists (which would hand prepareAgentStep a
+ * phantom active name). Tolerant of any stored shape — a non-array is ignored.
+ */
+export function seedActivatedTools(
+  metadata: Record<string, unknown> | undefined,
+  validDeferredNames: ReadonlySet<string>,
+): string[] {
+  const stored = metadata?.activatedTools;
+  if (!Array.isArray(stored)) return [];
+  const seen = new Set<string>();
+  const out: string[] = [];
+  for (const name of stored) {
+    if (typeof name === 'string' && validDeferredNames.has(name) && !seen.has(name)) {
+      seen.add(name);
+      out.push(name);
+    }
+  }
+  return out;
+}
+
+/**
+ * Whether the most recent assistant turn was rejected for CONTEXT OVERFLOW
+ * (#490): its row carries `metadata.replayOverflow` (stamped by the stream's
+ * onError). The next turn's budgeter reads this to trim aggressively — the
+ * reactive recovery. Only the LAST assistant turn matters (an older overflow was
+ * already recovered), so we stop at the first assistant row scanning backwards.
+ */
+export function lastAssistantReplayOverflow(
+  history: ReadonlyArray<AiChatMessage>,
+): boolean {
+  for (let i = history.length - 1; i >= 0; i--) {
+    const row = history[i];
+    if (row.role !== 'assistant') continue;
+    const meta = (row.metadata ?? {}) as { replayOverflow?: unknown };
+    return meta.replayOverflow === true;
+  }
+  return false;
+}
+
 /** The last message with role 'user' from a useChat payload, if any. */
 function lastUserMessage(
  messages: UIMessage[] | undefined,
@@ -2074,6 +2343,91 @@ function textPart(text: string): Array<{ type: 'text'; text: string }> {
  return text ? [{ type: 'text', text }] : [];
 }

+/**
+ * Part types accepted on an INCOMING user turn (#489). The client only ever
+ * sends `sendMessage({ text })` (a single text part); there is no file/attachment
+ * path. Everything else on a client-supplied user message — most dangerously a
+ * tool-part in `input-available` state — is untrusted data that would be
+ * persisted to `metadata.parts` verbatim and replayed through
+ * `convertToModelMessages` on every later turn, potentially bricking the chat.
+ */
+const ALLOWED_USER_PART_TYPES: ReadonlySet<string> = new Set(['text']);
+
+/**
+ * Keep only whitelisted parts on a client-supplied user message; report each
+ * dropped part's type via `onDrop` (the caller warns). Returns `undefined` when
+ * nothing survives (no parts / none whitelisted), so the caller persists a null
+ * metadata rather than an empty-parts object. Never throws.
+ */
+export function sanitizeUserParts(
+  parts: UIMessage['parts'] | undefined,
+  onDrop: (type: string) => void,
+): UIMessage['parts'] | undefined {
+  if (!Array.isArray(parts)) return undefined;
+  const kept = parts.filter((p) => {
+    const type =
+      typeof (p as { type?: unknown })?.type === 'string'
+        ? (p as { type: string }).type
+        : '';
+    if (ALLOWED_USER_PART_TYPES.has(type)) return true;
+    onDrop(type || '(unknown)');
+    return false;
+  });
+  return kept.length > 0 ? (kept as UIMessage['parts']) : undefined;
+}
+
+/** Marker for a history row whose tool parts could not be replayed (#489). */
+export const TOOL_CONTEXT_OMITTED_MARKER = '[tool context omitted]';
+
+/**
+ * Synthetic error text for a tool call that neither returned a result nor threw
+ * a `tool-error` — i.e. it was interrupted mid-step (an abort / server restart).
+ * Shared by `assistantParts` (the replayed `output-error` part) and
+ * `serializeSteps` (the `{ kind: 'interrupted' }` trace element) so the replay
+ * text and the trace stay in lockstep (#490).
+ */
+export const TOOL_CALL_INCOMPLETE_TEXT = 'Tool call did not complete.';
+
+/**
+ * Convert persisted UI history to model messages, tolerating a single poisoned
+ * row (#489). `convertToModelMessages` over the WHOLE array throws if ANY row is
+ * malformed (e.g. a tool-part left unbalanced / in `input-available` state),
+ * which would otherwise 500 every turn of the chat forever. On a batch failure we
+ * fall back to per-row conversion so the bad row is isolated: it is degraded to
+ * plain text carrying its readable text plus a `[tool context omitted]` marker
+ * (the model MUST see that its tool context was truncated — silent loss is not
+ * acceptable), while every healthy row converts normally. Because AI SDK v6
+ * carries a tool call and its result inside the SAME assistant UIMessage's parts,
+ * per-row conversion preserves call/result pairing.
+ */
+export async function convertHistoryResilient(
+  uiMessages: Array<Omit<UIMessage, 'id'> & { id: string }>,
+  onDegrade: (index: number, err: unknown) => void,
+): Promise<ModelMessage[]> {
+  try {
+    return await convertToModelMessages(uiMessages as UIMessage[]);
+  } catch {
+    const out: ModelMessage[] = [];
+    for (let i = 0; i < uiMessages.length; i++) {
+      const m = uiMessages[i];
+      try {
+        out.push(...(await convertToModelMessages([m as UIMessage])));
+      } catch (err) {
+        onDegrade(i, err);
+        const text = uiMessageText(m as UIMessage);
+        const degraded = text
+          ? `${text}\n\n${TOOL_CONTEXT_OMITTED_MARKER}`
+          : TOOL_CONTEXT_OMITTED_MARKER;
+        out.push({
+          role: m.role === 'assistant' ? 'assistant' : 'user',
+          content: degraded,
+        } as ModelMessage);
+      }
+    }
+    return out;
+  }
+}
+
 /**
 * Minimal shapes of the AI SDK v6 step objects we read to rebuild UIMessage
 * parts (see ai@6.0.134 `StepResult`: `text`, `toolCalls` -> TypedToolCall,
@@ -2241,71 +2595,97 @@ function normalizeToolError(error: unknown): string {
 */
 // Exported only so the unit tests can import these pure helpers; exporting
 // them does not change runtime behavior.
+/**
+ * Per-turn memo for {@link assistantParts}: a step's rebuilt parts keyed by the
+ * step OBJECT's identity (#490). A finished step in `capturedSteps` keeps a stable
+ * reference across every mid-stream flush, and `compactToolOutput` inside it does a
+ * `JSON.stringify` of the whole (often 50–200 KB) output — so without a memo each
+ * `onStepFinish` re-stringifies EVERY prior step's output (O(N²) stringify over a
+ * turn). Keyed by step identity => one stringify per step per turn. WeakMap so a
+ * turn's steps are GC'd with the turn.
+ */
+export type StepPartsCache = WeakMap<object, Array<Record<string, unknown>>>;
+
+/** Build the parts for ONE step (text + a part per tool call). Pure. */
+function buildStepParts(step: StepLike): Array<Record<string, unknown>> {
+  const parts: Array<Record<string, unknown>> = [];
+  if (step.text) {
+    parts.push({ type: 'text', text: step.text });
+  }
+  // Index this step's results by tool call id to pair calls with outputs.
+  const resultsById = new Map<string, unknown>();
+  for (const r of step.toolResults ?? []) {
+    if (r.toolCallId) resultsById.set(r.toolCallId, r.output);
+  }
+  // Index this step's THROWN tool failures (ai@6 `tool-error` content parts)
+  // by tool call id, so a call that failed replays with its real error text.
+  const errorsById = new Map<string, unknown>();
+  for (const part of step.content ?? []) {
+    if (part.type === 'tool-error' && part.toolCallId) {
+      errorsById.set(part.toolCallId, part.error);
+    }
+  }
+  for (const call of step.toolCalls ?? []) {
+    if (!call.toolName || !call.toolCallId) continue;
+    const hasResult = resultsById.has(call.toolCallId);
+    if (hasResult) {
+      // output-available: the tool returned; the next turn replays its result.
+      parts.push({
+        type: `tool-${call.toolName}`,
+        toolCallId: call.toolCallId,
+        state: 'output-available',
+        input: call.input,
+        output: compactToolOutput(resultsById.get(call.toolCallId)),
+      });
+    } else if (errorsById.has(call.toolCallId)) {
+      // The tool THREW: replay the REAL error so the model on the next turn
+      // knows WHY the call failed (and does not blindly repeat it). An
+      // output-error round-trips through convertToModelMessages as a balanced
+      // tool-call + tool-result, keeping the rebuilt history valid.
+      parts.push({
+        type: `tool-${call.toolName}`,
+        toolCallId: call.toolCallId,
+        state: 'output-error',
+        input: call.input,
+        errorText: normalizeToolError(errorsById.get(call.toolCallId)),
+      });
+    } else {
+      // No paired result AND no tool-error (e.g. aborted mid-step). Persisting
+      // a bare tool-call (input-available) would replay as an unpaired call and
+      // throw MissingToolResultsError on the next turn (convertToModelMessages
+      // emits no tool-result for it). Emit a SYNTHETIC paired result instead:
+      // an output-error round-trips through convertToModelMessages as a
+      // balanced tool-call + tool-result, keeping the rebuilt history valid.
+      parts.push({
+        type: `tool-${call.toolName}`,
+        toolCallId: call.toolCallId,
+        state: 'output-error',
+        input: call.input,
+        errorText: TOOL_CALL_INCOMPLETE_TEXT,
+      });
+    }
+  }
+  return parts;
+}
+
 export function assistantParts(
  steps: ReadonlyArray<StepLike> | undefined,
  fallbackText: string,
+  cache?: StepPartsCache,
 ): UIMessage['parts'] {
  const parts: Array<Record<string, unknown>> = [];
-  let sawText = false;
  for (const step of steps ?? []) {
-    if (step.text) {
-      parts.push({ type: 'text', text: step.text });
-      sawText = true;
-    }
-    // Index this step's results by tool call id to pair calls with outputs.
-    const resultsById = new Map<string, unknown>();
-    for (const r of step.toolResults ?? []) {
-      if (r.toolCallId) resultsById.set(r.toolCallId, r.output);
-    }
-    // Index this step's THROWN tool failures (ai@6 `tool-error` content parts)
-    // by tool call id, so a call that failed replays with its real error text.
-    const errorsById = new Map<string, unknown>();
-    for (const part of step.content ?? []) {
-      if (part.type === 'tool-error' && part.toolCallId) {
-        errorsById.set(part.toolCallId, part.error);
-      }
-    }
-    for (const call of step.toolCalls ?? []) {
-      if (!call.toolName || !call.toolCallId) continue;
-      const hasResult = resultsById.has(call.toolCallId);
-      if (hasResult) {
-        // output-available: the tool returned; the next turn replays its result.
-        parts.push({
-          type: `tool-${call.toolName}`,
-          toolCallId: call.toolCallId,
-          state: 'output-available',
-          input: call.input,
-          output: compactToolOutput(resultsById.get(call.toolCallId)),
-        });
-      } else if (errorsById.has(call.toolCallId)) {
-        // The tool THREW: replay the REAL error so the model on the next turn
-        // knows WHY the call failed (and does not blindly repeat it). An
-        // output-error round-trips through convertToModelMessages as a balanced
-        // tool-call + tool-result, keeping the rebuilt history valid.
-        parts.push({
-          type: `tool-${call.toolName}`,
-          toolCallId: call.toolCallId,
-          state: 'output-error',
-          input: call.input,
-          errorText: normalizeToolError(errorsById.get(call.toolCallId)),
-        });
-      } else {
-        // No paired result AND no tool-error (e.g. aborted mid-step). Persisting
-        // a bare tool-call (input-available) would replay as an unpaired call and
-        // throw MissingToolResultsError on the next turn (convertToModelMessages
-        // emits no tool-result for it). Emit a SYNTHETIC paired result instead:
-        // an output-error round-trips through convertToModelMessages as a
-        // balanced tool-call + tool-result, keeping the rebuilt history valid.
-        parts.push({
-          type: `tool-${call.toolName}`,
-          toolCallId: call.toolCallId,
-          state: 'output-error',
-          input: call.input,
-          errorText: 'Tool call did not complete.',
-        });
-      }
+    // Memoize per step object (#490): a finished step is immutable and keeps its
+    // reference across flushes, so its parts (and the costly output stringify) are
+    // built exactly once per turn. A cache miss (or no cache) just rebuilds.
+    let stepParts = cache?.get(step as object);
+    if (!stepParts) {
+      stepParts = buildStepParts(step);
+      cache?.set(step as object, stepParts);
    }
+    parts.push(...stepParts);
  }
+  const sawText = parts.some((p) => p.type === 'text');
  if (!sawText && fallbackText) {
    // No per-step text (e.g. a single final block): append the final text after
    // any tool parts so the natural call -> result -> answer order is preserved.
@@ -2468,6 +2848,16 @@ export function flushAssistant(
    maxContextTokens?: number;
    error?: string;
    pageChanged?: { title: string; diff: string } | null;
+    // Per-turn step->parts memo (#490): pass the SAME cache on every flush of a
+    // turn so each finished step's output is stringified once, not once per flush.
+    partsCache?: StepPartsCache;
+    // #490 observability: when the replay budgeter trimmed this turn's history,
+    // the (estimated) token size it trimmed to — the UI can show "replay truncated
+    // at N tokens". Omitted when nothing was trimmed.
+    replayTrimmedToTokens?: number;
+    // #490 reactive branch: set when the provider rejected this turn for context
+    // overflow. Stamped into metadata so the NEXT turn's budgeter trims aggressively.
+    replayOverflow?: boolean;
  },
 ): AssistantFlush {
  const finished = capturedSteps ?? [];
@@ -2477,13 +2867,20 @@ export function flushAssistant(
  // in-progress step's text (the partial answer cut off by an error/abort, or
  // simply not yet flushed mid-stream) as the last text part so the persisted
  // parts match what streamed to the client.
-  const parts = assistantParts(finished, '') as unknown as Array<
-    Record<string, unknown>
-  >;
+  const parts = assistantParts(
+    finished,
+    '',
+    extra?.partsCache,
+  ) as unknown as Array<Record<string, unknown>>;
  if (trailing) parts.push({ type: 'text', text: trailing });

  const metadata: Record<string, unknown> = {
    parts: parts as unknown as UIMessage['parts'],
+    // Era marker for the `tool_calls` trace shape (#490): v2 stores outcome flags
+    // ({ ok } / { error, kind }) and NO tool output (the output lives once in
+    // `parts`). Old rows have no marker and the legacy { output } shape; a
+    // dual-shape query branches on this. Old rows are deliberately NOT migrated.
+    toolTraceVersion: 2,
  };
  // finishReason: prefer an explicit one; else derive a sensible value from the
  // terminal status (so onError/onAbort records keep their historical reason).
@@ -2499,6 +2896,9 @@ export function flushAssistant(
  if (extra?.contextTokens) metadata.contextTokens = extra.contextTokens;
  if (extra?.maxContextTokens)
    metadata.maxContextTokens = extra.maxContextTokens;
+  if (extra?.replayTrimmedToTokens)
+    metadata.replayTrimmedToTokens = extra.replayTrimmedToTokens;
+  if (extra?.replayOverflow) metadata.replayOverflow = true;
  if (extra?.error) metadata.error = extra.error;
  // Persist the page-change diff the agent saw this turn (#274 observability),
  // so history / the Markdown export can show what the user changed. Only when
@@ -2524,42 +2924,85 @@ export function flushAssistant(

 /**
 * Reduce SDK step objects to a compact, JSON-serializable trace for the
- * `tool_calls` column. Stores only what the UI action-log and history need —
- * never raw provider payloads or keys.
+ * `tool_calls` column — trace format **v2** (#490).
+ *
+ * v2 stores, per call, ONLY the metadata a queryable trace needs — never the
+ * tool OUTPUT. Before #490 each output was persisted TWICE: once here (compacted)
+ * and once in `metadata.parts` (via `assistantParts`), so a 50-step run with
+ * 50–200 KB outputs wrote hundreds of MB per turn (each `onStepFinish` rewrote
+ * the whole row). The parts copy is the one the model replays and the UI/Markdown
+ * export render, so the trace copy of the output was pure duplication. v2 keeps
+ * the output ONLY in parts and reduces the trace to outcome flags.
+ *
+ * Element shapes (paired per call, in order):
+ *  - `{ toolName, input }`                       — the call
+ *  - `{ toolName, ok: true }`                     — it returned a result (success)
+ *  - `{ toolName, error, kind: 'thrown' }`        — it threw a `tool-error`
+ *  - `{ toolName, error, kind: 'interrupted' }`   — no result and no throw (an
+ *      abort / server restart mid-step). `kind` is MANDATORY: without it a
+ *      synthetic "Tool call did not complete." is indistinguishable from a real
+ *      hard-fail and pollutes any error-rate scan. The distinction is STRUCTURAL
+ *      (an `errorsById` hit vs the synthetic fallback branch), NOT a per-tool
+ *      classifier — soft failures stay OUT of the trace (they live in
+ *      `metadata.parts` outputs; a per-tool mirror would persist its own bugs).
+ *
+ * Rows carry `metadata.toolTraceVersion: 2` (set by {@link flushAssistant}) so a
+ * dual-shape query can branch on the era. Old rows are NOT migrated (rewriting
+ * giant jsonb is the very WAL churn this removes); see docs/reading-ai-logs.md.
 */
 export function serializeSteps(
  steps: ReadonlyArray<{
-    toolCalls?: ReadonlyArray<{ toolName?: string; input?: unknown }>;
-    toolResults?: ReadonlyArray<{ toolName?: string; output?: unknown }>;
+    toolCalls?: ReadonlyArray<{
+      toolCallId?: string;
+      toolName?: string;
+      input?: unknown;
+    }>;
+    toolResults?: ReadonlyArray<{ toolCallId?: string; toolName?: string }>;
    content?: ReadonlyArray<{
      type?: string;
+      toolCallId?: string;
      toolName?: string;
      error?: unknown;
    }>;
  }>,
 ): unknown {
-  const calls: Array<{
-    toolName?: string;
-    input?: unknown;
-    output?: unknown;
-    error?: string;
-  }> = [];
+  const calls: Array<
+    | { toolName?: string; input?: unknown }
+    | { toolName?: string; ok: true }
+    | { toolName?: string; error: string; kind: 'thrown' | 'interrupted' }
+  > = [];
  for (const step of steps ?? []) {
+    // Index this step's results + thrown errors by tool call id, so each call is
+    // paired with its outcome (mirrors assistantParts' pairing exactly).
+    const resultIds = new Set<string>();
+    for (const r of step.toolResults ?? []) {
+      if (r.toolCallId) resultIds.add(r.toolCallId);
+    }
+    const errorsById = new Map<string, unknown>();
+    for (const part of step.content ?? []) {
+      if (part.type === 'tool-error' && part.toolCallId) {
+        errorsById.set(part.toolCallId, part.error);
+      }
+    }
    for (const call of step.toolCalls ?? []) {
      calls.push({ toolName: call.toolName, input: call.input });
-    }
-    for (const r of step.toolResults ?? []) {
-      calls.push({ toolName: r.toolName, output: compactToolOutput(r.output) });
-    }
-    // ai@6 surfaces a THROWN tool failure as a `tool-error` content part, NOT as
-    // a `toolResults` entry. Record it as its own paired element (mirroring how a
-    // successful result is appended) so the failure and its reason survive in the
-    // trace instead of leaving an orphaned call with no result.
-    for (const part of step.content ?? []) {
-      if (part.type === 'tool-error') {
+      if (call.toolCallId && resultIds.has(call.toolCallId)) {
+        // Success: the output itself lives in metadata.parts, not here.
+        calls.push({ toolName: call.toolName, ok: true });
+      } else if (call.toolCallId && errorsById.has(call.toolCallId)) {
+        // Hard fail: the tool threw. Persist the real (bounded) reason.
        calls.push({
-          toolName: part.toolName,
-          error: normalizeToolError(part.error),
+          toolName: call.toolName,
+          error: normalizeToolError(errorsById.get(call.toolCallId)),
+          kind: 'thrown',
+        });
+      } else {
+        // Neither a result nor a throw: interrupted mid-step (abort/restart).
+        // Marked structurally so it never inflates a thrown-error count.
+        calls.push({
+          toolName: call.toolName,
+          error: TOOL_CALL_INCOMPLETE_TEXT,
+          kind: 'interrupted',
        });
      }
    }
@@ -0,0 +1,209 @@
+import { randomBytes } from 'crypto';
+import { Client } from 'pg';
+import { flushAssistant, serializeSteps } from './ai-chat.service';
+
+/**
+ * #490 write-volume regression — an OBSERVABLE-PROPERTY test on a LIVE Postgres,
+ * not "bytes through a mock repo" (a mock measures exactly the thing that does not
+ * hurt). It drives a realistic 50-step run where each step returns a ~100 KB tool
+ * output and, at every `onStepFinish`, UPDATEs the assistant row the way the
+ * service does — then reads the REAL write volume via the `pg_current_wal_lsn()`
+ * delta around the run.
+ *
+ * The property proven: v2 stores each tool OUTPUT only in `metadata.parts`, no
+ * longer ALSO in the `tool_calls` trace. So:
+ *   1. the trace (`tool_calls`) column's write volume is now O(Σ steps) — tiny,
+ *      linear outcome flags — vs the pre-#490 O(N²) that re-persisted every prior
+ *      output on every step; and
+ *   2. the FULL-row write volume drops sharply (the duplicated output copy is gone).
+ *
+ * Connects to the local gitmost test Postgres (docker `gitmost-test-pg` on :5432);
+ * SKIPS cleanly when that DB is not reachable so it never breaks a DB-less CI.
+ */
+const CONN =
+  process.env.WAL_TEST_DATABASE_URL ??
+  'postgresql://docmost:docmost_dev_pw@localhost:5432/docmost';
+
+// A step whose tool output is ~100 KB (a page read), in the SDK StepLike shape.
+// The body is INCOMPRESSIBLE random text — a `'x'.repeat()` filler would TOAST-
+// compress to nothing and hide the real write volume (a page body does not).
+function makeStep(i: number, outputBytes = 100_000) {
+  const body = randomBytes(Math.ceil(outputBytes * 0.75)).toString('base64');
+  return {
+    text: `step ${i} reasoning`,
+    toolCalls: [{ toolCallId: `c${i}`, toolName: 'getPage', input: { id: `p${i}` } }],
+    toolResults: [
+      {
+        toolCallId: `c${i}`,
+        toolName: 'getPage',
+        output: { id: `p${i}`, title: `Page ${i}`, body },
+      },
+    ],
+  };
+}
+
+// The pre-#490 (v1) trace: outputs stored a SECOND time in `tool_calls`
+// (the duplication #490 removed). Mirrors the OLD serializeSteps shape.
+function v1Trace(steps: ReturnType<typeof makeStep>[]): unknown {
+  const calls: unknown[] = [];
+  for (const s of steps) {
+    for (const c of s.toolCalls) calls.push({ toolName: c.toolName, input: c.input });
+    for (const r of s.toolResults)
+      calls.push({ toolName: r.toolName, output: r.output });
+  }
+  return calls;
+}
+
+async function walDelta(
+  client: Client,
+  fn: () => Promise<void>,
+): Promise<number> {
+  const before = (await client.query('SELECT pg_current_wal_lsn() AS l')).rows[0]
+    .l as string;
+  await fn();
+  // NOTE: do NOT pg_switch_wal() here — a segment switch pads the LSN to the next
+  // 16 MB boundary and would swamp the actual write delta. The raw LSN advances by
+  // the bytes of WAL emitted, which is exactly what we want to measure.
+  const after = (await client.query('SELECT pg_current_wal_lsn() AS l')).rows[0]
+    .l as string;
+  return Number(
+    (await client.query('SELECT pg_wal_lsn_diff($1,$2) AS d', [after, before]))
+      .rows[0].d,
+  );
+}
+
+describe('#490 write-volume on a live Postgres (pg_current_wal_lsn delta)', () => {
+  let client: Client | undefined;
+  let available = false;
+
+  beforeAll(async () => {
+    try {
+      client = new Client(CONN);
+      await client.connect();
+      await client.query('SELECT pg_current_wal_lsn()');
+      available = true;
+    } catch {
+      available = false;
+      client = undefined;
+    }
+  });
+
+  afterAll(async () => {
+    await client?.end().catch(() => undefined);
+  });
+
+  const STEPS = 50;
+
+  it('v2 trace write volume is O(Σ steps) — a tiny fraction of the v1 duplicate', async () => {
+    if (!available || !client) {
+      console.warn('SKIP: gitmost-test-pg not reachable; skipping WAL test.');
+      return;
+    }
+    const c = client;
+    // Isolated table so we measure only the tool_calls (trace) column's writes.
+    await c.query('DROP TABLE IF EXISTS _wal_trace');
+    await c.query('CREATE TABLE _wal_trace(id int primary key, tool_calls jsonb)');
+    await c.query("INSERT INTO _wal_trace VALUES (1, '[]'::jsonb)");
+
+    const steps: ReturnType<typeof makeStep>[] = [];
+
+    // v1: each step re-persists ALL prior outputs into the trace (the O(N²) churn).
+    const v1 = await walDelta(c, async () => {
+      const acc: ReturnType<typeof makeStep>[] = [];
+      for (let i = 0; i < STEPS; i++) {
+        acc.push(makeStep(i));
+        await c.query('UPDATE _wal_trace SET tool_calls=$1 WHERE id=1', [
+          JSON.stringify(v1Trace(acc)),
+        ]);
+      }
+      steps.push(...acc);
+    });
+
+    await c.query("UPDATE _wal_trace SET tool_calls='[]'::jsonb WHERE id=1");
+
+    // v2: the REAL serializeSteps — outcome flags only, NO outputs.
+    const v2 = await walDelta(c, async () => {
+      const acc: ReturnType<typeof makeStep>[] = [];
+      for (let i = 0; i < STEPS; i++) {
+        acc.push(makeStep(i));
+        await c.query('UPDATE _wal_trace SET tool_calls=$1 WHERE id=1', [
+          JSON.stringify(serializeSteps(acc)),
+        ]);
+      }
+    });
+
+    await c.query('DROP TABLE IF EXISTS _wal_trace');
+
+    // eslint-disable-next-line no-console
+    console.log(
+      `[#490 WAL] trace column over ${STEPS} steps: v1=${(v1 / 1e6).toFixed(1)}MB ` +
+        `v2=${(v2 / 1e6).toFixed(2)}MB (${(v1 / v2).toFixed(0)}x smaller)`,
+    );
+
+    // The trace no longer carries outputs: v2 is a tiny fraction of v1's WAL.
+    expect(v2).toBeLessThan(v1 * 0.1);
+    // And v2's trace WAL is small in absolute terms — O(Σ steps) of flags, not
+    // O(N² × output). 50 steps of ~40-byte flags is well under a few MB of WAL.
+    expect(v2).toBeLessThan(5_000_000);
+    // v1's duplicate alone is huge (≈ the 100 KB output re-written N² times).
+    expect(v1).toBeGreaterThan(50_000_000);
+  }, 120_000);
+
+  it('the full assistant row write drops sharply once the duplicate is gone', async () => {
+    if (!available || !client) return;
+    const c = client;
+    await c.query('DROP TABLE IF EXISTS _wal_full');
+    await c.query(
+      'CREATE TABLE _wal_full(id int primary key, content text, tool_calls jsonb, metadata jsonb, status text)',
+    );
+    await c.query("INSERT INTO _wal_full VALUES (1, '', '[]'::jsonb, '{}'::jsonb, 'streaming')");
+
+    const writeRow = async (patch: {
+      content: string;
+      toolCalls: unknown;
+      metadata: unknown;
+      status: string;
+    }) =>
+      c.query(
+        'UPDATE _wal_full SET content=$1, tool_calls=$2, metadata=$3, status=$4 WHERE id=1',
+        [
+          patch.content,
+          JSON.stringify(patch.toolCalls ?? null),
+          JSON.stringify(patch.metadata),
+          patch.status,
+        ],
+      );
+
+    // v2 (real flushAssistant): outputs live once, in metadata.parts.
+    const v2 = await walDelta(c, async () => {
+      const acc: ReturnType<typeof makeStep>[] = [];
+      for (let i = 0; i < STEPS; i++) {
+        acc.push(makeStep(i));
+        await writeRow(flushAssistant(acc as never, '', 'streaming'));
+      }
+    });
+
+    await c.query("UPDATE _wal_full SET content='', tool_calls='[]'::jsonb, metadata='{}'::jsonb WHERE id=1");
+
+    // v1: same row PLUS the duplicated outputs in the trace column.
+    const v1 = await walDelta(c, async () => {
+      const acc: ReturnType<typeof makeStep>[] = [];
+      for (let i = 0; i < STEPS; i++) {
+        acc.push(makeStep(i));
+        const f = flushAssistant(acc as never, '', 'streaming');
+        await writeRow({ ...f, toolCalls: v1Trace(acc) });
+      }
+    });
+
+    await c.query('DROP TABLE IF EXISTS _wal_full');
+
+    // eslint-disable-next-line no-console
+    console.log(
+      `[#490 WAL] full row over ${STEPS} steps: v1=${(v1 / 1e6).toFixed(1)}MB ` +
+        `v2=${(v2 / 1e6).toFixed(1)}MB (saved ${((1 - v2 / v1) * 100).toFixed(0)}%)`,
+    );
+
+    // Removing the duplicated trace copy is a large, real write-volume reduction.
+    expect(v2).toBeLessThan(v1 * 0.75);
+  }, 120_000);
+});
@@ -0,0 +1,261 @@
+import { errors } from 'undici';
+import {
+  McpClientsService,
+  isRetryableConnectError,
+} from './mcp-clients.service';
+
+/**
+ * #489 — external-MCP in-run transport recovery.
+ *
+ * The transport-error classification + retry gate are exercised against the REAL
+ * undici error CLASSES prod throws (`errors.SocketError` / `errors.BodyTimeoutError`,
+ * carrying the true `UND_ERR_*` codes and class names), wrapped EXACTLY as undici's
+ * `fetch` wraps them — a `TypeError('fetch failed'|'terminated')` whose `.cause` is
+ * the undici error. These are the real classes, not hand-rolled `{code:'...'}`
+ * mocks: constructing the genuine class is what makes this a faithful test of the
+ * prod predicate (epic root-cause #4 — a mock-shaped predicate would leave the
+ * evict/retry path silently dead in production while CI stays green). We construct
+ * rather than drive a live fetch because Jest's environment degrades the live-fetch
+ * error to a generic `Error` cause (no undici code), which would NOT be the prod
+ * shape.
+ */
+
+/** A REAL undici socket reset, wrapped as fetch wraps it. */
+function realSocketResetError(): unknown {
+  const err = new TypeError('fetch failed');
+  (err as { cause?: unknown }).cause = new errors.SocketError('other side closed');
+  return err;
+}
+
+/** A REAL undici body timeout, wrapped as fetch wraps it. */
+function realBodyTimeoutError(): unknown {
+  const err = new TypeError('terminated');
+  (err as { cause?: unknown }).cause = new errors.BodyTimeoutError();
+  return err;
+}
+
+type FakeServer = {
+  id: string;
+  name: string;
+  transport: 'http' | 'sse';
+  url: string;
+  headersEnc: string | null;
+  toolAllowlist: string[] | null;
+  instructions: string | null;
+};
+
+const server = (over: Partial<FakeServer> = {}): FakeServer => ({
+  id: 's1',
+  name: 'srv',
+  transport: 'http',
+  url: 'http://example.test/mcp',
+  headersEnc: null,
+  toolAllowlist: null,
+  instructions: null,
+  ...over,
+});
+
+function buildService(servers: FakeServer[], trusted = false) {
+  const repo = { listEnabled: jest.fn().mockResolvedValue(servers) };
+  const service = new McpClientsService(repo as never, {} as never);
+  // Seed a DETERMINISTIC write-class map so the retry gate is controlled here
+  // (the production map loads from @docmost/mcp via a dynamic ESM import). getPage
+  // is a read, patchNode is a write — the real classifications.
+  (
+    service as unknown as { writeClassMapPromise: Promise<unknown> }
+  ).writeClassMapPromise = Promise.resolve({
+    getPage: 'readOnly',
+    patchNode: 'write',
+  });
+  // The service only APPLIES that map to a TRUSTED internal Docmost server
+  // (isInternalDocmostServer, really false for every third-party row). A retry
+  // test needs a trusted server to exercise the readOnly-retry path at all, so it
+  // passes trusted=true to model a Docmost-origin server; the third-party
+  // double-apply test leaves it at the real value (false).
+  if (trusted) {
+    jest
+      .spyOn(
+        service as unknown as {
+          isInternalDocmostServer: (s: FakeServer) => boolean;
+        },
+        'isInternalDocmostServer',
+      )
+      .mockReturnValue(true);
+  }
+  return { service, repo };
+}
+
+/** Spy the private `connect` so each call yields a controlled fake client whose
+ *  single tool's execute is the supplied function. Returns the connect spy. */
+function stubConnect(
+  service: McpClientsService,
+  toolName: string,
+  execs: Array<(...a: unknown[]) => Promise<unknown>>,
+) {
+  let n = 0;
+  return jest
+    .spyOn(
+      service as unknown as { connect: (s: FakeServer) => Promise<unknown> },
+      'connect',
+    )
+    .mockImplementation(async () => {
+      const exec = execs[Math.min(n, execs.length - 1)];
+      n += 1;
+      return {
+        tools: async () => ({ [toolName]: { description: 'x', execute: exec } }),
+        close: jest.fn().mockResolvedValue(undefined),
+      };
+    });
+}
+
+const opts = (abortSignal?: AbortSignal) =>
+  ({ toolCallId: 't', messages: [], abortSignal }) as never;
+
+describe('isRetryableConnectError (#489, REAL error shapes)', () => {
+  it('classifies a real undici socket reset and body timeout as retryable', async () => {
+    const socketErr = await realSocketResetError();
+    const bodyErr = await realBodyTimeoutError();
+    expect(isRetryableConnectError(socketErr)).toBe(true);
+    expect(isRetryableConnectError(bodyErr)).toBe(true);
+    // Unwraps a wrapped cause chain (e.g. an MCPClientError around the socket err).
+    const wrapped = new Error('mcp call failed');
+    (wrapped as { cause?: unknown }).cause = socketErr;
+    expect(isRetryableConnectError(wrapped)).toBe(true);
+  });
+
+  it('does NOT classify an application-level error as a transport break', () => {
+    expect(isRetryableConnectError(new Error('validation failed'))).toBe(false);
+    expect(isRetryableConnectError({ name: 'HttpError', status: 400 })).toBe(false);
+    expect(isRetryableConnectError(undefined)).toBe(false);
+    expect(isRetryableConnectError('boom')).toBe(false);
+  });
+});
+
+describe('McpClientsService in-run transport recovery (#489)', () => {
+  afterEach(() => jest.restoreAllMocks());
+
+  it('a readOnly tool whose transport breaks reconnects and retries WITHIN the same run', async () => {
+    const realErr = await realSocketResetError();
+    const { service } = buildService([server()], true);
+    const first = jest.fn().mockRejectedValue(realErr);
+    const second = jest.fn().mockResolvedValue({ ok: true });
+    const connectSpy = stubConnect(service, 'getPage', [first, second]);
+
+    const toolset = await service.toolsFor('ws-1');
+    const tool = toolset.tools['srv_getPage'];
+    const result = await (tool.execute as (a: unknown, o: unknown) => Promise<unknown>)(
+      { pageId: 'p' },
+      opts(),
+    );
+
+    // The repeat call within the run got a LIVE client and succeeded.
+    expect(result).toEqual({ ok: true });
+    expect(first).toHaveBeenCalledTimes(1);
+    expect(second).toHaveBeenCalledTimes(1);
+    // Exactly one reconnect was minted (initial build connect + one recovery).
+    expect(connectSpy).toHaveBeenCalledTimes(2);
+    // The run accumulated BOTH leases (old + reconnected) — released together at end.
+    expect(toolset.clients).toHaveLength(2);
+    await Promise.all(toolset.clients.map((c) => c.close()));
+  });
+
+  it('a WRITE tool does NOT auto-retry on a transport error (indeterminate)', async () => {
+    const realErr = await realSocketResetError();
+    const { service } = buildService([server()], true);
+    const exec = jest.fn().mockRejectedValue(realErr);
+    const connectSpy = stubConnect(service, 'patchNode', [exec]);
+
+    const toolset = await service.toolsFor('ws-2');
+    const tool = toolset.tools['srv_patchNode'];
+    await expect(
+      (tool.execute as (a: unknown, o: unknown) => Promise<unknown>)(
+        { pageId: 'p' },
+        opts(),
+      ),
+    ).rejects.toThrow(/MAY have already applied/);
+
+    // Called exactly once — NO blind retry (avoids double-apply, the #435 class).
+    expect(exec).toHaveBeenCalledTimes(1);
+    // No fresh connection was minted for a write.
+    expect(connectSpy).toHaveBeenCalledTimes(1);
+    await Promise.all(toolset.clients.map((c) => c.close()));
+  });
+
+  it('does NOT retry (or reconnect) after the run is aborted (Stop)', async () => {
+    const realErr = await realSocketResetError();
+    const { service } = buildService([server()], true);
+    const controller = new AbortController();
+    // The transport error arrives, but the run was Stopped in the same tick.
+    const first = jest.fn().mockImplementation(async () => {
+      controller.abort();
+      throw realErr;
+    });
+    const second = jest.fn().mockResolvedValue({ ok: true });
+    const connectSpy = stubConnect(service, 'getPage', [first, second]);
+
+    const toolset = await service.toolsFor('ws-3');
+    const tool = toolset.tools['srv_getPage'];
+    await expect(
+      (tool.execute as (a: unknown, o: unknown) => Promise<unknown>)(
+        { pageId: 'p' },
+        opts(controller.signal),
+      ),
+    ).rejects.toBeDefined();
+
+    // getPage IS readOnly, but the Stop blocks the retry — no second call, no mint.
+    expect(second).not.toHaveBeenCalled();
+    expect(connectSpy).toHaveBeenCalledTimes(1);
+    await Promise.all(toolset.clients.map((c) => c.close()));
+  });
+
+  it('an app-level (non-transport) tool error is surfaced verbatim, never retried', async () => {
+    const { service } = buildService([server()], true);
+    const appErr = new Error('tool says: bad input');
+    const exec = jest.fn().mockRejectedValue(appErr);
+    const connectSpy = stubConnect(service, 'getPage', [exec]);
+
+    const toolset = await service.toolsFor('ws-4');
+    const tool = toolset.tools['srv_getPage'];
+    await expect(
+      (tool.execute as (a: unknown, o: unknown) => Promise<unknown>)(
+        { pageId: 'p' },
+        opts(),
+      ),
+    ).rejects.toThrow('tool says: bad input');
+    expect(exec).toHaveBeenCalledTimes(1);
+    expect(connectSpy).toHaveBeenCalledTimes(1); // no reconnect for an app error
+    await Promise.all(toolset.clients.map((c) => c.close()));
+  });
+
+  // #489 (review, MEDIUM) — the Docmost write-class map keys by DOCMOST tool
+  // names; a THIRD-PARTY server may name a WRITE tool `getPage` (a Docmost read
+  // name). It must NOT inherit readOnly and must NOT auto-retry on a transport
+  // error — a blind retry of that write is a double-apply (the #435 class). Here
+  // the server is UNTRUSTED (buildService default, isInternalDocmostServer=false),
+  // so the map is not applied and `getPage` classifies as a write.
+  //
+  // MUTATION-VERIFY: forcing the server "trusted" (buildService(..., true)) makes
+  // `getPage` inherit readOnly -> it WOULD reconnect+retry (connect twice) and the
+  // assertions below fail — i.e. removing the trust scope re-opens the bug.
+  it('a THIRD-PARTY WRITE tool named like a Docmost read does NOT auto-retry (no double-apply)', async () => {
+    const realErr = await realSocketResetError();
+    // Untrusted: default trusted=false — a real third-party server.
+    const { service } = buildService([server()]);
+    const exec = jest.fn().mockRejectedValue(realErr);
+    const connectSpy = stubConnect(service, 'getPage', [exec, exec]);
+
+    const toolset = await service.toolsFor('ws-5');
+    const tool = toolset.tools['srv_getPage'];
+    await expect(
+      (tool.execute as (a: unknown, o: unknown) => Promise<unknown>)(
+        { pageId: 'p' },
+        opts(),
+      ),
+    ).rejects.toThrow(/MAY have already applied/);
+
+    // Exactly one call, NO reconnect — the name collision granted no readOnly-retry.
+    expect(exec).toHaveBeenCalledTimes(1);
+    expect(connectSpy).toHaveBeenCalledTimes(1);
+    await Promise.all(toolset.clients.map((c) => c.close()));
+  });
+});
@@ -106,8 +106,11 @@ describe('McpClientsService.decryptHeaders', () => {

 describe('McpClientsService.guardedFetch (SSRF per-request guard)', () => {
  // The bound guardedFetch closure lives on the instance as a private field.
+  // #489 split it into per-transport HTTP/SSE bindings (they differ only in the
+  // dispatcher's bodyTimeout); the SSRF guard is identical, so testing the HTTP
+  // one is sufficient.
  const guardedFetchOf = (service: McpClientsService) =>
-    (service as unknown as { guardedFetch: typeof fetch }).guardedFetch;
+    (service as unknown as { guardedFetchHttp: typeof fetch }).guardedFetchHttp;

  let fetchSpy: jest.SpiedFunction<typeof fetch>;

@@ -1,5 +1,6 @@
 import { isIP } from 'node:net';
 import { lookup as dnsLookup, type LookupAddress } from 'node:dns';
+import { pathToFileURL } from 'node:url';
 import { Injectable, Logger } from '@nestjs/common';
 import { type Tool, type ToolCallOptions } from 'ai';
 import { createMCPClient } from '@ai-sdk/mcp';
@@ -10,9 +11,29 @@ import {
  streamingDispatcherOptions,
  mcpStreamTimeoutMs,
  mcpCallTimeoutMs,
+  mcpSseBodyTimeoutMs,
 } from '../../../integrations/ai/ai-streaming-fetch';
 import { SecretBoxService } from '../../../integrations/crypto/secret-box';
 import { isUrlAllowed, isIpAllowed } from './ssrf-guard';
+// TYPE-ONLY (erased at compile): @docmost/mcp is ESM-only and cannot be a runtime
+// `require()` from this commonjs module (same constraint as docmost-client.loader).
+// The write-class MAP is loaded lazily via the dynamic-import trick below.
+import type { ToolWriteClass } from '@docmost/mcp';
+
+// TS(commonjs) downlevels a literal `import()` to `require()`, which cannot load
+// the ESM-only @docmost/mcp. Indirect through Function so the real dynamic
+// `import()` survives compilation (same trick as docmost-client.loader.ts).
+const esmImport = new Function(
+  'specifier',
+  'return import(specifier)',
+) as (specifier: string) => Promise<unknown>;
+
+/** Local read-only predicate — avoids a value import of the ESM-only package.
+ *  Only a pure read is retry-safe after a transport break (a write is
+ *  indeterminate). Kept in lockstep with @docmost/mcp's isRetryableWriteClass. */
+function isReadOnlyWriteClass(writeClass: ToolWriteClass | undefined): boolean {
+  return writeClass === 'readOnly';
+}

 /** A closable external MCP client handle. */
 export interface Closable {
@@ -81,12 +102,52 @@ const MAX_TOOL_NAME_LENGTH = 64;
 * close until the turn releases it, so a TTL expiry mid-turn never closes a
 * client a stream is still executing against.
 */
+/**
+ * Where a merged (namespaced) tool came from, so the per-run recovery wrapper
+ * (#489) can, on a transport error, reconnect THAT server and re-resolve the SAME
+ * underlying tool by its raw name. `writeClass` gates the single auto-retry (a
+ * read is retry-safe; a write is indeterminate). `serverIndex` indexes the
+ * entry's `servers` array (which server config to reconnect).
+ */
+interface ToolProvenance {
+  serverIndex: number;
+  rawName: string;
+  writeClass: ToolWriteClass | undefined;
+}
+
+/** A live reconnected server (its fresh client + raw call-timeout-wrapped tools). */
+interface RecoveredServerState {
+  client: McpClient;
+  tools: Record<string, Tool>;
+}
+
+/**
+ * Per-run, per-server recovery binding (#489). `current` is the server's LIVE
+ * target for this run: `null` means "use the ORIGINAL cached client/template";
+ * a non-null value is a reconnected throwaway client all this server's tools now
+ * call. `reconnecting` dedupes concurrent reconnects so only ONE fresh client is
+ * minted per death (a losing concurrent call awaits it and retries on the SAME
+ * new client — the CAS-by-identity rule).
+ */
+interface ServerBinding {
+  current: RecoveredServerState | null;
+  reconnecting?: Promise<RecoveredServerState>;
+}
+
 interface CacheEntry {
  tools: Record<string, Tool>;
  clients: McpClient[];
  outcomes: ServerOutcome[];
  /** Prompt guidance for qualifying servers (see McpServerInstruction). */
  instructions: McpServerInstruction[];
+  /**
+   * The enabled server configs used to build this entry (#489), so the per-run
+   * recovery wrapper can reconnect a specific server by index. Parallel to the
+   * indices referenced by {@link toolMeta}.
+   */
+  servers: AiMcpServer[];
+  /** merged-tool-key -> provenance (#489), for the per-run recovery wrapper. */
+  toolMeta: Record<string, ToolProvenance>;
  expiresAt: number;
  /** Active leases (turns currently using these clients). */
  refCount: number;
@@ -120,20 +181,82 @@ export class McpClientsService {
   */
  private readonly cache = new Map<string, Promise<CacheEntry>>();
  /**
-   * A single shared SSRF-pinned dispatcher for ALL outbound external-MCP fetches.
-   * Its custom connect.lookup runs per connection, so one instance safely guards
-   * every server's connections (we never connect to an unvalidated IP).
+   * SSRF-pinned dispatchers for outbound external-MCP fetches. Both use the SAME
+   * custom connect.lookup (so every connection is IP-validated), but carry a
+   * DIFFERENT `bodyTimeout` (#489): the HTTP (streamable) transport opens a fresh
+   * request per call, so it keeps the tight silence timeout; the SSE transport
+   * holds ONE long-lived body open across many calls, so a >1-min idle BETWEEN
+   * calls is LEGITIMATE and must not break the socket — it gets a much larger
+   * bodyTimeout. (headersTimeout stays tight on both.)
   */
-  private readonly dispatcher: Dispatcher = buildPinnedDispatcher();
-  /** guardedFetch bound to the pinned dispatcher; reused by every transport. */
-  private readonly guardedFetch: typeof fetch = (input, init) =>
-    guardedFetch(this.dispatcher, input, init);
+  private readonly dispatcherHttp: Dispatcher = buildPinnedDispatcher(
+    mcpStreamTimeoutMs(),
+  );
+  private readonly dispatcherSse: Dispatcher = buildPinnedDispatcher(
+    mcpSseBodyTimeoutMs(),
+  );
+  /** guardedFetch bound to each dispatcher; picked by transport type in connect(). */
+  private readonly guardedFetchHttp: typeof fetch = (input, init) =>
+    guardedFetch(this.dispatcherHttp, input, init);
+  private readonly guardedFetchSse: typeof fetch = (input, init) =>
+    guardedFetch(this.dispatcherSse, input, init);
+
+  /**
+   * Memoized write-class map (#489), loaded lazily from @docmost/mcp via the
+   * dynamic-import trick. Keyed by tool name (=== mcpName). A tool NOT in the map
+   * (any third-party external MCP tool) classifies as `undefined` -> treated as a
+   * write by the retry gate (the safe default: never blind-retry an unknown tool).
+   * On any load failure the map is `{}` (every tool -> no auto-retry), so a
+   * missing/older @docmost/mcp build only DISABLES retries, never mis-retries.
+   */
+  private writeClassMapPromise: Promise<Record<string, ToolWriteClass>> | null =
+    null;

  constructor(
    private readonly repo: AiMcpServerRepo,
    private readonly secretBox: SecretBoxService,
  ) {}

+  /**
+   * Whether an external MCP server is the TRUSTED internal Docmost MCP server —
+   * the only server whose tools may be classified by the Docmost write-class map
+   * (#489 review). Today this is ALWAYS false: every `ai_mcp_servers` row is an
+   * admin-configured THIRD-PARTY endpoint (there is no builtin/self flag, sentinel
+   * URL, or synthetic server in this path — Docmost's OWN tools are exposed via the
+   * separate in-app tools path, never through this external-MCP client). So no
+   * third-party tool can inherit `readOnly` by a name collision with a Docmost read
+   * tool, and none is ever auto-retried on a transport error (which would risk a
+   * double-apply — the #435 class). Flip this (an explicit `kind`/`isBuiltin`
+   * column, or a configured self-MCP URL) if a trusted internal server is ever
+   * introduced. A method (not a free function) so it is a single, mockable seam.
+   */
+  private isInternalDocmostServer(_server: AiMcpServer): boolean {
+    return false;
+  }
+
+  /** Lazily load + memoize the shared write-class map (see the field doc). */
+  private getWriteClassMap(): Promise<Record<string, ToolWriteClass>> {
+    if (!this.writeClassMapPromise) {
+      this.writeClassMapPromise = (async () => {
+        try {
+          const entry = require.resolve('@docmost/mcp');
+          const mod = (await esmImport(pathToFileURL(entry).href)) as {
+            SHARED_TOOL_WRITE_CLASS?: Record<string, ToolWriteClass>;
+          };
+          return mod.SHARED_TOOL_WRITE_CLASS ?? {};
+        } catch (err) {
+          this.logger.warn(
+            `Could not load MCP write-class map (auto-retry disabled): ${shortError(
+              err,
+            )}`,
+          );
+          return {};
+        }
+      })();
+    }
+    return this.writeClassMapPromise;
+  }
+
  /**
   * Build (or reuse a cached) external toolset for a workspace. Returns the
   * merged tools, the open client handles to release, and per-server outcomes.
@@ -162,11 +285,37 @@ export class McpClientsService {
        }
      },
    };
-    // One release handle drives the whole leased entry; closing it releases all
-    // underlying clients together (they share the same lease lifecycle).
+
+    // #489: the run accumulates a SET of leases — the primary cache lease PLUS any
+    // throwaway client minted by an in-run transport-recovery reconnect. They are
+    // NEVER released mid-run (releasing a swapped-out client while a concurrent
+    // in-flight call still holds it would INDUCE a second failure); the caller
+    // releases the WHOLE set together at turn-end. A recovery reconnect pushes its
+    // lease onto this live array, which the consumer closes over.
+    const leaseSet: Closable[] = [release];
+
+    // #489: per-RUN transport-recovery binding, one per server, SHARED by all of
+    // that server's tools so a swap by one call is seen by the next (CAS by
+    // identity). Kept per-run (here, not in the cached entry) because the binding
+    // + lease-set state is per-run.
+    const bindings = new Map<number, ServerBinding>();
+    const capMs = mcpCallTimeoutMs();
+
+    // Wrap each cached tool with the recovery layer. On a transport error a
+    // declared readOnly tool reconnects its server and retries ONCE; a write is
+    // never blind-retried (indeterminate — may have applied before the reset). A
+    // tool without provenance (a minimal stub entry in a test) passes through raw.
+    const tools: Record<string, Tool> = {};
+    for (const [key, tool] of Object.entries(entry.tools)) {
+      const meta = entry.toolMeta?.[key];
+      tools[key] = meta
+        ? this.wrapWithTransportRecovery(entry, meta, tool, leaseSet, bindings, capMs)
+        : tool;
+    }
+
    return {
-      tools: entry.tools,
-      clients: [release],
+      tools,
+      clients: leaseSet,
      outcomes: entry.outcomes,
      instructions: entry.instructions,
    };
@@ -254,6 +403,16 @@ export class McpClientsService {
    // Per-call total wall-clock cap, read once for this build (env-overridable).
    const callTimeoutMs = mcpCallTimeoutMs();
    const instructions: McpServerInstruction[] = [];
+    // merged-key -> provenance for the per-run recovery wrapper (#489).
+    const toolMeta: Record<string, ToolProvenance> = {};
+    // Shared Docmost write-class map (#489) — classifies a tool by its raw name.
+    // Loaded ONLY when at least one server is a TRUSTED internal Docmost server
+    // (see isInternalDocmostServer): for third-party servers the map is never
+    // applied (a name collision must not grant readOnly-retry), so we skip the
+    // dynamic ESM load entirely in that (currently universal) case.
+    const writeClassMap = servers.some((s) => this.isInternalDocmostServer(s))
+      ? await this.getWriteClassMap()
+      : null;

    // Per-server connect+tools result, still tagged with its server so the merge
    // below can be applied in the SAME order as `servers` (see the parallel note).
@@ -327,11 +486,23 @@ export class McpClientsService {
      // against names already merged from earlier servers, so no external
      // tool is silently overwritten on collision. The returned count drives
      // whether this server's prompt guidance is included (≥1 tool merged).
+      // #489 (review): the Docmost write-class map keys by DOCMOST tool names and
+      // may ONLY be trusted for a server KNOWN to be the internal Docmost MCP
+      // server. Every row here is an admin-configured THIRD-PARTY endpoint, so a
+      // third-party WRITE tool that happens to be named like a Docmost read
+      // (getPage, listPages, ...) must NOT inherit readOnly — that would auto-retry
+      // a mutation on a transport error (double-apply, the #435 class). Gate the
+      // map on the trust check; untrusted servers get writeClass=undefined -> the
+      // recovery wrapper treats them as writes and never auto-retries.
+      const trustWriteClass = this.isInternalDocmostServer(server);
      const merged = this.mergeNamespaced(
        tools,
        result.guarded,
        server.name,
        server.id,
+        toolMeta,
+        i,
+        trustWriteClass ? writeClassMap : null,
      );
      outcomes.push({ name: server.name, ok: true });
      // Include this server's guidance ONLY when it actually contributed at
@@ -353,6 +524,8 @@ export class McpClientsService {
      clients,
      outcomes,
      instructions,
+      servers,
+      toolMeta,
      expiresAt: Date.now() + CACHE_TTL_MS,
      refCount: 0,
      evicted: false,
@@ -379,18 +552,33 @@ export class McpClientsService {
    picked: Record<string, Tool>,
    serverName: string,
    serverId: string,
+    toolMeta: Record<string, ToolProvenance>,
+    serverIndex: number,
+    // The Docmost write-class map, or `null` for an UNTRUSTED (third-party)
+    // server whose tools must all default to write (never auto-retried).
+    writeClassMap: Record<string, ToolWriteClass> | null,
  ): { count: number; prefix: string } {
    let count = 0;
-    for (const [name, tool] of Object.entries(namespace(picked, serverName))) {
-      let key = name;
+    for (const { full, raw, tool } of namespace(picked, serverName)) {
+      let key = full;
      if (key in target) {
        const original = key;
-        key = disambiguate(name, serverId, (candidate) => candidate in target);
+        key = disambiguate(full, serverId, (candidate) => candidate in target);
        this.logger.debug(
          `External MCP tool name "${original}" collided; renamed to "${key}"`,
        );
      }
      target[key] = tool;
+      // Record provenance so the per-run recovery wrapper (#489) can reconnect
+      // this tool's server and re-resolve it by its raw name. writeClass is set
+      // ONLY from a TRUSTED (internal-Docmost) map; for a third-party server the
+      // map is null -> writeClass stays undefined -> the wrapper treats the tool
+      // as a write and never auto-retries it (no double-apply on name collision).
+      toolMeta[key] = {
+        serverIndex,
+        rawName: raw,
+        writeClass: writeClassMap ? writeClassMap[raw] : undefined,
+      };
      count += 1;
    }
    return { count, prefix: namespacePrefix(serverName) };
@@ -424,7 +612,10 @@ export class McpClientsService {
        // Defense in depth: re-validate the actual request host on EVERY fetch
        // AND pin the socket to a validated IP via the dispatcher's connect
        // lookup, closing the DNS-rebinding TOCTOU between check and connect.
-        fetch: this.guardedFetch,
+        // #489: the SSE transport uses the raised-bodyTimeout dispatcher (idle
+        // between calls is legit); HTTP uses the tight one.
+        fetch:
+          transportType === 'sse' ? this.guardedFetchSse : this.guardedFetchHttp,
      },
    })) as unknown as McpClient;
    return client;
@@ -505,6 +696,176 @@ export class McpClientsService {
    }
  }

+  /**
+   * Wrap one merged external tool with the per-run transport-recovery layer (#489).
+   *
+   * attempt 1 runs on the server's CURRENT binding (the cached client, or a client
+   * a sibling tool already reconnected this run). On a REAL transport error
+   * (undici/@ai-sdk socket/body-timeout shapes — {@link isRetryableConnectError},
+   * NOT a mock) and ONLY for a declared readOnly tool, it reconnects the server
+   * and retries EXACTLY ONCE on the fresh client; a write is surfaced as an
+   * indeterminate error (it may have applied before the reset — never
+   * blind-retried). A single per-call cap bounds BOTH attempts + the reconnect,
+   * and the run's abort signal is checked before the retry AND before minting a
+   * fresh connection (no connection is opened for a stopped run).
+   */
+  private wrapWithTransportRecovery(
+    entry: CacheEntry,
+    meta: ToolProvenance,
+    template: Tool,
+    leaseSet: Closable[],
+    bindings: Map<number, ServerBinding>,
+    capMs: number,
+  ): Tool {
+    const original = template.execute;
+    if (typeof original !== 'function') return template;
+    const service = this;
+    const { serverIndex, rawName, writeClass } = meta;
+
+    let binding = bindings.get(serverIndex);
+    if (!binding) {
+      binding = { current: null };
+      bindings.set(serverIndex, binding);
+    }
+    const boundBinding = binding;
+
+    const execute = async (args: unknown, options: ToolCallOptions) => {
+      // The per-call cap governs the WHOLE sequence (attempt1 + reconnect +
+      // attempt2). Compose it with the run's abort signal so a Stop or the cap
+      // ends any awaited call — @ai-sdk/mcp does not settle on abort, so we RACE.
+      const capController = new AbortController();
+      const capTimer = setTimeout(() => {
+        capController.abort(new Error(`MCP tool call timed out after ${capMs}ms`));
+      }, capMs);
+      capTimer.unref?.();
+      const runSignal = options?.abortSignal;
+      const composed = runSignal
+        ? AbortSignal.any([runSignal, capController.signal])
+        : capController.signal;
+      const stopped = () => runSignal?.aborted === true || capController.signal.aborted;
+
+      const callOn = async (
+        exec: NonNullable<Tool['execute']>,
+      ): Promise<unknown> => {
+        const aborted = new Promise<never>((_, reject) => {
+          const fail = () => reject(abortReason(composed));
+          if (composed.aborted) fail();
+          else composed.addEventListener('abort', fail, { once: true });
+        });
+        return Promise.race([exec(args, { ...options, abortSignal: composed }), aborted]);
+      };
+
+      const execFor = (
+        state: RecoveredServerState | null,
+      ): NonNullable<Tool['execute']> | undefined =>
+        state ? (state.tools[rawName]?.execute as NonNullable<Tool['execute']>) : original;
+
+      try {
+        // Snapshot the target BEFORE the call so a swap by a concurrent call is
+        // detected by identity in the catch.
+        const attemptState = boundBinding.current;
+        const attemptExec = execFor(attemptState);
+        if (typeof attemptExec !== 'function') {
+          throw new Error(`external MCP tool "${rawName}" is not callable`);
+        }
+        try {
+          return await callOn(attemptExec);
+        } catch (err) {
+          // Never retry on a Stop or an exhausted cap.
+          if (stopped()) throw err;
+          // Only a genuine transport break is a recovery candidate.
+          if (!isRetryableConnectError(err)) throw err;
+          // A write tool is INDETERMINATE on a transport error (may have applied
+          // before the reset) — surface that; do NOT auto-retry (double-apply is
+          // the #435 incident class).
+          if (!isReadOnlyWriteClass(writeClass)) {
+            throw new Error(
+              `external MCP tool "${rawName}" hit a transport error and MAY have already ` +
+                `applied on the server — not retried automatically; verify state before ` +
+                `retrying. (${shortError(err)})`,
+            );
+          }
+          // Abort check BEFORE minting a fresh connection (no socket for a
+          // stopped run). LIMITATION (#489, LOW): the reconnect's own connect is
+          // bounded by CONNECT_TIMEOUT_MS but does NOT itself observe `composed`,
+          // so a Stop that lands DURING the handshake is only honored at the next
+          // `stopped()` gate (before the retry) — a bounded ≤5s late-abort window;
+          // the throwaway client is closed at turn-end regardless. Threading
+          // `composed` into the SHARED (CAS-deduped) reconnect is deliberately
+          // avoided: it would let the first caller's abort tear down a reconnect a
+          // concurrent still-live caller depends on.
+          if (stopped()) throw err;
+          // CAS-swap by IDENTITY: mint+swap only if nobody swapped since this
+          // call's snapshot; a losing concurrent call awaits the same reconnect
+          // and retries on the SAME fresh client.
+          let target: RecoveredServerState;
+          if (boundBinding.current === attemptState) {
+            if (!boundBinding.reconnecting) {
+              boundBinding.reconnecting = (async () => {
+                const server = entry.servers[serverIndex];
+                const fresh = await service.reconnectServer(server, capMs);
+                leaseSet.push(fresh.lease); // accumulate; released at turn-end
+                boundBinding.current = fresh.state;
+                return fresh.state;
+              })();
+              // Clear the in-flight marker once it settles (success or failure) so
+              // a LATER death of the new client can reconnect again.
+              void boundBinding.reconnecting.then(
+                () => (boundBinding.reconnecting = undefined),
+                () => (boundBinding.reconnecting = undefined),
+              );
+            }
+            target = await boundBinding.reconnecting;
+          } else {
+            target = boundBinding.current as RecoveredServerState;
+          }
+          // Abort check BEFORE the retry.
+          if (stopped()) throw err;
+          const retryExec = execFor(target);
+          if (typeof retryExec !== 'function') throw err;
+          return await callOn(retryExec);
+        }
+      } finally {
+        clearTimeout(capTimer);
+      }
+    };
+    return { ...template, execute } as unknown as Tool;
+  }
+
+  /**
+   * Reconnect ONE server for an in-run recovery (#489): open a fresh client and
+   * list+wrap its tools. The throwaway client is NOT cached — it is owned by the
+   * RUN via the returned lease (closed at turn-end), independent of the shared
+   * cache entry (whose TTL rebuild heals future turns). On a failure the fresh
+   * client is closed so its socket never leaks.
+   */
+  private async reconnectServer(
+    server: AiMcpServer,
+    capMs: number,
+  ): Promise<{ state: RecoveredServerState; lease: Closable }> {
+    const client = await this.connectWithTimeout(server, CONNECT_TIMEOUT_MS);
+    let tools: Record<string, Tool>;
+    try {
+      const raw = await withTimeout(client.tools(), CONNECT_TIMEOUT_MS);
+      const allow = server.toolAllowlist;
+      const picked =
+        Array.isArray(allow) && allow.length > 0 ? pick(raw, allow) : raw;
+      tools = wrapToolsWithCallTimeout(picked, capMs);
+    } catch (err) {
+      void client.close().catch(() => undefined);
+      throw err;
+    }
+    let released = false;
+    const lease: Closable = {
+      close: async () => {
+        if (released) return;
+        released = true;
+        await client.close().catch(() => undefined);
+      },
+    };
+    return { state: { client, tools }, lease };
+  }
+
  /** Mark an entry evicted; close its clients now if nothing is leasing them. */
  private evict(entry: CacheEntry): void {
    clearTimeout(entry.timer);
@@ -554,22 +915,21 @@ export function validateResolvedAddresses(addrs: readonly LookupAddress[]): {
 * certificate validation still uses the real hostname (we never rewrite the URL
 * to an IP literal).
 */
-function buildPinnedDispatcher(): Agent {
-  // External-MCP traffic uses a DEDICATED, shorter silence timeout
+function buildPinnedDispatcher(bodyTimeoutMs: number): Agent {
+  // External-MCP traffic uses a DEDICATED, shorter HEADERS silence timeout
  // (`AI_MCP_STREAM_TIMEOUT_MS`, default 1 min) — deliberately tighter than the
  // chat provider's 15-min `streamTimeoutMs()` — so a byte-silent/hung MCP
  // upstream is broken in ~1 min instead of 15. We keep the keep-alive options
-  // from `streamingDispatcherOptions()` but OVERRIDE headers/body timeouts.
-  // Accepted trade-off: a legitimately long but byte-silent single tool call,
-  // and an SSE transport idling >1 min BETWEEN tool calls, are also cut here; the
-  // per-call total cap (wrapToolsWithCallTimeout, `AI_MCP_CALL_TIMEOUT_MS`) is the
-  // complementary guard for chatty-but-stuck calls that keep the socket warm yet
-  // never return.
-  const mcpSilenceMs = mcpStreamTimeoutMs();
+  // from `streamingDispatcherOptions()` but OVERRIDE the timeouts. `bodyTimeout`
+  // is passed in per-transport (#489): tight for HTTP (fresh request per call),
+  // raised for SSE (one long-lived body across calls — idle BETWEEN calls is
+  // legit). The per-call total cap (`AI_MCP_CALL_TIMEOUT_MS`) is the complementary
+  // guard for chatty-but-stuck calls that keep the socket warm yet never return.
+  const headersMs = mcpStreamTimeoutMs();
  return new Agent({
    ...streamingDispatcherOptions(),
-    headersTimeout: mcpSilenceMs,
-    bodyTimeout: mcpSilenceMs,
+    headersTimeout: headersMs,
+    bodyTimeout: bodyTimeoutMs,
    connect: {
      lookup: (hostname, _options, callback) => {
        // Always resolve ALL addresses ourselves; do not trust the caller's
@@ -669,18 +1029,22 @@ function pick(
 function namespace(
  tools: Record<string, Tool>,
  serverName: string,
-): Record<string, Tool> {
+): Array<{ full: string; raw: string; tool: Tool }> {
  const prefix = namespacePrefix(serverName);
-  const out: Record<string, Tool> = {};
+  const out: Array<{ full: string; raw: string; tool: Tool }> = [];
+  const taken: Record<string, true> = {};
  for (const [name, t] of Object.entries(tools)) {
    const safe = sanitizeName(name);
    let full = capName(`${prefix}_${safe}`);
    // Duplicate names within ONE server can still collide after sanitize/
    // truncate — suffix-disambiguate so the second tool is not overwritten.
-    if (full in out) {
-      full = disambiguate(full, '', (candidate) => candidate in out);
+    if (full in taken) {
+      full = disambiguate(full, '', (candidate) => candidate in taken);
    }
-    out[full] = t;
+    taken[full] = true;
+    // Keep the RAW (un-namespaced) name alongside the merged key so the per-run
+    // recovery wrapper (#489) can re-resolve the same tool on a fresh client.
+    out.push({ full, raw: name, tool: t });
  }
  return out;
 }
@@ -804,6 +1168,69 @@ export function wrapToolWithCallTimeout(tool: Tool, ms: number): Tool {
  return { ...tool, execute } as unknown as Tool;
 }

+/**
+ * undici / Node network error CODES that mean the connection broke (not an
+ * application-level error) — a transient transport failure a readOnly call may
+ * safely retry after reconnecting. Matched against the REAL error shapes (#489):
+ * a socket reset surfaces as `TypeError: fetch failed` whose `.cause` is an
+ * undici `SocketError { code:'UND_ERR_SOCKET' }`; a body-timeout as
+ * `TypeError: terminated` whose `.cause` is `BodyTimeoutError`. Classifying by
+ * these real codes/names (not by mock errors) is essential — a mock-shaped
+ * predicate would leave eviction silently dead in production while CI is green.
+ */
+const RETRYABLE_TRANSPORT_ERROR_CODES: ReadonlySet<string> = new Set([
+  'ECONNRESET',
+  'ECONNREFUSED',
+  'ECONNABORTED',
+  'EPIPE',
+  'ETIMEDOUT',
+  'EAI_AGAIN',
+  'ENETUNREACH',
+  'EHOSTUNREACH',
+  'UND_ERR_SOCKET',
+  'UND_ERR_CONNECT_TIMEOUT',
+  'UND_ERR_HEADERS_TIMEOUT',
+  'UND_ERR_BODY_TIMEOUT',
+  'UND_ERR_CLOSED',
+  'UND_ERR_DESTROYED',
+]);
+
+/** undici error CLASS names for the same transport-break conditions. */
+const RETRYABLE_TRANSPORT_ERROR_NAMES: ReadonlySet<string> = new Set([
+  'SocketError',
+  'BodyTimeoutError',
+  'HeadersTimeoutError',
+  'ConnectTimeoutError',
+  'ClientClosedError',
+  'ClientDestroyedError',
+]);
+
+/**
+ * Whether `err` is a retryable TRANSPORT break (a broken socket / body timeout),
+ * classified by the REAL undici/@ai-sdk error shapes (#489). undici surfaces a
+ * reset as `TypeError('fetch failed'|'terminated')` with the real error in
+ * `.cause`, and @ai-sdk/mcp may wrap it again in an `MCPClientError` (cause
+ * chain), so we walk `.cause` (bounded depth) checking `.code` and `.name`. An
+ * app-level tool error (a 4xx, a validation failure) is NOT retryable and returns
+ * false — only a connection-level failure heals with a reconnect.
+ */
+export function isRetryableConnectError(err: unknown, depth = 0): boolean {
+  if (!err || typeof err !== 'object' || depth > 6) return false;
+  const e = err as {
+    code?: unknown;
+    name?: unknown;
+    cause?: unknown;
+  };
+  if (typeof e.code === 'string' && RETRYABLE_TRANSPORT_ERROR_CODES.has(e.code)) {
+    return true;
+  }
+  if (typeof e.name === 'string' && RETRYABLE_TRANSPORT_ERROR_NAMES.has(e.name)) {
+    return true;
+  }
+  if (e.cause != null) return isRetryableConnectError(e.cause, depth + 1);
+  return false;
+}
+
 /** The signal's reason as an Error (informative thrown value on abort/timeout). */
 function abortReason(signal: AbortSignal): Error {
  const r = signal.reason;
@@ -0,0 +1,266 @@
+import type { ModelMessage } from 'ai';
+import {
+  resolveReplayBudget,
+  isContextOverflowError,
+  estimateMessagesTokens,
+  trimHistoryForReplay,
+  REPLAY_BUDGET_DEFAULT_TOKENS,
+  REPLAY_TRUNCATION_MARKER,
+  REPLAY_TURN_COLLAPSED_MARKER,
+} from './history-budget';
+
+describe('resolveReplayBudget', () => {
+  it('uses min(default, 0.7 x window) for a configured window', () => {
+    // 0.7 x 60k = 42k < 100k
+    expect(resolveReplayBudget(60_000)).toEqual({
+      thresholdTokens: 42_000,
+      usedDefault: false,
+    });
+    // 0.7 x 1M = 700k, capped to the 100k default
+    expect(resolveReplayBudget(1_000_000)).toEqual({
+      thresholdTokens: REPLAY_BUDGET_DEFAULT_TOKENS,
+      usedDefault: false,
+    });
+  });
+
+  it('accepts the raw ::text stored form', () => {
+    expect(resolveReplayBudget('60000').thresholdTokens).toBe(42_000);
+  });
+
+  // The crux (#490): a chat with NO context window configured must STILL be
+  // budgeted — those are exactly the installs that hit terminal overflow.
+  it('applies the flat default when the window is unset/empty', () => {
+    expect(resolveReplayBudget(undefined)).toEqual({
+      thresholdTokens: REPLAY_BUDGET_DEFAULT_TOKENS,
+      usedDefault: true,
+    });
+    expect(resolveReplayBudget('')).toEqual({
+      thresholdTokens: REPLAY_BUDGET_DEFAULT_TOKENS,
+      usedDefault: true,
+    });
+    expect(resolveReplayBudget('   ')).toEqual({
+      thresholdTokens: REPLAY_BUDGET_DEFAULT_TOKENS,
+      usedDefault: true,
+    });
+  });
+
+  it('treats an explicit 0 as the off-switch (distinct from unset)', () => {
+    expect(resolveReplayBudget(0)).toEqual({
+      thresholdTokens: null,
+      usedDefault: false,
+    });
+    expect(resolveReplayBudget('0')).toEqual({
+      thresholdTokens: null,
+      usedDefault: false,
+    });
+  });
+
+  it('falls back to the default on a negative/garbage value', () => {
+    expect(resolveReplayBudget(-5).usedDefault).toBe(true);
+    expect(resolveReplayBudget('abc').usedDefault).toBe(true);
+  });
+});
+
+describe('isContextOverflowError', () => {
+  it('classifies a real provider 400 context-overflow shape', () => {
+    // OpenAI-compatible shape.
+    expect(
+      isContextOverflowError({
+        statusCode: 400,
+        message:
+          "This model's maximum context length is 128000 tokens. However, your messages resulted in 214000 tokens. Please reduce the length of the messages.",
+      }),
+    ).toBe(true);
+    // Anthropic-style wording.
+    expect(
+      isContextOverflowError({
+        status: 400,
+        message: 'prompt is too long: 250000 tokens > 200000 maximum',
+      }),
+    ).toBe(true);
+    // Nested body + string status.
+    expect(
+      isContextOverflowError({
+        response: { status: '400' },
+        message: 'input is too long for the requested model',
+      }),
+    ).toBe(true);
+    // Error instance with the cause carrying the body.
+    const e = new Error('Bad request');
+    (e as any).statusCode = 400;
+    (e as any).cause = new Error('maximum context window exceeded');
+    expect(isContextOverflowError(e)).toBe(true);
+  });
+
+  it('does NOT classify unrelated 400s or auth/rate-limit errors', () => {
+    expect(
+      isContextOverflowError({ statusCode: 400, message: 'invalid tool schema' }),
+    ).toBe(false);
+    expect(
+      isContextOverflowError({
+        statusCode: 429,
+        message: 'context length exceeded but rate limited',
+      }),
+    ).toBe(false);
+    expect(isContextOverflowError({ statusCode: 500, message: 'server error' })).toBe(
+      false,
+    );
+    expect(isContextOverflowError(undefined)).toBe(false);
+    expect(isContextOverflowError('some random string')).toBe(false);
+  });
+});
+
+// Helpers to build ModelMessage fixtures in the ai@6 shape.
+const userMsg = (text: string): ModelMessage =>
+  ({ role: 'user', content: [{ type: 'text', text }] }) as ModelMessage;
+const assistantMsg = (
+  text: string,
+  toolCallId?: string,
+  toolName?: string,
+): ModelMessage =>
+  ({
+    role: 'assistant',
+    content: [
+      { type: 'text', text },
+      ...(toolCallId
+        ? [{ type: 'tool-call', toolCallId, toolName, input: {} }]
+        : []),
+    ],
+  }) as ModelMessage;
+const toolMsg = (
+  toolCallId: string,
+  toolName: string,
+  value: unknown,
+): ModelMessage =>
+  ({
+    role: 'tool',
+    content: [
+      { type: 'tool-result', toolCallId, toolName, output: { type: 'json', value } },
+    ],
+  }) as ModelMessage;
+
+describe('trimHistoryForReplay', () => {
+  it('null budget disables trimming (returns the same reference)', () => {
+    const msgs = [userMsg('hi'), assistantMsg('yo')];
+    const r = trimHistoryForReplay(msgs, null);
+    expect(r.trimmed).toBe(false);
+    expect(r.messages).toBe(msgs);
+  });
+
+  it('leaves history under budget untouched (same reference)', () => {
+    const msgs = [userMsg('hi'), assistantMsg('a short answer')];
+    const r = trimHistoryForReplay(msgs, 100_000);
+    expect(r.trimmed).toBe(false);
+    expect(r.messages).toBe(msgs);
+  });
+
+  it('truncates OLD tool outputs but keeps recent turns full', () => {
+    const big = 'X'.repeat(40_000); // ~16k tokens on its own
+    const msgs: ModelMessage[] = [];
+    // 6 OLD turns (indices 0..5), each with a huge tool output.
+    for (let i = 0; i < 6; i++) {
+      msgs.push(userMsg(`old q${i}`));
+      msgs.push(assistantMsg('looking', `c${i}`, 'getPage'));
+      msgs.push(toolMsg(`c${i}`, 'getPage', { body: big }));
+      msgs.push(assistantMsg(`old a${i}`));
+    }
+    // 3 small recent turns, then the CURRENT turn with its own huge tool output.
+    // With REPLAY_KEEP_RECENT_TURNS=4 the last 4 user-turns stay full, so only
+    // these small recent turns + the current big one are kept full; the 6 old
+    // turns above fall in the trim region.
+    for (let i = 0; i < 3; i++) {
+      msgs.push(userMsg(`recent q${i}`));
+      msgs.push(assistantMsg(`recent a${i}`));
+    }
+    msgs.push(userMsg('current q'));
+    msgs.push(assistantMsg('looking', 'cR', 'getPage'));
+    msgs.push(toolMsg('cR', 'getPage', { body: big }));
+    msgs.push(assistantMsg('current a'));
+
+    // Budget large enough that phase-1 tool truncation alone brings it under.
+    const r = trimHistoryForReplay(msgs, 30_000);
+    expect(r.trimmed).toBe(true);
+    const flat = JSON.stringify(r.messages);
+    // The CURRENT turn's tool output survives in full.
+    expect(flat).toContain(big);
+    // Old outputs were truncated with the marker.
+    expect(flat).toContain(REPLAY_TRUNCATION_MARKER);
+    // Phase 1 sufficed: the oldest turns were NOT collapsed.
+    expect(flat).not.toContain(REPLAY_TURN_COLLAPSED_MARKER);
+    expect(estimateMessagesTokens(r.messages)).toBeLessThan(
+      estimateMessagesTokens(msgs),
+    );
+  });
+
+  it('collapses the oldest turns when tool truncation is not enough', () => {
+    // Many turns with LARGE assistant TEXT (not tool output) so phase 1 can't help.
+    const bigText = 'слово '.repeat(8_000); // large Cyrillic text per turn
+    const msgs: ModelMessage[] = [];
+    for (let i = 0; i < 12; i++) {
+      msgs.push(userMsg(`q${i}`));
+      msgs.push(assistantMsg(bigText));
+    }
+    const r = trimHistoryForReplay(msgs, 30_000);
+    expect(r.trimmed).toBe(true);
+    // Oldest turns collapsed; result fits (best-effort) and is much smaller.
+    expect(estimateMessagesTokens(r.messages)).toBeLessThan(
+      estimateMessagesTokens(msgs),
+    );
+    // The LAST turn's text is preserved in full (recent turns stay full).
+    expect(JSON.stringify(r.messages[r.messages.length - 1])).toContain(bigText);
+  });
+
+  it('is deterministic / byte-stable for identical inputs', () => {
+    const big = 'Y'.repeat(30_000);
+    const build = (): ModelMessage[] => {
+      const m: ModelMessage[] = [];
+      for (let i = 0; i < 10; i++) {
+        m.push(userMsg(`q${i}`));
+        m.push(assistantMsg('t', `c${i}`, 'getPage'));
+        m.push(toolMsg(`c${i}`, 'getPage', { body: big }));
+      }
+      return m;
+    };
+    const a = trimHistoryForReplay(build(), 15_000);
+    const b = trimHistoryForReplay(build(), 15_000);
+    expect(JSON.stringify(a.messages)).toBe(JSON.stringify(b.messages));
+  });
+
+  it('never leaves an unpaired tool-call after collapsing (balanced history)', () => {
+    const big = 'Z'.repeat(40_000);
+    const msgs: ModelMessage[] = [];
+    for (let i = 0; i < 10; i++) {
+      msgs.push(userMsg(`q${i}`));
+      msgs.push(assistantMsg('t', `c${i}`, 'getPage'));
+      msgs.push(toolMsg(`c${i}`, 'getPage', { body: big }));
+    }
+    const r = trimHistoryForReplay(msgs, 8_000);
+    // Count tool-call vs tool-result parts in the trimmed output.
+    let calls = 0;
+    let results = 0;
+    for (const m of r.messages) {
+      if (!Array.isArray(m.content)) continue;
+      for (const p of m.content as Array<{ type?: string }>) {
+        if (p.type === 'tool-call') calls++;
+        if (p.type === 'tool-result' || p.type === 'tool-error') results++;
+      }
+    }
+    // Every surviving tool-call has a surviving result (collapsing drops BOTH).
+    expect(calls).toBe(results);
+    // Collapsed turns carry the marker.
+    expect(JSON.stringify(r.messages)).toContain(REPLAY_TURN_COLLAPSED_MARKER);
+  });
+
+  it('respects the provider fact: under-budget contextTokens skips trimming', () => {
+    const big = 'W'.repeat(60_000);
+    const msgs = [
+      userMsg('q'),
+      assistantMsg('t', 'c1', 'getPage'),
+      toolMsg('c1', 'getPage', { body: big }),
+    ];
+    // char-estimate is high, but the provider says we are well under budget.
+    const r = trimHistoryForReplay(msgs, 100_000, 5_000);
+    expect(r.trimmed).toBe(false);
+    expect(r.messages).toBe(msgs);
+  });
+});
@@ -0,0 +1,356 @@
+/**
+ * History-replay token budget (#490).
+ *
+ * The whole persisted conversation is replayed to the provider on EVERY turn, so
+ * a long chat eventually exceeds the model's context window and the provider 400s
+ * on every turn — terminally (the chat "bricks"). This module bounds the replayed
+ * history at REPLAY TIME only: it never mutates what is persisted (the DB stays
+ * the full record), and its output is a deterministic, byte-stable function of its
+ * input so the trimmed prefix is identical turn to turn (provider prompt-cache
+ * friendliness — real money on long chats).
+ *
+ * The PRIMARY signal is the provider's own fact: `metadata.contextTokens` from the
+ * last turn. The chars-based {@link estimateTokens} (shared with the client) is
+ * used only for the DELTA of not-yet-sent messages, to decide WHAT to trim, and as
+ * the fallback for chats with no usage yet.
+ */
+import type { ModelMessage } from 'ai';
+import { estimateTokens } from '@docmost/token-estimate';
+
+/** Flat default budget when no context window is configured (tokens). */
+export const REPLAY_BUDGET_DEFAULT_TOKENS = 100_000;
+/** Fraction of a configured context window used as the budget. */
+export const REPLAY_BUDGET_WINDOW_FRACTION = 0.7;
+/**
+ * Fraction of the normal budget used for the REACTIVE re-trim after a provider
+ * context-overflow 400 — the preventive estimate under-counted, so cut harder.
+ */
+export const REPLAY_AGGRESSIVE_FRACTION = 0.5;
+/**
+ * Turns (a user message + its assistant/tool replies) kept FULL at the tail,
+ * including the current one — never trimmed. Older turns are compacted first.
+ */
+export const REPLAY_KEEP_RECENT_TURNS = 4;
+/** Leading chars kept from a truncated old tool output. */
+export const REPLAY_TOOL_OUTPUT_HEAD = 800;
+/** Trailing chars kept from a truncated old tool output. */
+export const REPLAY_TOOL_OUTPUT_TAIL = 300;
+/** Marker inserted where an old tool output was truncated for replay. */
+export const REPLAY_TRUNCATION_MARKER =
+  '[…truncated for replay; call the tool again to read the full output]';
+/** Marker for a whole old turn collapsed to its text. */
+export const REPLAY_TURN_COLLAPSED_MARKER =
+  '[earlier tool activity omitted for replay]';
+
+export interface ReplayBudget {
+  /** Token threshold above which replay history is trimmed; `null` = OFF. */
+  thresholdTokens: number | null;
+  /** True when the flat default was used (no context window configured). */
+  usedDefault: boolean;
+}
+
+/**
+ * Resolve the replay budget from the RAW stored `chatContextWindow` (text/number).
+ *  - a positive value  -> `min(default, floor(fraction × window))`
+ *  - explicit `0`       -> OFF (admin opt-out; `null` threshold)
+ *  - unset/empty/invalid-> the flat default (still protects — the installations
+ *    that hit terminal overflow are exactly the ones that never set a window)
+ *
+ * Note the raw value is needed because the parsed `chatContextWindow` collapses
+ * both `0` and unset to `undefined`, which would erase the explicit off-switch.
+ */
+export function resolveReplayBudget(rawContextWindow: unknown): ReplayBudget {
+  let n: number | undefined;
+  if (typeof rawContextWindow === 'number') {
+    n = rawContextWindow;
+  } else if (typeof rawContextWindow === 'string') {
+    const t = rawContextWindow.trim();
+    n = t === '' ? undefined : Number(t);
+  }
+  // Unset / empty / non-numeric / negative -> flat default (the protective case).
+  if (n === undefined || !Number.isFinite(n) || n < 0) {
+    return { thresholdTokens: REPLAY_BUDGET_DEFAULT_TOKENS, usedDefault: true };
+  }
+  // Explicit 0 -> off-switch.
+  if (n === 0) {
+    return { thresholdTokens: null, usedDefault: false };
+  }
+  return {
+    thresholdTokens: Math.min(
+      REPLAY_BUDGET_DEFAULT_TOKENS,
+      Math.floor(REPLAY_BUDGET_WINDOW_FRACTION * n),
+    ),
+    usedDefault: false,
+  };
+}
+
+/**
+ * True when a provider error is a CONTEXT-OVERFLOW rejection (the prompt exceeds
+ * the model's window). Providers surface this as an HTTP 400 with a recognizable
+ * message; match both the status and the message patterns robustly across
+ * OpenAI-compatible / Anthropic / Gemini wordings, since the exact shape varies.
+ */
+export function isContextOverflowError(error: unknown): boolean {
+  const status = extractStatus(error);
+  const msg = extractMessage(error).toLowerCase();
+  // Message patterns seen across providers for "prompt too long".
+  const overflowPattern =
+    /context (?:length|window)|maximum context|too many tokens|too large for|reduce the length|prompt is too long|input (?:is )?too long|exceeds? the (?:maximum )?(?:context|token)|maximum.*tokens|string too long/;
+  if (!overflowPattern.test(msg)) return false;
+  // A 400/413 with an overflow-shaped message is an overflow. Some providers
+  // omit/rewrite the status, so accept the message match when the status is
+  // unknown, but reject it for auth/rate-limit statuses that never mean overflow.
+  if (status === 400 || status === 413) return true;
+  if (status === 401 || status === 403 || status === 429) return false;
+  return true;
+}
+
+function extractStatus(error: unknown): number | undefined {
+  if (!error || typeof error !== 'object') return undefined;
+  const e = error as Record<string, unknown>;
+  for (const k of ['statusCode', 'status']) {
+    const v = e[k];
+    if (typeof v === 'number') return v;
+    if (typeof v === 'string' && /^\d+$/.test(v)) return Number(v);
+  }
+  // Nested (e.g. { response: { status } } / { cause: { statusCode } }).
+  for (const k of ['response', 'cause', 'data']) {
+    const nested = e[k];
+    if (nested && typeof nested === 'object') {
+      const s = extractStatus(nested);
+      if (s !== undefined) return s;
+    }
+  }
+  return undefined;
+}
+
+function extractMessage(error: unknown): string {
+  if (error == null) return '';
+  if (typeof error === 'string') return error;
+  if (error instanceof Error) {
+    // Include nested causes (provider libs wrap the real body in `cause`).
+    const cause = (error as { cause?: unknown }).cause;
+    return `${error.message} ${cause ? extractMessage(cause) : ''}`;
+  }
+  if (typeof error === 'object') {
+    const e = error as Record<string, unknown>;
+    const parts: string[] = [];
+    for (const k of ['message', 'error', 'body', 'responseBody', 'data']) {
+      const v = e[k];
+      if (typeof v === 'string') parts.push(v);
+      else if (v && typeof v === 'object') parts.push(extractMessage(v));
+    }
+    return parts.join(' ');
+  }
+  return String(error);
+}
+
+/** Rough token size of a ModelMessage array via the shared chars estimator. */
+export function estimateMessagesTokens(
+  messages: ReadonlyArray<ModelMessage>,
+): number {
+  let total = 0;
+  for (const m of messages) {
+    total += estimateTokens(serializeContent(m.content));
+  }
+  return total;
+}
+
+function serializeContent(content: unknown): string {
+  if (typeof content === 'string') return content;
+  try {
+    return JSON.stringify(content) ?? '';
+  } catch {
+    return '';
+  }
+}
+
+/** Deep JSON string of an arbitrary value, bounded so estimation never throws. */
+function stringifyValue(value: unknown): string {
+  if (typeof value === 'string') return value;
+  try {
+    return JSON.stringify(value) ?? String(value);
+  } catch {
+    return String(value);
+  }
+}
+
+export interface TrimResult {
+  messages: ModelMessage[];
+  /** Whether any trimming was applied. */
+  trimmed: boolean;
+  /** Estimated tokens of the returned messages (chars-based). */
+  estimatedTokens: number;
+}
+
+/**
+ * Bound the replayed history to `budgetTokens`, deterministically. Returns the
+ * SAME array reference (no copy) when nothing needs trimming, so the common case
+ * is free and byte-identical. Trimming order (spec #490):
+ *   1. truncate OLD turns' tool outputs (head+tail + marker) — the bulk of the size
+ *   2. mechanically collapse the OLDEST turns to their text (concatenation, no LLM)
+ *   3. the current + last {@link REPLAY_KEEP_RECENT_TURNS} turns stay FULL
+ *
+ * `budgetTokens === null` disables trimming. `priorContextTokens` (the provider's
+ * fact from last turn) short-circuits the decision: when it is known and already
+ * under budget we skip trimming even if the char-estimate is higher (the provider
+ * count is authoritative). The char-estimate drives WHAT to cut.
+ */
+export function trimHistoryForReplay(
+  messages: ModelMessage[],
+  budgetTokens: number | null,
+  priorContextTokens?: number,
+): TrimResult {
+  if (budgetTokens == null) {
+    return { messages, trimmed: false, estimatedTokens: 0 };
+  }
+  const estimated = estimateMessagesTokens(messages);
+  // Decision signal: prefer the provider's fact (last turn's contextTokens) plus
+  // the estimated delta of the messages appended since; fall back to the pure
+  // char-estimate for a chat with no usage yet.
+  const projected =
+    priorContextTokens != null
+      ? Math.max(priorContextTokens, estimated)
+      : estimated;
+  if (projected <= budgetTokens) {
+    return { messages, trimmed: false, estimatedTokens: estimated };
+  }
+
+  // The tail we always keep full: from the Nth-from-last user message onward.
+  const boundary = recentBoundaryIndex(messages, REPLAY_KEEP_RECENT_TURNS);
+  const tail = messages.slice(boundary);
+  let head = messages.slice(0, boundary).map(cloneMessage);
+
+  // Phase 1: truncate old tool outputs.
+  for (const m of head) {
+    if (m.role === 'tool') truncateToolMessage(m);
+  }
+  let out = [...head, ...tail];
+  let est = estimateMessagesTokens(out);
+  if (est <= budgetTokens) {
+    return { messages: out, trimmed: true, estimatedTokens: est };
+  }
+
+  // Phase 2: collapse the oldest turns (in `head`) to their text, one at a time,
+  // from the oldest, until we fit or the whole head is collapsed.
+  const turns = splitTurns(head);
+  const collapsed: ModelMessage[] = [];
+  let i = 0;
+  for (; i < turns.length; i++) {
+    if (est <= budgetTokens) break;
+    collapsed.push(...collapseTurn(turns[i]));
+    // Re-estimate the whole prospective output.
+    const remaining = turns.slice(i + 1).flat();
+    out = [...collapsed, ...remaining, ...tail];
+    est = estimateMessagesTokens(out);
+  }
+  // Include any turns we didn't need to collapse.
+  const remaining = turns.slice(i).flat();
+  out = [...collapsed, ...remaining, ...tail];
+  est = estimateMessagesTokens(out);
+  return { messages: out, trimmed: true, estimatedTokens: est };
+}
+
+/** Index of the first message of the Nth-from-last user turn (0 if fewer). */
+function recentBoundaryIndex(
+  messages: ReadonlyArray<ModelMessage>,
+  keepTurns: number,
+): number {
+  const userIdx: number[] = [];
+  for (let i = 0; i < messages.length; i++) {
+    if (messages[i].role === 'user') userIdx.push(i);
+  }
+  if (userIdx.length <= keepTurns) return 0;
+  return userIdx[userIdx.length - keepTurns];
+}
+
+/** Split a message list into turns; each turn starts at a `user` message. */
+function splitTurns(messages: ModelMessage[]): ModelMessage[][] {
+  const turns: ModelMessage[][] = [];
+  for (const m of messages) {
+    if (m.role === 'user' || turns.length === 0) turns.push([m]);
+    else turns[turns.length - 1].push(m);
+  }
+  return turns;
+}
+
+/**
+ * Collapse a whole turn to its plain text (mechanical concatenation, not an LLM
+ * summary). Keeps the user message; replaces the assistant/tool messages with a
+ * single assistant text message = the assistant's concatenated text + a marker
+ * when tool activity was dropped. Dropping BOTH the tool-call and tool-result
+ * parts together keeps the rebuilt history balanced (no unpaired calls).
+ */
+function collapseTurn(turn: ModelMessage[]): ModelMessage[] {
+  const out: ModelMessage[] = [];
+  let assistantText = '';
+  let hadTools = false;
+  for (const m of turn) {
+    if (m.role === 'user') {
+      out.push(m);
+    } else if (m.role === 'assistant') {
+      const { text, tools } = extractAssistantText(m.content);
+      assistantText += text;
+      hadTools = hadTools || tools;
+    } else if (m.role === 'tool') {
+      hadTools = true;
+    } else {
+      out.push(m);
+    }
+  }
+  const note =
+    (assistantText ? assistantText.trimEnd() : '') +
+    (hadTools
+      ? `${assistantText ? '\n\n' : ''}${REPLAY_TURN_COLLAPSED_MARKER}`
+      : '');
+  if (note) out.push({ role: 'assistant', content: note } as ModelMessage);
+  return out;
+}
+
+function extractAssistantText(content: unknown): {
+  text: string;
+  tools: boolean;
+} {
+  if (typeof content === 'string') return { text: content, tools: false };
+  if (!Array.isArray(content)) return { text: '', tools: false };
+  let text = '';
+  let tools = false;
+  for (const part of content) {
+    const type = (part as { type?: string })?.type;
+    if (type === 'text') text += (part as { text?: string }).text ?? '';
+    else if (type === 'tool-call') tools = true;
+  }
+  return { text, tools };
+}
+
+/** Truncate every tool-result output in a `tool` message to head+tail+marker. */
+function truncateToolMessage(message: ModelMessage): void {
+  const content = message.content;
+  if (!Array.isArray(content)) return;
+  for (const part of content) {
+    const p = part as { type?: string; output?: { type?: string; value?: unknown } };
+    if (p.type !== 'tool-result' && p.type !== 'tool-error') continue;
+    if (!p.output) continue;
+    const raw = stringifyValue(p.output.value);
+    const budget = REPLAY_TOOL_OUTPUT_HEAD + REPLAY_TOOL_OUTPUT_TAIL;
+    if (raw.length <= budget + REPLAY_TRUNCATION_MARKER.length) continue;
+    const truncated =
+      raw.slice(0, REPLAY_TOOL_OUTPUT_HEAD) +
+      `\n${REPLAY_TRUNCATION_MARKER}\n` +
+      raw.slice(raw.length - REPLAY_TOOL_OUTPUT_TAIL);
+    // Represent the shrunk output as a text output (a valid tool-result output).
+    p.output = { type: 'text', value: truncated };
+  }
+}
+
+/** Shallow-ish clone so trimming never mutates the caller's (persisted-derived)
+ *  message objects — only the OLD region is cloned before it is edited. */
+function cloneMessage(m: ModelMessage): ModelMessage {
+  if (typeof m.content === 'string') return { ...m };
+  return {
+    ...m,
+    content: (m.content as unknown[]).map((p) =>
+      p && typeof p === 'object' ? { ...(p as object) } : p,
+    ),
+  } as ModelMessage;
+}
@@ -0,0 +1,24 @@
+import { type Kysely, sql } from 'kysely';
+
+export async function up(db: Kysely<any>): Promise<void> {
+  // Chat-level metadata bag (#490). First use: the deferred-tool ACTIVATION set
+  // (`activatedTools`) is persisted here so it survives across turns — previously
+  // the set was reset every turn, forcing the model to re-run loadTools and pay a
+  // fresh round-trip to re-activate the same tools each turn. On load the stored
+  // set is intersected with the current valid deferred names, so an allowlist /
+  // role change can never inject a now-nonexistent tool.
+  //
+  // jsonb, defaulted to '{}' so every row (incl. pre-migration ones, backfilled
+  // by the default) is a readable object — the app never has to null-guard the
+  // bag itself, only individual keys.
+  await db.schema
+    .alterTable('ai_chats')
+    .addColumn('metadata', 'jsonb', (col) =>
+      col.notNull().defaultTo(sql`'{}'::jsonb`),
+    )
+    .execute();
+}
+
+export async function down(db: Kysely<any>): Promise<void> {
+  await db.schema.alterTable('ai_chats').dropColumn('metadata').execute();
+}
@@ -606,6 +606,9 @@ export interface AiChats {
  // The document the chat was created in (open page at first message). NULL =>
  // started outside any document. ON DELETE SET NULL on the page FK.
  pageId: string | null;
+  // Chat-level metadata bag (#490). jsonb, defaulted to '{}'. First key:
+  // `activatedTools` — the deferred-tool activation set persisted across turns.
+  metadata: Generated<Json>;
  createdAt: Generated<Timestamp>;
  updatedAt: Generated<Timestamp>;
  deletedAt: Timestamp | null;
@@ -245,6 +245,9 @@ export class AiSettingsService {
      // Max context window for the chat header badge denominator. Stored as
      // ::text; 0/unset/invalid = no limit (undefined).
      chatContextWindow: parsePositiveInt(provider.chatContextWindow),
+      // RAW stored value (#490): the replay budgeter reads this to distinguish an
+      // explicit `0` (off-switch) from unset, which parsePositiveInt cannot.
+      chatContextWindowRaw: provider.chatContextWindow,
      // Plain passthrough; getChatModel defaults unset to 'openai-compatible'.
      chatApiStyle: provider.chatApiStyle,
      // Cheap model id for the anonymous public-share assistant; reuses the chat
@@ -129,6 +129,12 @@ const DEFAULT_MCP_STREAM_TIMEOUT_MS = 60_000;
 /** Default total wall-clock cap for ONE external MCP tool call (2 min). */
 const DEFAULT_MCP_CALL_TIMEOUT_MS = 120_000;

+/**
+ * Default `bodyTimeout` for the EXTERNAL-MCP SSE transport (10 min) — #489.
+ * Deliberately much LARGER than {@link DEFAULT_MCP_STREAM_TIMEOUT_MS}.
+ */
+const DEFAULT_MCP_SSE_BODY_TIMEOUT_MS = 600_000;
+
 /**
 * SILENCE timeout (ms) for EXTERNAL-MCP transport ONLY. Override with
 * `AI_MCP_STREAM_TIMEOUT_MS`; a missing/invalid/non-positive value falls back to
@@ -164,6 +170,26 @@ export function mcpCallTimeoutMs(): number {
  return positiveEnv('AI_MCP_CALL_TIMEOUT_MS', DEFAULT_MCP_CALL_TIMEOUT_MS);
 }

+/**
+ * `bodyTimeout` (ms) for the EXTERNAL-MCP **SSE** transport ONLY — #489. Override
+ * with `AI_MCP_SSE_BODY_TIMEOUT_MS`; a missing/invalid/non-positive value falls
+ * back to {@link DEFAULT_MCP_SSE_BODY_TIMEOUT_MS} (10 min).
+ *
+ * The SSE transport holds ONE long-lived response body open across many tool
+ * calls, so undici's `bodyTimeout` (time between body bytes) counts the LEGITIMATE
+ * silence BETWEEN calls, not just a hung single call. At the tight HTTP silence
+ * timeout ({@link mcpStreamTimeoutMs}, 1 min) a normal >1-min gap between the
+ * model's tool calls would break the SSE socket, and the cache would then serve a
+ * dead client until TTL. So the SSE transport gets its OWN, RAISED bodyTimeout;
+ * the per-call total cap ({@link mcpCallTimeoutMs}) still bounds a single stuck
+ * call, and the app-level transport-error retry heals a socket that does break.
+ * The HTTP (streamable) transport keeps the tight timeout — it opens a fresh
+ * request per call, so idle-between-calls does not apply there.
+ */
+export function mcpSseBodyTimeoutMs(): number {
+  return positiveEnv('AI_MCP_SSE_BODY_TIMEOUT_MS', DEFAULT_MCP_SSE_BODY_TIMEOUT_MS);
+}
+
 /**
 * undici `Agent` options for streaming AI traffic — the (generous, finite)
 * silence timeouts plus the keep-alive recycle window. Shared by the chat
@@ -105,6 +105,10 @@ export interface ResolvedAiConfig extends Partial<AiProviderSettings> {
  // Max context window in tokens; surfaced to the chat header badge as the
  // "current / max" denominator. 0/unset = no limit.
  chatContextWindow?: number;
+  // RAW stored context window (::text), BEFORE parsePositiveInt collapses `0` and
+  // unset to `undefined`. The #490 replay budgeter needs the raw value to honor an
+  // explicit `0` off-switch distinctly from "unset -> flat default".
+  chatContextWindowRaw?: string | number;
  // Cheap model id for the public-share assistant; reuses the chat creds.
  publicShareChatModel?: string;
  // Agent-role id whose persona the public-share assistant adopts (empty/unset
@@ -8,19 +8,35 @@ real pain (a "which tools fail most?" analysis that confidently answered

 Read the **Gotchas** section before you trust any error count.

+> **TWO ERAS — check the marker first.** The `tool_calls` shape changed in **#490
+> (trace v2)**. A row written by v2 carries `metadata.toolTraceVersion = 2`; older
+> rows have no such key. The two shapes store DIFFERENT things (v2 dropped the tool
+> OUTPUT from the trace), so **every query below is dual-shape** — branch on the
+> marker. **Never compare an aggregate or trend across the era boundary**: a metric
+> jump on the cut-over week is an artifact of the shape change, not a behavior
+> change.
+
 ## TL;DR

 - Agent chats live in Postgres, DB `docmost`, tables `ai_chat_*`.
- Each tool invocation is stored as **two** array elements (a `tool-call` part and
-  a `tool-result` part), so naive counting double-counts.
- **A tool that *throws* writes no result part.** Since the #407 fix its error is
-  persisted as a dedicated `{toolName, error}` element in `tool_calls` (queryable +
-  replayed to the model). **Rows written before #407 still drop it** — the error is
-  nowhere in the DB and shows only in the live UI. So `isError` / `success=false`
-  scans under-report by design, and pre-#407 thrown errors are invisible.
- To find where agents fail: (1) soft-failure markers in `tool_calls`, (2) the new
-  `error` field for thrown errors (new rows) / the orphan-gap proxy (old rows),
-  (3) server logs / the live UI for full stack traces beyond the truncated message.
+- **Era marker:** `metadata.toolTraceVersion = 2` ⇒ v2 (#490) row; absent ⇒ legacy row.
+- Each tool invocation is stored as **two** consecutive array elements — a
+  `tool-call` part then an OUTCOME part — so naive counting double-counts.
+  - **v2 (#490):** outcome is `{toolName, ok: true}` on success, or
+    `{toolName, error, kind: 'thrown'|'interrupted'}` on failure. The tool **OUTPUT
+    is NOT in `tool_calls`** any more — it lives once in `metadata.parts` (this
+    removed a hundreds-of-MB-per-run write duplication). Soft-failure analysis
+    therefore reads `metadata.parts`, not `tool_calls`.
+  - **legacy:** outcome is `{toolName, output}` on success; a **thrown** failure is
+    a `{toolName, error}` element **only on rows after #407**, and is dropped
+    entirely (silent orphan) on pre-#407 rows.
+- **A tool that *throws* writes no result part.** In v2 it is a
+  `{error, kind:'thrown'}` element; an interrupted/aborted call is a distinct
+  `{error, kind:'interrupted'}`. `isError`/`success=false` scans read the *output*
+  and so under-report thrown failures in every era.
+- To find where agents fail: (1) soft-failure markers in `metadata.parts` outputs
+  (v2) / `tool_calls` outputs (legacy), (2) the `error`/`kind` fields for thrown
+  failures (v2 + post-#407), (3) server logs / the live UI for full stack traces.

 ## Where the data lives

@@ -53,33 +69,67 @@ are rows in `workspaces`, not separate deployments.
 separate `tool` role), `content` (text), `tool_calls` (jsonb array), `metadata`
 (jsonb, holds run `error` + rendered `parts`), `status`, `tsv` (full-text index).

+## Era marker — check this before every query
+
+```sql
+-- how many rows are in each era?
+SELECT COALESCE((metadata->>'toolTraceVersion'), 'legacy') AS era, count(*)
+FROM ai_chat_messages
+WHERE role = 'assistant' AND jsonb_typeof(tool_calls) = 'array'
+GROUP BY 1 ORDER BY 2 DESC;
+```
+
+- `toolTraceVersion = '2'` → **v2** (#490): outcome flags, **no output in the trace**.
+- `NULL` (`'legacy'`) → pre-#490: outcome carries the tool `output` inline.
+
+**Do not trend a metric across the cut-over.** The shape change alone shifts counts
+(e.g. "elements with `output`" collapses to zero for v2), so a week that straddles
+the boundary shows an artifact, not a behavior change. Segment by era, or restrict to
+one era, before comparing.
+
 ## How tool calls are stored — READ THIS

 Tool calls are **not** one-object-per-call. Each logical invocation is split into
-two consecutive elements of the `tool_calls` array:
+two consecutive elements of the `tool_calls` array — a **call** then an **outcome**.
+The outcome shape is era-dependent:

 ```text
-index 0: { "toolName": "getPage",  "input":  { "pageId": "…" } }   ← tool-call   (has input, NO output)
-index 1: { "toolName": "getPage",  "output": { … } }                ← tool-result (has output, NO input)
+# v2 (#490) — metadata.toolTraceVersion = 2
+index 0: { "toolName":"getPage", "input":{...} }                       ← call     (has input)
+index 1: { "toolName":"getPage", "ok":true }                          ← success  (NO output here)
+   or  : { "toolName":"getPage", "error":"…", "kind":"thrown" }       ← threw
+   or  : { "toolName":"getPage", "error":"…", "kind":"interrupted" }  ← aborted mid-step
+
+# legacy — no toolTraceVersion
+index 0: { "toolName":"getPage", "input":{...} }                       ← call     (has input, NO output)
+index 1: { "toolName":"getPage", "output":{...} }                     ← success  (has output)
+   or  : { "toolName":"getPage", "error":"…" }                        ← threw (post-#407 only)
 ```

-The keys that appear on an element are `toolName`, `input`, `output`, and — for a
-**thrown** failure on rows written after the #407 fix — `error` (the tool's error
-message; see the "Hard failures" section below). There is no `state`, no `errorText`,
-no `type`. On pre-#407 rows a thrown failure has NO paired result element at all
-(silent orphan). Consequences:
+The keys that can appear: `toolName`, `input` (call), and on the outcome — **v2:**
+`ok` **or** `error`+`kind`; **legacy:** `output` **or** (post-#407) `error`. There is
+no `state`, no `errorText`, no `type` in `tool_calls` (those live on `metadata.parts`).
+Consequences:

-1. **Real invocation count = elements that have `output` or `error`.** Counting every
-   element double-counts (you get ~2× and a spurious "~50% of every tool has no output").
-2. **Pairing:** a call = a `tool-call` part followed by its result part. A success
-   carries `output`; a thrown failure (post-#407) carries `error` instead. Both carry
-   `toolName`, so you can group by tool on either.
+1. **Real invocation count** — count the OUTCOME elements, not every element (else you
+   double-count): **v2** = elements with `ok` or `error`; **legacy** = elements with
+   `output` or `error`.
+2. **Pairing:** a call (`input`) is followed by its outcome. `toolName` is on both, so
+   you can group by tool on either. In v2 the `kind` field separates a real hard-fail
+   (`thrown`) from an aborted call (`interrupted`) — a distinction legacy rows cannot
+   make (both are orphans; see below).
+3. **The tool OUTPUT is only in `metadata.parts` on v2 rows.** To inspect what a tool
+   returned (soft-error markers, page bodies) on a v2 row, read the parts
+   (`part->>'type' LIKE 'tool-%'`, `part->>'state' = 'output-available'`, `part->'output'`),
+   not `tool_calls`.

 ## The two classes of failure (and which the DB can see)

 ### 1. Soft failures — tool RAN and returned an error-shaped result → PERSISTED ✅

-These are visible in the `tool-result` `output`. The marker differs per tool:
+These are visible in the tool `output` — **on v2 rows in `metadata.parts`** (the
+`output-available` part's `output`), on **legacy rows in the `tool_calls` outcome
+element's `output`**. The marker differs per tool:

 | Tool(s) | Error marker in `output` |
 | --- | --- |
@@ -91,37 +141,32 @@ These are visible in the `tool-result` `output`. The marker differs per tool:
 Note `editPageText` returns `failed: []` on success — filtering on the *presence*
 of the key gives false positives; filter on **non-empty**.

-### 2. Hard failures — tool THREW → NOW PERSISTED ✅ (since the #407 fix)
+### 2. Hard failures — tool THREW → PERSISTED ✅

 When a tool throws (the classic one is `patchNode` / `insertNode` / `tableUpdateCell`
 → `Failed to encode document to Yjs (fromJSON): Unknown node type: undefined`), the
-runtime still writes **no `tool-result` part** — the failure is an ai@6 `tool-error`
-content part instead. **Since the #407 fix, that error is persisted**: `serializeSteps`
-appends a dedicated element `{toolName, error: "<message>"}` right after the failed
-call, mirroring how a successful `{toolName, output}` element is appended. So a thrown
-error now leaves a queryable `error` field carrying its (truncated) reason, and the
-same real text is replayed to the model on the next turn (an `output-error` part with
-the real `errorText`, no longer the `'Tool call did not complete.'` placeholder).
+runtime writes **no `tool-result` part** — the failure is an ai@6 `tool-error` content
+part. How that lands in `tool_calls` depends on the era:

-**Cutover caveat — old rows keep the old blind shape.** Rows written **before** this
-change have the two-part shape (`call` + `output` only) and simply **drop** thrown
-errors, leaving a silent **orphan** (a `call` with no `output` *and* no `error`). Rows
-written **after** the fix additionally carry the `error` element. So:
+- **v2 (#490):** a `{toolName, error, kind:'thrown'}` outcome element. An interrupted /
+  aborted mid-step call is a **distinct** `{toolName, error:'Tool call did not
+  complete.', kind:'interrupted'}` element — so you can tell a real hard-fail from an
+  abort **directly, without the orphan heuristic**. Query `kind = 'thrown'`.
+- **post-#407 legacy:** a `{toolName, error}` element (no `kind`) right after the call.
+- **pre-#407 legacy:** the error is **dropped** — a silent **orphan** (a `call` with no
+  `output` *and* no `error`).

- **New rows:** query the `error` field directly (see the hard-error query below) — no
-  orphan heuristic needed for thrown failures.
- **Old rows (pre-#407):** the only DB-side proxy is still an **orphan**: a `tool-call`
-  part with no matching `tool-result` *and* no `error`. Orphans also appear when a run
-  is **aborted** mid-flight (server restart), so a high-volume tool (`createComment`,
-  `searchInPage`, `Search_web_search`) shows orphans from aborts, not real errors on
-  old rows. Treat the orphan gap as an *upper bound*, and cross-check the tool: a gap on
-  a structural editor (`patchNode`, `insertNode`, `updatePageJson`, `transformPage`) is
-  almost certainly a thrown Yjs-encode error; a gap on `createComment` is mostly aborts.
+The same real error text is replayed to the model on the next turn (an `output-error`
+part with the real `errorText`, from `metadata.parts`), in every era.

-A note on the aborted-call fallback: a call with **neither** a result **nor** a
-`tool-error` (genuinely interrupted mid-step) still replays with the
-`'Tool call did not complete.'` placeholder and persists as an orphan — that path is
-unchanged, and is distinct from a real thrown error, which now carries `error`.
+**Cutover caveat.** Only pre-#407 legacy rows need the orphan proxy: an orphan is a
+`tool-call` with no matching outcome. Orphans there also appear when a run is **aborted**
+mid-flight (server restart), so a high-volume tool (`createComment`, `searchInPage`,
+`Search_web_search`) shows orphans from aborts, not real errors. Treat the orphan gap as
+an *upper bound* and cross-check the tool: a gap on a structural editor (`patchNode`,
+`insertNode`, `updatePageJson`, `transformPage`) is almost certainly a thrown Yjs-encode
+error; a gap on `createComment` is mostly aborts. **On v2 rows this ambiguity is gone**
+— `kind` labels each outcome.

 ### 3. Run-level failures → `ai_chat_runs`

@@ -134,22 +179,34 @@ the wild: `Run interrupted by a server restart.` (aborts) and

 Run all of these via `docker exec gitmost-postgresql psql -U docmost -d docmost -P pager=off -c "…"`.

-**Real invocation count per tool** (result parts only — the correct denominator):
+**Real invocation count per tool** (outcome parts only — the correct denominator).
+Dual-shape: a v2 outcome has `ok` or `error`; a legacy outcome has `output` or `error`:

 ```sql
 SELECT elem->>'toolName' AS tool, count(*) AS calls
 FROM ai_chat_messages m, jsonb_array_elements(m.tool_calls) elem
-WHERE jsonb_typeof(m.tool_calls) = 'array' AND elem ? 'output'
+WHERE jsonb_typeof(m.tool_calls) = 'array'
+  AND (elem ? 'ok' OR elem ? 'output' OR elem ? 'error')
 GROUP BY 1 ORDER BY 2 DESC;
 ```

-**Soft errors per tool** (everything the DB can honestly see):
+**Soft errors per tool.** The soft-error marker lives in the tool OUTPUT — which on
+**v2 rows is in `metadata.parts`**, on **legacy rows is in the `tool_calls` outcome
+element**. This query UNIONs both eras, projecting each output as `o`:

 ```sql
 WITH res AS (
+  -- v2 (#490): output is in metadata.parts (output-available tool parts)
+  SELECT part->>'type' AS tool, part->'output' AS o
+  FROM ai_chat_messages m, jsonb_array_elements(m.metadata->'parts') part
+  WHERE (m.metadata->>'toolTraceVersion') = '2'
+    AND part->>'type' LIKE 'tool-%' AND part->>'state' = 'output-available'
+  UNION ALL
+  -- legacy: output is inline in the tool_calls outcome element
  SELECT elem->>'toolName' AS tool, elem->'output' AS o
  FROM ai_chat_messages m, jsonb_array_elements(m.tool_calls) elem
-  WHERE jsonb_typeof(m.tool_calls) = 'array' AND elem ? 'output'
+  WHERE (m.metadata->>'toolTraceVersion') IS NULL
+    AND jsonb_typeof(m.tool_calls) = 'array' AND elem ? 'output'
 )
 SELECT tool, count(*) AS calls,
       sum(COALESCE(
@@ -167,13 +224,23 @@ FROM res GROUP BY tool HAVING sum(COALESCE(
 ORDER BY soft_errors DESC;
 ```

-**`editPageText` failure reasons** (the most common real agent mistake — bad `find`):
+Note the v2 `tool` label is the part type (`tool-editPageText`); strip the `tool-`
+prefix if you join it against the legacy `toolName`.
+
+**`editPageText` failure reasons** (the most common real agent mistake — bad `find`).
+Same dual-shape output source:

 ```sql
 WITH res AS (
+  SELECT part->'output' AS o
+  FROM ai_chat_messages m, jsonb_array_elements(m.metadata->'parts') part
+  WHERE (m.metadata->>'toolTraceVersion') = '2'
+    AND part->>'type' = 'tool-editPageText' AND part->>'state' = 'output-available'
+  UNION ALL
  SELECT elem->'output' AS o
  FROM ai_chat_messages m, jsonb_array_elements(m.tool_calls) elem
-  WHERE jsonb_typeof(m.tool_calls) = 'array'
+  WHERE (m.metadata->>'toolTraceVersion') IS NULL
+    AND jsonb_typeof(m.tool_calls) = 'array'
    AND elem->>'toolName' = 'editPageText' AND elem ? 'output'
 )
 SELECT f->>'reason' AS reason, count(*)
@@ -182,30 +249,43 @@ WHERE jsonb_typeof(o->'failed') = 'array'
 GROUP BY 1 ORDER BY 2 DESC;
 ```

-**Hard errors — persisted `error` field per tool (NEW rows, since #407)** — thrown
-tool failures now carry their real reason, so query them directly:
+**Hard errors — persisted `error` field per tool (v2 + post-#407 rows)** — thrown tool
+failures carry their real reason, so query them directly. On **v2** rows exclude the
+`interrupted` kind so an aborted call is not counted as a hard-fail:

 ```sql
 SELECT elem->>'toolName' AS tool, count(*) AS thrown_errors,
       min(elem->>'error') AS sample_error
 FROM ai_chat_messages m, jsonb_array_elements(m.tool_calls) elem
 WHERE jsonb_typeof(m.tool_calls) = 'array' AND elem ? 'error'
+  -- v2 rows label the kind; a legacy error element has no kind (count it).
+  AND COALESCE(elem->>'kind', 'thrown') = 'thrown'
+GROUP BY 1 ORDER BY 2 DESC;
+```
+
+Aborted mid-step calls on v2 rows are a distinct, directly countable population:
+
+```sql
+SELECT elem->>'toolName' AS tool, count(*) AS interrupted
+FROM ai_chat_messages m, jsonb_array_elements(m.tool_calls) elem
+WHERE jsonb_typeof(m.tool_calls) = 'array' AND elem->>'kind' = 'interrupted'
 GROUP BY 1 ORDER BY 2 DESC;
 ```

 **Hard-error proxy for OLD rows (pre-#407) — orphan gap per tool, WITH a spread column**
-(call parts minus result parts, plus how many distinct chats the gap is spread across).
-This covers rows written before thrown errors were persisted; on new rows a thrown
-failure now has its own `error` element (use the query above) and an orphan means only
-a genuinely aborted mid-step call:
+(call parts minus outcome parts, plus how many distinct chats the gap is spread across).
+This is needed ONLY for pre-#407 legacy rows (v2 and post-#407 rows carry the error /
+`kind` directly — use the queries above). The `WHERE` restricts to the legacy era so v2
+rows (where an `ok` outcome is not an `output`) never produce phantom orphans:

 ```sql
 WITH parts AS (
  SELECT m.chat_id, elem->>'toolName' AS tool,
-         (elem ? 'input' AND NOT (elem ? 'output')) AS is_call,
-         (elem ? 'output' OR elem ? 'error')        AS is_result
+         (elem ? 'input' AND NOT (elem ? 'output') AND NOT (elem ? 'ok')) AS is_call,
+         (elem ? 'output' OR elem ? 'error' OR elem ? 'ok')               AS is_result
  FROM ai_chat_messages m, jsonb_array_elements(m.tool_calls) elem
  WHERE jsonb_typeof(m.tool_calls) = 'array' AND m.role = 'assistant'
+    AND (m.metadata->>'toolTraceVersion') IS NULL
 ),
 per_chat AS (
  SELECT tool, chat_id, sum(is_call::int) - sum(is_result::int) AS gap
@@ -261,11 +341,21 @@ WHERE tsv @@ websearch_to_tsquery('english', 'some phrase') LIMIT 20;

 ## Don't blow up your context

-A single `tool_calls` row can be **300–400 KB** (results embed full page content and
-search payloads). Never `SELECT tool_calls` (or `jsonb_pretty(tool_calls)`) raw.
-Always project just the keys you need and truncate:
+Tool outputs embed full page content and search payloads (hundreds of KB per row).
+On **legacy** rows they are in `tool_calls`; on **v2** rows they moved to
+`metadata->'parts'` (the `tool_calls` trace itself is now small). Never `SELECT
+tool_calls` / `metadata` (or `jsonb_pretty(...)`) raw — project just the keys you need
+and truncate:

 ```sql
+-- v2: outputs live in metadata.parts
+SELECT part->>'type',
+       left(regexp_replace((part->'output')::text, '\s+', ' ', 'g'), 200)
+FROM ai_chat_messages m, jsonb_array_elements(m.metadata->'parts') part
+WHERE (m.metadata->>'toolTraceVersion') = '2'
+  AND part->>'state' = 'output-available' LIMIT 5;
+
+-- legacy: outputs live in tool_calls
 SELECT elem->>'toolName',
       left(regexp_replace((elem->'output')::text, '\s+', ' ', 'g'), 200)
 FROM ai_chat_messages m, jsonb_array_elements(m.tool_calls) elem
@@ -280,26 +370,32 @@ docker compose -p gitmost logs -f --tail=100 # whole stack
 ```

 Logging is `json-file`, `max-size=10m max-file=5` → ~50 MB retained, then rotated,
-and **wiped on container recreate**. Since the #407 fix, thrown-tool error text is
-**persisted in the `error` field** of `tool_calls` (see the hard-error query above), so
-you no longer depend on live logs for it. Logs/live UI remain useful for **pre-#407
-rows** (whose thrown errors were dropped) and for full stack traces beyond the
-truncated stored message. A per-tool `tool_calls_total{tool,status}` metric to
-VictoriaMetrics is still a possible future add for aggregate dashboards.
+and **wiped on container recreate**. Thrown-tool error text is **persisted** — in the
+`error` field of `tool_calls` (v2 `kind:'thrown'` / post-#407 legacy) — so you no longer
+depend on live logs for it. Logs/live UI remain useful for **pre-#407 rows** (whose
+thrown errors were dropped) and for full stack traces beyond the truncated stored
+message. A per-tool `tool_calls_total{tool,status}` metric to VictoriaMetrics is still a
+possible future add for aggregate dashboards.

 ## Gotchas checklist

- [ ] Counting every `tool_calls` element → **overcount**. Count `output` elements; add `error` elements for thrown failures (new rows), but don't count both as invocations.
- [ ] `isError` / `success=false` ≈ 0 does **not** mean "no errors" — thrown errors are a separate `error` element (new rows) or dropped entirely (pre-#407 rows).
- [ ] Thrown errors persist only on rows written **after the #407 fix** — pre-#407 rows still drop them (orphan only). Mind the cutover when trending over time.
+- [ ] **Check `metadata.toolTraceVersion` first.** v2 (`= 2`) has no output in `tool_calls`; legacy has it inline. Never trend a metric across the era boundary.
+- [ ] Counting every `tool_calls` element → **overcount**. Count OUTCOME elements — v2: `ok` or `error`; legacy: `output` or `error` — never both call+outcome as invocations.
+- [ ] `isError` / `success=false` ≈ 0 does **not** mean "no errors" — thrown errors are an `error` element (v2 `kind:'thrown'` / post-#407), not in the output.
+- [ ] **v2:** soft-error markers (the tool output) are in `metadata.parts`, NOT `tool_calls`. Legacy: they are in the `tool_calls` outcome `output`.
+- [ ] **v2:** `kind` splits a real hard-fail (`thrown`) from an aborted call (`interrupted`) directly — no orphan heuristic needed. The orphan gap is a pre-#407-legacy-only proxy.
 - [ ] `editPageText.failed` is `[]` on success — test for **non-empty**, not presence.
- [ ] Orphan gap on OLD rows mixes thrown errors **and** aborted runs — split by tool. On NEW rows a thrown error is its own `error` element, so a gap ≈ aborted call.
 - [ ] `aborted` runs = server restarts, `failed` runs = provider overload — not agent mistakes.
- [ ] Never dump a raw `tool_calls` cell — it can be hundreds of KB.
- [ ] Logs are ephemeral (≤50 MB, wiped on recreate) — grab hard-error text live.
+- [ ] Never dump a raw `tool_calls` **or** `metadata.parts` cell — outputs are hundreds of KB.
+- [ ] Logs are ephemeral (≤50 MB, wiped on recreate) — grab pre-#407 hard-error text live.

 ## Snapshot (2026-07-07, illustrative — rerun the queries for current numbers)

+> All rows in this snapshot predate #490, so they are **legacy-era** (outputs inline in
+> `tool_calls`, orphan proxy for thrown errors). Do not trend these numbers against v2
+> rows — segment by `toolTraceVersion` first.
+
+
 - 226 chats, 732 messages, 46 runs; ~4 400 real tool invocations.
 - Soft errors (persisted): `editPageText` 4/79 (bad/non-unique `find`) + 9 markdown-in-`find` warnings; `semanticSearch` 3/4 (`unavailable`); `Habr_update_draft_from_docmost` 1/2 (`doc` sent as object, not string).
 - Missing-result proxy, read WITH the spread column:
@@ -59,6 +59,39 @@ import {
  mergeFootnoteDefinitions,
 } from "../lib/transforms.js";

+// Max concurrent per-page comment fetches in checkNewComments (#490). The scan is
+// O(N) independent REST reads over the working set; running them one-at-a-time made
+// a large space linear in round-trips. A small cap parallelizes without hammering
+// the server (or exhausting sockets). 6 is a conservative middle of the 5–8 band.
+const COMMENT_SCAN_CONCURRENCY = 6;
+
+/**
+ * Map `items` through `fn` with at most `limit` in flight, preserving INPUT ORDER
+ * in the returned array. A tiny bounded pool (no p-limit dependency): `limit`
+ * workers pull the next index off a shared cursor until the list is drained.
+ */
+async function mapWithConcurrency<T, R>(
+  items: readonly T[],
+  limit: number,
+  fn: (item: T, index: number) => Promise<R>,
+): Promise<R[]> {
+  const results = new Array<R>(items.length);
+  let cursor = 0;
+  const worker = async (): Promise<void> => {
+    for (;;) {
+      const i = cursor++;
+      if (i >= items.length) return;
+      results[i] = await fn(items[i], i);
+    }
+  };
+  const workers = Array.from(
+    { length: Math.max(1, Math.min(limit, items.length)) },
+    () => worker(),
+  );
+  await Promise.all(workers);
+  return results;
+}
+
 // Public method surface of CommentsMixin (issue #450) — a NAMED type so the factory
 // return type is expressible in the emitted .d.ts (the anonymous mixin class
 // carries the base's protected shared state, which would otherwise trip TS4094).
@@ -655,27 +688,32 @@ export function CommentsMixin<TBase extends GConstructor<DocmostClientContext>>(
      parentPageId,
    );

-    // 2. Fetch comments for each page, keep ones created after since
-    const results: any[] = [];
-    for (const page of pagesInScope) {
-      try {
-        // Full feed (incl. resolved): a "new comments since" scan reports all
-        // recent activity; the active-only filter is scoped to listComments.
-        const comments = (await this.listComments(page.id, true)).items;
-        const newComments = comments.filter(
-          (c: any) => new Date(c.createdAt) > sinceDate,
-        );
-        if (newComments.length > 0) {
-          results.push({
-            pageId: page.id,
-            pageTitle: page.title,
-            comments: newComments,
-          });
+    // 2. Fetch comments for each page, keep ones created after since. Runs with
+    // bounded concurrency (#490) instead of one-at-a-time — the per-page reads are
+    // independent, so a large working set no longer costs O(N) serial round-trips.
+    // Order is preserved (mapWithConcurrency keeps input order), so the output is
+    // deterministic regardless of which fetch finishes first.
+    const perPage = await mapWithConcurrency(
+      pagesInScope,
+      COMMENT_SCAN_CONCURRENCY,
+      async (page: any) => {
+        try {
+          // Full feed (incl. resolved): a "new comments since" scan reports all
+          // recent activity; the active-only filter is scoped to listComments.
+          const comments = (await this.listComments(page.id, true)).items;
+          const newComments = comments.filter(
+            (c: any) => new Date(c.createdAt) > sinceDate,
+          );
+          return newComments.length > 0
+            ? { pageId: page.id, pageTitle: page.title, comments: newComments }
+            : null;
+        } catch (e: any) {
+          // Skip pages with errors (e.g. deleted between calls)
+          return null;
        }
-      } catch (e: any) {
-        // Skip pages with errors (e.g. deleted between calls)
-      }
-    }
+      },
+    );
+    const results: any[] = perPage.filter((r): r is any => r !== null);

    const totalNewComments = results.reduce(
      (sum, r) => sum + r.comments.length,
@@ -30,6 +30,13 @@ export { destroyAllSessions } from "./lib/collab-session.js";
 // internals directly; it goes through loadDocmostMcp()).
 export { SHARED_TOOL_SPECS } from "./tool-specs.js";
 export type { SharedToolSpec } from "./tool-specs.js";
+// #489 — write-class registry consumed by the in-app external-MCP retry gate.
+export {
+  SHARED_TOOL_WRITE_CLASS,
+  isRetryableWriteClass,
+  assertEverySpecDeclaresWriteClass,
+} from "./tool-specs.js";
+export type { ToolWriteClass } from "./tool-specs.js";

 // Re-export the build-time REGISTRY_STAMP (issue #447): a deterministic hash of
 // the tool-specs registry content, generated into src/registry-stamp.generated.ts
@@ -127,6 +127,18 @@ export interface SharedToolSpec {
  mcpName: string;
  /** camelCase key in the ai-SDK tools object (the in-app layer). */
  inAppKey: string;
+  /**
+   * Write-class of the tool (#489), declared on EVERY spec (a registration-time
+   * assert enforces completeness; `satisfies Record<string, SharedToolSpec>`
+   * makes it a compile error to omit). 'readOnly' = a pure read that mutates
+   * NOTHING durable, so it is safe to auto-retry once after a transport break.
+   * 'write' = anything that mutates a page/comment/share/diagram/etc — a
+   * transport error is INDETERMINATE (the server may have applied it before the
+   * connection reset), so it is NEVER blind-retried (a retry would double-apply,
+   * the #435 incident class). Consumed by the external-MCP retry path
+   * (mcp-clients.service.ts) to gate its single auto-retry.
+   */
+  writeClass: 'readOnly' | 'write';
  /** Single canonical model-facing description used by both layers. */
  description: string;
  /**
@@ -240,6 +252,7 @@ export const SHARED_TOOL_SPECS = {
  getWorkspace: {
    mcpName: 'getWorkspace',
    inAppKey: 'getWorkspace',
+    writeClass: 'readOnly',
    description: 'Fetch metadata about the current workspace (name, settings).',
    tier: 'core',
    catalogLine: 'getWorkspace — fetch current workspace metadata (name, settings).',
@@ -249,6 +262,7 @@ export const SHARED_TOOL_SPECS = {
  listSpaces: {
    mcpName: 'listSpaces',
    inAppKey: 'listSpaces',
+    writeClass: 'readOnly',
    description:
      'List the spaces the current user can access. Returns the array of ' +
      'spaces (id, name, slug, ...).',
@@ -260,6 +274,7 @@ export const SHARED_TOOL_SPECS = {
  listShares: {
    mcpName: 'listShares',
    inAppKey: 'listShares',
+    writeClass: 'readOnly',
    description:
      'List all public shares in the workspace with page titles and public URLs.',
    tier: 'deferred',
@@ -272,6 +287,7 @@ export const SHARED_TOOL_SPECS = {
  getPageJson: {
    mcpName: 'getPageJson',
    inAppKey: 'getPageJson',
+    writeClass: 'readOnly',
    description:
      'Get page details with the raw ProseMirror JSON content (lossless: ' +
      'includes block ids, callouts, tables, link/image attributes) plus the ' +
@@ -289,6 +305,7 @@ export const SHARED_TOOL_SPECS = {
  getOutline: {
    mcpName: 'getOutline',
    inAppKey: 'getOutline',
+    writeClass: 'readOnly',
    description:
      "Return a COMPACT outline of a page's top-level blocks ({index, type, " +
      'id, level, firstText}; tables add rows/cols/header; lists add item ' +
@@ -309,6 +326,7 @@ export const SHARED_TOOL_SPECS = {
  getNode: {
    mcpName: 'getNode',
    inAppKey: 'getNode',
+    writeClass: 'readOnly',
    description:
      "Fetch a single block for editing. `nodeId` is a block id from the page " +
      'outline or page-JSON view (works for headings/paragraphs/callouts/images), OR ' +
@@ -350,6 +368,7 @@ export const SHARED_TOOL_SPECS = {
  searchInPage: {
    mcpName: 'searchInPage',
    inAppKey: 'searchInPage',
+    writeClass: 'readOnly',
    description:
      'Find every occurrence of a string (or regex) INSIDE one page and get ' +
      'WHERE each is — instead of pulling blocks one-by-one with getNode. ' +
@@ -413,6 +432,7 @@ export const SHARED_TOOL_SPECS = {
  deleteNode: {
    mcpName: 'deleteNode',
    inAppKey: 'deleteNode',
+    writeClass: 'write',
    description:
      'Remove a single block by its attrs.id (from the page outline or ' +
      'page-JSON view) WITHOUT resending the whole document.',
@@ -438,6 +458,7 @@ export const SHARED_TOOL_SPECS = {
  patchNode: {
    mcpName: 'patchNode',
    inAppKey: 'patchNode',
+    writeClass: 'write',
    description:
      'Replace a single content block identified by its attrs.id, WITHOUT ' +
      'resending the whole document; the replacement keeps the same block id. ' +
@@ -505,6 +526,7 @@ export const SHARED_TOOL_SPECS = {
  insertNode: {
    mcpName: 'insertNode',
    inAppKey: 'insertNode',
+    writeClass: 'write',
    description:
      'Insert content before/after another block (by attrs.id or anchor text) ' +
      'or append it at the end (top level). For before/after you MUST provide ' +
@@ -597,6 +619,7 @@ export const SHARED_TOOL_SPECS = {
  sharePage: {
    mcpName: 'sharePage',
    inAppKey: 'sharePage',
+    writeClass: 'write',
    // CANONICAL: merges the MCP copy's URL-format + idempotency detail with the
    // in-app copy's reversibility note; keeps the security framing both had.
    description:
@@ -626,6 +649,7 @@ export const SHARED_TOOL_SPECS = {
  unsharePage: {
    mcpName: 'unsharePage',
    inAppKey: 'unsharePage',
+    writeClass: 'write',
    description: 'Remove the public share of a page (revokes the public URL).',
    tier: 'deferred',
    catalogLine: "unsharePage — revoke a page's public share (removes the public URL).",
@@ -640,6 +664,7 @@ export const SHARED_TOOL_SPECS = {
  diffPageVersions: {
    mcpName: 'diffPageVersions',
    inAppKey: 'diffPageVersions',
+    writeClass: 'readOnly',
    description:
      'Diff two versions of a page and return a Docmost-equivalent change set ' +
      '(inserted/deleted text, integrity counts for images/links/tables/' +
@@ -672,6 +697,7 @@ export const SHARED_TOOL_SPECS = {
  listPageHistory: {
    mcpName: 'listPageHistory',
    inAppKey: 'listPageHistory',
+    writeClass: 'readOnly',
    description:
      "List a page's saved versions (Docmost auto-snapshots on every save), " +
      'newest first, cursor-paginated. Returns { items, nextCursor }; each ' +
@@ -693,6 +719,7 @@ export const SHARED_TOOL_SPECS = {
  restorePageVersion: {
    mcpName: 'restorePageVersion',
    inAppKey: 'restorePageVersion',
+    writeClass: 'write',
    description:
      'Restore a page to a saved version: writes that version\'s content back ' +
      'as the page\'s current content (Docmost has no restore endpoint, so ' +
@@ -713,6 +740,7 @@ export const SHARED_TOOL_SPECS = {
  importPageMarkdown: {
    mcpName: 'importPageMarkdown',
    inAppKey: 'importPageMarkdown',
+    writeClass: 'write',
    // IN-APP ONLY (issue #411): the external /mcp surface no longer exposes
    // importPageMarkdown — the registry loop in index.ts skips inAppOnly specs,
    // so this stays available to the in-app agent (round-tripping an EXPORTED
@@ -742,6 +770,7 @@ export const SHARED_TOOL_SPECS = {
  copyPageContent: {
    mcpName: 'copyPageContent',
    inAppKey: 'copyPageContent',
+    writeClass: 'write',
    description:
      "Replace targetPageId's content with a copy of sourcePageId's content, " +
      'entirely server-side — the document is NOT sent through the model. The ' +
@@ -770,6 +799,7 @@ export const SHARED_TOOL_SPECS = {
  editPageText: {
    mcpName: 'editPageText',
    inAppKey: 'editPageText',
+    writeClass: 'write',
    description:
      "Surgical find/replace inside a page's text, preserving all block " +
      'ids and marks. A find MAY cross bold/italic/link boundaries; the ' +
@@ -819,6 +849,7 @@ export const SHARED_TOOL_SPECS = {
  stashPage: {
    mcpName: 'stashPage',
    inAppKey: 'stashPage',
+    writeClass: 'readOnly',
    description:
      'Serialize a whole page (the full ProseMirror JSON, as getPageJson ' +
      'returns) into an ephemeral in-memory blob and return ONLY a short ' +
@@ -880,6 +911,7 @@ export const SHARED_TOOL_SPECS = {
  getPage: {
    mcpName: 'getPage',
    inAppKey: 'getPage',
+    writeClass: 'readOnly',
    description:
      'Fetch a single page as Markdown by its id. Returns the page title and ' +
      'its Markdown content. The converter is canonical (round-trips text and ' +
@@ -919,6 +951,7 @@ export const SHARED_TOOL_SPECS = {
  listPages: {
    mcpName: 'listPages',
    inAppKey: 'listPages',
+    writeClass: 'readOnly',
    description:
      'List the most recent pages (ordered by updatedAt, descending), ' +
      'optionally scoped to a single space. Returns a bounded list (default ' +
@@ -965,6 +998,7 @@ export const SHARED_TOOL_SPECS = {
  getTree: {
    mcpName: 'getTree',
    inAppKey: 'getTree',
+    writeClass: 'readOnly',
    description:
      "Get a space's page hierarchy (or one subtree) as a nested tree in a " +
      'SINGLE request — completely and without loss. Each node is ' +
@@ -1009,6 +1043,7 @@ export const SHARED_TOOL_SPECS = {
  getPageContext: {
    mcpName: 'getPageContext',
    inAppKey: 'getPageContext',
+    writeClass: 'readOnly',
    description:
      'Given a pageId, get its LOCATION and immediate surroundings (metadata ' +
      'only, no page content) in one call — answers "where am I / what is ' +
@@ -1038,6 +1073,7 @@ export const SHARED_TOOL_SPECS = {
  createPage: {
    mcpName: 'createPage',
    inAppKey: 'createPage',
+    writeClass: 'write',
    description:
      'Create a new page with a Markdown body in a space, optionally under a ' +
      'parent page (omit parentPageId to create at the space root). Returns ' +
@@ -1088,6 +1124,7 @@ export const SHARED_TOOL_SPECS = {
  movePage: {
    mcpName: 'movePage',
    inAppKey: 'movePage',
+    writeClass: 'write',
    description:
      'Move a page under a new parent page, or to the space root when no ' +
      'parent is given. Reversible: move it back at any time.',
@@ -1181,6 +1218,7 @@ export const SHARED_TOOL_SPECS = {
  renamePage: {
    mcpName: 'renamePage',
    inAppKey: 'renamePage',
+    writeClass: 'write',
    description:
      'Rename a page (change its title only; the body is untouched, never ' +
      'resent). Reversible: rename back at any time.',
@@ -1202,6 +1240,7 @@ export const SHARED_TOOL_SPECS = {
  deletePage: {
    mcpName: 'deletePage',
    inAppKey: 'deletePage',
+    writeClass: 'write',
    description:
      'Move a page to the trash — SOFT delete only: the page can be restored ' +
      'from trash and nothing is ever permanently deleted.',
@@ -1235,6 +1274,7 @@ export const SHARED_TOOL_SPECS = {
  updatePageJson: {
    mcpName: 'updatePageJson',
    inAppKey: 'updatePageJson',
+    writeClass: 'write',
    description:
      "Replace a page's content with a raw ProseMirror JSON document (lossless " +
      'write: preserves the block ids, callouts, tables and attributes you pass ' +
@@ -1291,6 +1331,7 @@ export const SHARED_TOOL_SPECS = {
  updatePageMarkdown: {
    mcpName: 'updatePageMarkdown',
    inAppKey: 'updatePageMarkdown',
+    writeClass: 'write',
    description:
      "Replace a page's body with new Markdown content (and optionally its " +
      'title). The whole body is re-imported from the markdown (block ids ' +
@@ -1325,6 +1366,7 @@ export const SHARED_TOOL_SPECS = {
  exportPageMarkdown: {
    mcpName: 'exportPageMarkdown',
    inAppKey: 'exportPageMarkdown',
+    writeClass: 'readOnly',
    // CANONICAL: the MCP copy (a strict superset of the terse in-app wording).
    description:
      'Export a page to a single self-contained Docmost-flavoured Markdown ' +
@@ -1373,6 +1415,7 @@ export const SHARED_TOOL_SPECS = {
  createComment: {
    mcpName: 'createComment',
    inAppKey: 'createComment',
+    writeClass: 'write',
    // CANONICAL: the in-app copy (the more-maintained one). It keeps the same
    // rules as the MCP copy — inline-only, top-level requires a `selection`, no
    // page-level comments, replies inherit the anchor, suggestedText must be
@@ -1505,6 +1548,7 @@ export const SHARED_TOOL_SPECS = {
  listComments: {
    mcpName: 'listComments',
    inAppKey: 'listComments',
+    writeClass: 'readOnly',
    // CANONICAL: the two copies are near-identical; the MCP copy is the
    // superset (it keeps the "(pagination is handled internally)" note the
    // in-app copy dropped), so it is used verbatim.
@@ -1532,6 +1576,7 @@ export const SHARED_TOOL_SPECS = {
  resolveComment: {
    mcpName: 'resolveComment',
    inAppKey: 'resolveComment',
+    writeClass: 'write',
    // CANONICAL: the MCP copy's richer wording, minus its reference
    // to `deleteComment` (a sibling tool that does NOT exist in the in-app
    // layer) — rephrased transport-neutrally per the registry convention.
@@ -1572,6 +1617,7 @@ export const SHARED_TOOL_SPECS = {
  checkNewComments: {
    mcpName: 'checkNewComments',
    inAppKey: 'checkNewComments',
+    writeClass: 'readOnly',
    // CANONICAL: the MCP copy (the more detailed of the two). The MCP layer's
    // execute-side guard that rejects an unparseable `since` timestamp stays in
    // its execute body (per-layer logic), not in the shared schema.
@@ -1651,6 +1697,7 @@ export const SHARED_TOOL_SPECS = {
  tableInsertRow: {
    mcpName: 'tableInsertRow',
    inAppKey: 'tableInsertRow',
+    writeClass: 'write',
    description:
      'Insert a row of plain-text cells into a table. `table` is `#<index>` ' +
      'from the page outline, or a block id inside it. `cells` is the text per ' +
@@ -1684,6 +1731,7 @@ export const SHARED_TOOL_SPECS = {
  tableDeleteRow: {
    mcpName: 'tableDeleteRow',
    inAppKey: 'tableDeleteRow',
+    writeClass: 'write',
    description:
      'Delete the row at 0-based `index` from a table (`table` is `#<index>` ' +
      'from the page outline, or a block id inside it). Refuses to delete the ' +
@@ -1707,6 +1755,7 @@ export const SHARED_TOOL_SPECS = {
  tableUpdateCell: {
    mcpName: 'tableUpdateCell',
    inAppKey: 'tableUpdateCell',
+    writeClass: 'write',
    description:
      'Set the plain-text content of cell [row, col] (0-based) in a table ' +
      '(`table` is `#<index>` from the page outline, or a block id inside it). ' +
@@ -1747,6 +1796,7 @@ export const SHARED_TOOL_SPECS = {
  insertFootnote: {
    mcpName: 'insertFootnote',
    inAppKey: 'insertFootnote',
+    writeClass: 'write',
    description:
      'Insert an AUTHOR-INLINE footnote: you specify only WHERE (anchorText) ' +
      'and WHAT (text). The footnote marker is placed right after anchorText in ' +
@@ -1784,6 +1834,7 @@ export const SHARED_TOOL_SPECS = {
  insertImage: {
    mcpName: 'insertImage',
    inAppKey: 'insertImage',
+    writeClass: 'write',
    description:
      'Download an image from a web (http/https) URL and insert it into ' +
      'a page in one step. By default ' +
@@ -1828,6 +1879,7 @@ export const SHARED_TOOL_SPECS = {
  replaceImage: {
    mcpName: 'replaceImage',
    inAppKey: 'replaceImage',
+    writeClass: 'write',
    description:
      'Replace an existing image on a page with a new image fetched from a web ' +
      '(http/https) URL: uploads the new file as a NEW ' +
@@ -1866,6 +1918,7 @@ export const SHARED_TOOL_SPECS = {
  drawioGet: {
    mcpName: 'drawioGet',
    inAppKey: 'drawioGet',
+    writeClass: 'readOnly',
    description:
      'Read a draw.io diagram on a page as mxGraph XML (default) or as its raw ' +
      '`.drawio.svg`. `node` is the drawio node\'s attrs.id (from getOutline / ' +
@@ -1899,6 +1952,7 @@ export const SHARED_TOOL_SPECS = {
  drawioCreate: {
    mcpName: 'drawioCreate',
    inAppKey: 'drawioCreate',
+    writeClass: 'write',
    description:
      'Create a draw.io diagram from mxGraph XML and insert it as a diagram ' +
      'block. `xml` is a bare `<mxGraphModel>` OR a list of `<mxCell>` elements ' +
@@ -1969,6 +2023,7 @@ export const SHARED_TOOL_SPECS = {
  drawioUpdate: {
    mcpName: 'drawioUpdate',
    inAppKey: 'drawioUpdate',
+    writeClass: 'write',
    description:
      'Replace a draw.io diagram\'s content with new mxGraph XML (same lint ' +
      'pipeline as drawioCreate). `baseHash` is MANDATORY: pass the hash from ' +
@@ -2020,6 +2075,7 @@ export const SHARED_TOOL_SPECS = {
  drawioEditCells: {
    mcpName: 'drawioEditCells',
    inAppKey: 'drawioEditCells',
+    writeClass: 'write',
    description:
      'Make TARGETED, id-based edits to an existing draw.io diagram instead of ' +
      'resending the whole XML (a full-XML diff is fragile — draw.io reorders ' +
@@ -2078,6 +2134,7 @@ export const SHARED_TOOL_SPECS = {
  drawioFromGraph: {
    mcpName: 'drawioFromGraph',
    inAppKey: 'drawioFromGraph',
+    writeClass: 'write',
    description:
      'Build a draw.io diagram from a SEMANTIC graph — you describe nodes, groups ' +
      'and edges by MEANING and the server picks every coordinate, color and icon ' +
@@ -2202,6 +2259,7 @@ export const SHARED_TOOL_SPECS = {
  drawioFromMermaid: {
    mcpName: 'drawioFromMermaid',
    inAppKey: 'drawioFromMermaid',
+    writeClass: 'write',
    description:
      'Convert Mermaid `flowchart` text into an EDITABLE draw.io diagram (LLMs ' +
      'write Mermaid reliably). Best for STANDARD flowcharts/decision trees: ' +
@@ -2248,6 +2306,7 @@ export const SHARED_TOOL_SPECS = {
  drawioShapes: {
    mcpName: 'drawioShapes',
    inAppKey: 'drawioShapes',
+    writeClass: 'readOnly',
    description:
      'Look up VERIFIED draw.io stencil style-strings so you never guess a ' +
      '`shape=mxgraph.*` name (a wrong name renders as an EMPTY BOX). Searches a ' +
@@ -2294,6 +2353,7 @@ export const SHARED_TOOL_SPECS = {
  drawioGuide: {
    mcpName: 'drawioGuide',
    inAppKey: 'drawioGuide',
+    writeClass: 'readOnly',
    description:
      'Progressive-disclosure draw.io authoring reference. Call with a `section` ' +
      'to pull one focused, <=4KB chapter instead of bloating context: ' +
@@ -2323,3 +2383,53 @@ export const SHARED_TOOL_SPECS = {
    inlineBothHosts: true,
  },
 } satisfies Record<string, SharedToolSpec>;
+
+// --- write-class registry (#489) ------------------------------------------
+
+/** A tool's retry-safety class. 'readOnly' may be auto-retried once after a
+ *  transport break; 'write' is indeterminate and must never be blind-retried. */
+export type ToolWriteClass = 'readOnly' | 'write';
+
+/**
+ * Name → write-class map for the shared registry, keyed by mcpName (=== inAppKey).
+ * The external-MCP retry path (mcp-clients.service.ts) looks a tool up here by its
+ * RAW (un-namespaced) name to decide whether a transport failure may be retried.
+ * A tool NOT in this map (a third-party external MCP tool) is treated as 'write'
+ * by the consumer — the safe default (never blind-retry an unknown tool).
+ */
+export const SHARED_TOOL_WRITE_CLASS: Record<string, ToolWriteClass> =
+  Object.fromEntries(
+    Object.values(SHARED_TOOL_SPECS).map((spec) => [spec.mcpName, spec.writeClass]),
+  );
+
+/** Whether a write-class permits a single automatic retry after a transport
+ *  break. Only a pure read is retry-safe; everything mutating is indeterminate. */
+export function isRetryableWriteClass(
+  writeClass: ToolWriteClass | undefined,
+): boolean {
+  return writeClass === 'readOnly';
+}
+
+/**
+ * Registration-time assert (#489): EVERY spec must declare a valid write-class.
+ * `satisfies Record<string, SharedToolSpec>` already makes an omission a compile
+ * error, but this guards a raw/cast construction path and documents the invariant
+ * at the point of use. Runs once on import — both hosts import this module, so
+ * both get the check. Throws (fails startup) rather than silently mis-gating a
+ * retry in production.
+ */
+export function assertEverySpecDeclaresWriteClass(): void {
+  for (const [key, spec] of Object.entries(SHARED_TOOL_SPECS)) {
+    const wc = (spec as SharedToolSpec).writeClass;
+    if (wc !== 'readOnly' && wc !== 'write') {
+      throw new Error(
+        `tool-specs: spec "${key}" must declare writeClass ('readOnly' | 'write'), got ${JSON.stringify(
+          wc,
+        )}`,
+      );
+    }
+  }
+}
+
+// Enforce at module load (registration time) on both hosts.
+assertEverySpecDeclaresWriteClass();
@@ -442,3 +442,71 @@ test("checkNewComments subtree includes the root without a separate getPageRaw",
  assert.equal(result.checkedPages, 2, "root + one descendant scanned");
  assert.equal(result.totalNewComments, 1, "the root's fresh comment found");
 });
+
+// -----------------------------------------------------------------------------
+// 6) checkNewComments parallelism (#490): the per-page comment fetches run with
+//    bounded concurrency (not one-at-a-time), and the results still preserve the
+//    page order deterministically regardless of which fetch finishes first.
+// -----------------------------------------------------------------------------
+test("checkNewComments fetches pages concurrently (bounded) and preserves order", async () => {
+  // A subtree with 12 descendants so the scan has plenty to parallelize.
+  const NODES = [{ id: "parent", title: "Parent", parentPageId: null, hasChildren: true }];
+  for (let i = 0; i < 12; i++) {
+    NODES.push({ id: `k${i}`, title: `Kid ${i}`, parentPageId: "parent", hasChildren: false });
+  }
+
+  let inFlight = 0;
+  let maxInFlight = 0;
+
+  const { baseURL } = await spawn(async (req, res) => {
+    const raw = await readBody(req);
+    if (handleLogin(req, res)) return;
+    if (req.url === "/api/pages/tree") {
+      sendJson(res, 200, { success: true, data: { items: NODES } });
+      return;
+    }
+    if (req.url === "/api/comments") {
+      const body = JSON.parse(raw || "{}");
+      inFlight++;
+      maxInFlight = Math.max(maxInFlight, inFlight);
+      // Hold the response briefly so concurrent fetches actually overlap.
+      setTimeout(() => {
+        inFlight--;
+        // Every page carries one fresh comment so ordering is observable.
+        sendJson(res, 200, {
+          success: true,
+          data: {
+            items: [
+              { id: `c-${body.pageId}`, createdAt: "2030-01-01T00:00:00.000Z", content: null },
+            ],
+            meta: { nextCursor: null },
+          },
+        });
+      }, 25);
+      return;
+    }
+    sendJson(res, 404, {});
+  });
+
+  const client = new DocmostClient(baseURL, "user@example.com", "pw");
+  const result = await client.checkNewComments(
+    "space-1",
+    "2020-01-01T00:00:00.000Z",
+    "parent",
+  );
+
+  // 13 pages (parent + 12 kids) were scanned; each had a fresh comment.
+  assert.equal(result.checkedPages, 13, "all pages scanned");
+  assert.equal(result.totalNewComments, 13, "one fresh comment per page");
+  // Parallelism: more than one request was in flight at once, but never above the
+  // cap (6). A serial implementation would show maxInFlight === 1.
+  assert.ok(maxInFlight > 1, `expected concurrent fetches, saw max ${maxInFlight}`);
+  assert.ok(maxInFlight <= 6, `concurrency must be bounded, saw ${maxInFlight}`);
+  // Deterministic order: results follow the page-enumeration order (parent first).
+  assert.equal(result.comments[0].pageId, "parent", "results preserve page order");
+  assert.deepEqual(
+    result.comments.map((r) => r.pageId),
+    ["parent", ...Array.from({ length: 12 }, (_, i) => `k${i}`)],
+    "result order matches the enumeration order regardless of finish order",
+  );
+});
@@ -2,7 +2,12 @@ import { test } from "node:test";
 import assert from "node:assert/strict";
 import { z } from "zod";

-import { SHARED_TOOL_SPECS } from "../../build/tool-specs.js";
+import {
+  SHARED_TOOL_SPECS,
+  SHARED_TOOL_WRITE_CLASS,
+  isRetryableWriteClass,
+  assertEverySpecDeclaresWriteClass,
+} from "../../build/tool-specs.js";

 // The shared registry is consumed by BOTH the zod-v3 MCP server and the zod-v4
 // in-app AI-SDK service, so every spec must carry the cross-layer wiring
@@ -43,6 +48,41 @@ test("mcpName and inAppKey are each unique across the registry", () => {
  }
 });

+// #489 — every spec must declare its write-class so the external-MCP retry path
+// can gate a single auto-retry ONLY on a pure read (a blind retry of a write =
+// double-apply). The declaration is enforced at registration time.
+test("#489: every spec declares a valid writeClass ('readOnly' | 'write')", () => {
+  for (const [key, spec] of Object.entries(SHARED_TOOL_SPECS)) {
+    assert.ok(
+      spec.writeClass === "readOnly" || spec.writeClass === "write",
+      `${key}: missing/invalid writeClass: ${JSON.stringify(spec.writeClass)}`,
+    );
+  }
+  // The registration-time assert must not throw for the shipped registry.
+  assert.doesNotThrow(() => assertEverySpecDeclaresWriteClass());
+});
+
+test("#489: SHARED_TOOL_WRITE_CLASS maps every mcpName to its class; helper gates on readOnly", () => {
+  const specs = Object.values(SHARED_TOOL_SPECS);
+  assert.equal(Object.keys(SHARED_TOOL_WRITE_CLASS).length, specs.length);
+  for (const spec of specs) {
+    assert.equal(SHARED_TOOL_WRITE_CLASS[spec.mcpName], spec.writeClass);
+  }
+  // Only a readOnly tool is retry-eligible; a write tool and an unknown tool are not.
+  assert.equal(isRetryableWriteClass("readOnly"), true);
+  assert.equal(isRetryableWriteClass("write"), false);
+  assert.equal(isRetryableWriteClass(undefined), false);
+});
+
+test("#489: representative reads are readOnly and representative writes are write", () => {
+  for (const name of ["getPage", "getTree", "searchInPage", "listComments"]) {
+    assert.equal(SHARED_TOOL_SPECS[name].writeClass, "readOnly", `${name} should be readOnly`);
+  }
+  for (const name of ["patchNode", "createPage", "deletePage", "createComment", "drawioCreate"]) {
+    assert.equal(SHARED_TOOL_SPECS[name].writeClass, "write", `${name} should be write`);
+  }
+});
+
 test("buildShape (when present) returns a usable ZodRawShape with a real zod", () => {
  for (const [key, spec] of Object.entries(SHARED_TOOL_SPECS)) {
    if (!spec.buildShape) continue;
@@ -0,0 +1,19 @@
+{
+  "name": "@docmost/token-estimate",
+  "version": "0.1.0",
+  "description": "Shared, provider-agnostic token estimator (chars/2.5) used by the AI-chat client counter and the server history-replay budgeter, so the two never diverge.",
+  "private": true,
+  "main": "./dist/index.js",
+  "types": "./dist/index.d.ts",
+  "scripts": {
+    "build": "tsc",
+    "watch": "tsc --watch",
+    "test": "vitest run",
+    "test:watch": "vitest"
+  },
+  "license": "MIT",
+  "devDependencies": {
+    "typescript": "^5.0.0",
+    "vitest": "4.1.6"
+  }
+}
@@ -0,0 +1,31 @@
+import { describe, it, expect } from 'vitest';
+import { estimateTokens, CHARS_PER_TOKEN } from './index';
+
+describe('estimateTokens (shared chars/2.5)', () => {
+  it('returns 0 for empty / nullish input', () => {
+    expect(estimateTokens('')).toBe(0);
+    expect(estimateTokens(null)).toBe(0);
+    expect(estimateTokens(undefined)).toBe(0);
+  });
+
+  it('uses the chars/2.5 ratio, ceiled', () => {
+    expect(CHARS_PER_TOKEN).toBe(2.5);
+    // 5 chars / 2.5 = 2
+    expect(estimateTokens('abcde')).toBe(2);
+    // any non-empty string is at least 1 token (ceil)
+    expect(estimateTokens('a')).toBe(1);
+    // 100 chars / 2.5 = 40
+    expect(estimateTokens('x'.repeat(100))).toBe(40);
+  });
+
+  it('counts Cyrillic ~2x higher than the old chars/4 rule (no undercount)', () => {
+    const cyr = 'привет мир как дела'; // 19 chars
+    expect(estimateTokens(cyr)).toBe(Math.ceil(19 / 2.5)); // 8
+    expect(estimateTokens(cyr)).toBeGreaterThan(Math.ceil(19 / 4)); // > 5
+  });
+
+  it('is deterministic / byte-stable (same input => same output)', () => {
+    const s = 'the quick brown fox';
+    expect(estimateTokens(s)).toBe(estimateTokens(s));
+  });
+});
@@ -0,0 +1,35 @@
+/**
+ * Shared, provider-agnostic token estimator (#490).
+ *
+ * No provider exposes an exact tokenizer we can afford to run on the hot path (a
+ * real BPE pass is O(n²)-ish, bloats the client bundle, and is wrong for
+ * Gemini/Ollama anyway), so both the client's in-body counter AND the server's
+ * history-replay budgeter use this ONE cheap chars-based heuristic. Keeping it in
+ * a single shared module is deliberate: two independent estimators drift, and then
+ * "the badge shows 60%" while "the budgeter already trimmed" — the exact confusion
+ * this package prevents.
+ *
+ * Ratio: **chars / 2.5**. Most content here is Cyrillic, where a token is ~2.5
+ * characters; the common English `chars/4` rule of thumb UNDER-counts Cyrillic by
+ * ~2×, which for a budget check is the dangerous direction (it lets the context
+ * overflow). 2.5 slightly over-estimates pure English/code, which is the SAFE
+ * direction for a budget. This is an estimate, never an exact count — the
+ * authoritative figure is always the provider's reported usage; the estimate is
+ * for UI affordances, the delta of not-yet-sent messages, and deciding what to
+ * trim.
+ */
+
+/** Characters per token for the shared estimate. See the module comment. */
+export const CHARS_PER_TOKEN = 2.5;
+
+/**
+ * Rough token estimate for a piece of text (chars / {@link CHARS_PER_TOKEN}).
+ * Returns 0 for empty/nullish input, and ceils so any non-empty text counts as at
+ * least one token. Pure and deterministic (byte-stable), so the same text always
+ * yields the same estimate — which the server budgeter relies on to keep replay
+ * trimming stable turn to turn (provider prompt-cache friendliness).
+ */
+export function estimateTokens(text: string | null | undefined): number {
+  if (!text) return 0;
+  return Math.ceil(text.length / CHARS_PER_TOKEN);
+}
@@ -0,0 +1,16 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "CommonJS",
+    "moduleResolution": "Node",
+    "outDir": "./dist",
+    "rootDir": "./src",
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "forceConsistentCasingInFileNames": true,
+    "declaration": true
+  },
+  "include": ["src/**/*"],
+  "exclude": ["src/**/*.test.ts"]
+}
@@ -287,6 +287,9 @@ importers:
      '@docmost/prosemirror-markdown':
        specifier: workspace:*
        version: link:../../packages/prosemirror-markdown
+      '@docmost/token-estimate':
+        specifier: workspace:*
+        version: link:../../packages/token-estimate
      '@excalidraw/excalidraw':
        specifier: 0.18.0-3a5ef40
        version: 0.18.0-3a5ef40(@types/react-dom@18.3.1)(@types/react@18.3.12)(react-dom@18.3.1(react@18.3.1))(react@18.3.1)
@@ -561,6 +564,9 @@ importers:
      '@docmost/prosemirror-markdown':
        specifier: workspace:*
        version: link:../../packages/prosemirror-markdown
+      '@docmost/token-estimate':
+        specifier: workspace:*
+        version: link:../../packages/token-estimate
      '@fastify/compress':
        specifier: ^9.0.0
        version: 9.0.0
@@ -1148,6 +1154,15 @@ importers:
        specifier: 4.1.6
        version: 4.1.6(@opentelemetry/api@1.9.0)(@types/node@20.19.43)(@vitest/coverage-v8@4.1.6)(happy-dom@20.8.9)(jsdom@25.0.0)(vite@8.0.5(@types/node@20.19.43)(esbuild@0.28.0)(jiti@2.4.2)(less@4.2.0)(sugarss@5.0.1(postcss@8.5.14))(terser@5.39.0)(tsx@4.21.0)(yaml@2.8.3))

+  packages/token-estimate:
+    devDependencies:
+      typescript:
+        specifier: ^5.0.0
+        version: 5.9.3
+      vitest:
+        specifier: 4.1.6
+        version: 4.1.6(@opentelemetry/api@1.9.0)(@types/node@25.5.0)(@vitest/coverage-v8@4.1.6)(happy-dom@20.8.9)(jsdom@27.4.0(@noble/hashes@2.0.1))(vite@8.0.5(@types/node@25.5.0)(esbuild@0.28.0)(jiti@2.4.2)(less@4.2.0)(sugarss@5.0.1(postcss@8.5.14))(terser@5.39.0)(tsx@4.21.0)(yaml@2.8.3))
+
 packages:

  '@aashutoshrathi/word-wrap@1.2.6':