diff --git a/apps/client/src/features/ai-chat/components/chat-thread.test.tsx b/apps/client/src/features/ai-chat/components/chat-thread.test.tsx
index 5fbcebaa..47e047cd 100644
--- a/apps/client/src/features/ai-chat/components/chat-thread.test.tsx
+++ b/apps/client/src/features/ai-chat/components/chat-thread.test.tsx
@@ -1089,6 +1089,43 @@ describe("ChatThread — live reconnect + stalled", () => {
     );
   });
 
+  it("#491 regression (#137/#161 dup): getRun REJECT on a live disconnect drops the live partial + nulls the anchor", async () => {
+    // The re-seed source (getRun) FAILS — a flaky-network blip (SSE + getRun both
+    // fail, network recovers in ~1s). The OLD .catch just re-entered the ladder with
+    // NO re-seed and NO filter, so the reconnect could tail-apply the registry's
+    // frames onto the live partial that ALREADY has those steps -> duplicated text.
+    renderLive();
+    h.state.getRun.mockReset();
+    h.state.getRun.mockRejectedValue(new Error("network"));
+    await disconnect(); // live partial = liveMsg (id "a2")
+    expect(h.state.getRun).toHaveBeenCalledWith("c1");
+    // THE GUARANTEE: on the getRun failure the live partial (a2) is FILTERED from the
+    // store, so the reconnect can never tail-apply already-present steps onto it.
+    // MUTATION-VERIFY: revert the .catch fix (enterReconnect only, no filter) and no
+    // setMessages call removes a2 -> this reddens.
+    const removedLivePartial = (
+      h.state.setMessages as unknown as {
+        mock: { calls: [unknown][] };
+      }
+    ).mock.calls.some(([updater]) => {
+      if (typeof updater !== "function") return false;
+      const out = (updater as (p: { id: string }[]) => { id: string }[])([
+        { id: "a2" },
+        { id: "u1" },
+      ]);
+      return !out.some((m) => m.id === "a2");
+    });
+    expect(removedLivePartial).toBe(true);
+    expect(screen.getByText(/reconnecting/i)).toBeTruthy();
+    advanceToAttempt(1);
+    expect(h.state.resumeStream).toHaveBeenCalledTimes(1);
+    // Anchor was nulled -> replay-from-start (no params) / 204 -> poll; never a stale
+    // ?anchor=&n= over the live partial.
+    expect(h.state.transport!.prepareReconnectToStreamRequest!().api).toBe(
+      "/api/ai-chat/runs/c1/stream",
+    );
+  });
+
   it("#488 (browser QA): the reconnect banner is SHOWN, not masked by the residual useChat error", async () => {
     // The drop sets useChat `error` (real SDK), and the terminal errorView describes
     // it ("Lost connection to the server"). The FSM phase-gate must let the
diff --git a/apps/client/src/features/ai-chat/components/chat-thread.tsx b/apps/client/src/features/ai-chat/components/chat-thread.tsx
index 01e703ce..6eb7fdaa 100644
--- a/apps/client/src/features/ai-chat/components/chat-thread.tsx
+++ b/apps/client/src/features/ai-chat/components/chat-thread.tsx
@@ -720,13 +720,33 @@ export default function ChatThread({
         const runId = extractRunId(message ?? undefined) ?? "pending";
         const enterReconnect = (fact: string): void => {
           if (!mountedRef.current) return;
-          dispatch({ type: "RUN_FACT", runFact: { runId: fact } });
+          // Epoch-stamp the run-fact too (I1): the getRun rtt widens the
+          // onFinish->dispatch window, so a concurrent SEND_LOCAL during it must be
+          // able to drop this stale RUN_FACT (else it clobbers the new turn's
+          // runFact.runId). Consistent with the postRun RUN_FACT stamp.
+          dispatch({ type: "RUN_FACT", runFact: { runId: fact }, epoch: stampEpoch });
           dispatch({
             type: "FINISH_DISCONNECT",
             hasVisibleContent: msgHasVisible,
             epoch: stampEpoch,
           });
         };
+        // Restore the STRUCTURAL guarantee that the live partial is never the
+        // tail-apply base: drop the live partial from the store by id and null the
+        // anchor, so the reconnect replays from step 0 into a CLEAN store (a full
+        // rebuild) or, past any rotation, 204s -> degraded poll. Used on BOTH the
+        // no-persisted-row and getRun-FAILURE paths — after this there is no path
+        // where the attach tail-applies frames onto a row that already has them
+        // (the #137/#161 duplication class).
+        const dropLivePartialAndReplayFromStart = (): void => {
+          if (message?.role === "assistant" && typeof message.id === "string") {
+            const liveId = message.id;
+            setMessagesRef.current?.((prev) =>
+              prev.filter((m) => m.id !== liveId),
+            );
+          }
+          anchorRef.current = null;
+        };
         if (cid) {
           void getRun(cid)
             .then((res) => {
@@ -738,23 +758,29 @@ export default function ChatThread({
                   stepsPersisted: stepsPersistedOf(persisted),
                 };
                 // Replace the live partial with the persisted row IN PLACE by id —
-                // the re-seed from persist.
+                // the re-seed from persist. The attach's tail (steps >= N) then
+                // appends to a store holding EXACTLY steps 0..N-1: no duplication.
                 setMessages((prev) => mergeById(prev, rowToUiMessage(persisted)));
               } else {
-                // No persisted assistant row (pre-first-frame break): no anchor; the
-                // reconnect attaches with no params and replays from start.
-                anchorRef.current = null;
+                // No persisted assistant row (pre-first-frame break): drop the live
+                // partial + replay from start (no anchor/n) so nothing is duplicated.
+                dropLivePartialAndReplayFromStart();
               }
               enterReconnect(res.run?.id ?? runId);
             })
             .catch(() => {
-              // Persist read failed: keep whatever anchor we had and still enter the
-              // ladder so a live detached run is not stranded (the attach 204s ->
-              // degraded poll if it cannot cover).
+              if (!mountedRef.current) return;
+              // Persist read FAILED: we cannot re-seed from fresh persist, and a
+              // stale mount-time anchor over the live partial would tail-apply
+              // already-present steps -> duplication (a flaky-network blip:
+              // SSE + getRun both fail, network recovers in ~1s, the registry still
+              // covers from the mount frontier). Restore the removed-filter guarantee
+              // instead: drop the live partial + replay from start / 204 -> poll.
+              dropLivePartialAndReplayFromStart();
               enterReconnect(runId);
             });
         } else {
-          anchorRef.current = null;
+          dropLivePartialAndReplayFromStart();
           enterReconnect(runId);
         }
         setStopNotice(null);
diff --git a/apps/client/src/features/ai-chat/state/run-fsm.spec.md b/apps/client/src/features/ai-chat/state/run-fsm.spec.md
index 4f70f744..c40f3fbf 100644
--- a/apps/client/src/features/ai-chat/state/run-fsm.spec.md
+++ b/apps/client/src/features/ai-chat/state/run-fsm.spec.md
@@ -121,8 +121,7 @@ holds. **Pending column: empty.**
 | 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 |
+| 14–15 | `anchorRef {id, stepsPersisted}` | **data** (attachStrategy) | #491 tail-only: replaced `stripRef`/`strippedRowRef`. The PERSISTED assistant row that pins the run (server invariant 6) + its step frontier N; feeds `?anchor=<id>&n=<stepsPersisted>`. No strip — the seed keeps every row; entering reconnecting re-seeds from persist |
 | 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 |
@@ -178,6 +177,9 @@ Pessimism rule: a stale-but-positive fact PERMITS entering recovery (attach); th
   /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.
+- **attachStrategy** is behind the `resumeStream` effect; #491 swapped it to
+  tail-only (`?anchor=&n=`, `anchorRef` data) WITHOUT touching the FSM. Entering
+  reconnecting always re-seeds from persist; on a getRun failure the live partial
+  is dropped + replay-from-start so it is never the tail-apply base (no #137/#161
+  duplication).
 - **Queue** stays a data structure; flush/interrupt decisions are transitions.