feat(ai-chat): live-follow a still-running run on chat reopen (#184)

Reopening a chat whose agent run is still going showed a frozen snapshot
from the moment it was opened. Add a passive-observer reconnect-poll path:
when this tab did NOT start the run locally, poll POST /ai-chat/run every
2s while the run is pending/running and merge its incrementally-persisted
assistant message into the thread, so new steps/tool-calls and the growing
text appear live. Polling stops on terminal status (refetchInterval keyed
on run.status, mirroring the reindex polling); a final messages invalidate
shows the persisted end state.

Observer-vs-streamer detection: ChatThread reports its local useChat
streaming status up; the window only polls/merges while NOT locally
streaming (the streamer's SSE owns the view — no double-render). Gated by
settings.ai.autonomousRuns; the query is disabled when the feature is off
so the flag-gated endpoint is never hit, and a failed fetch can't loop
(retry:false -> refetchInterval(undefined)=false).

Pure decisions (poll interval, observe gate, message merge) extracted to
run-polling.ts and unit-tested; added query enable-gating and ChatThread
observer-merge tests. Client-only change — the reconnect endpoint already
returns the run plus the assistant message with its metadata.parts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/ai-chat/components/ai-chat-window.tsx

@@ -17,7 +17,7 @@ import {
   IconPlus,
   IconX,
 } from "@tabler/icons-react";
-import { useAtom, useSetAtom } from "jotai";
+import { useAtom, useAtomValue, useSetAtom } from "jotai";
 import { useMatch } from "react-router-dom";
 import { useTranslation } from "react-i18next";
 import { useQueryClient } from "@tanstack/react-query";
@@ -34,9 +34,12 @@ import {
   AI_CHATS_RQ_KEY,
   AI_CHAT_MESSAGES_RQ_KEY,
   useAiChatMessagesQuery,
+  useAiChatRunQuery,
   useAiChatsQuery,
   useAiRolesQuery,
 } from "@/features/ai-chat/queries/ai-chat-query.ts";
+import { shouldObserveRun } from "@/features/ai-chat/utils/run-polling.ts";
+import { workspaceAtom } from "@/features/user/atoms/current-user-atom";
 import ConversationList from "@/features/ai-chat/components/conversation-list.tsx";
 import ChatThread from "@/features/ai-chat/components/chat-thread.tsx";
 import { exportAiChat } from "@/features/ai-chat/services/ai-chat-service.ts";
@@ -162,6 +165,59 @@ export default function AiChatWindow() {
   const { data: messageRows, isLoading: messagesLoading } =
     useAiChatMessagesQuery(activeChatId ?? undefined);
 
+  // #184 reconnect-and-live-follow. Whether detached agent runs are enabled for
+  // this workspace; the reconnect endpoint is flag-gated server-side, so we must
+  // not poll it when the feature is off.
+  const workspace = useAtomValue(workspaceAtom);
+  const autonomousRunsEnabled =
+    workspace?.settings?.ai?.autonomousRuns === true;
+
+  // Whether THIS tab is the one actively streaming the open chat's run locally
+  // (it started the run here and holds the SSE). Reported up from ChatThread. We
+  // are the STREAMER while true and a passive OBSERVER while false — the basis of
+  // the observer-vs-streamer detection. Reset to false by the fresh ChatThread's
+  // mount effect on every chat switch.
+  const [localStreaming, setLocalStreaming] = useState(false);
+  const onStreamingChange = useCallback((streaming: boolean) => {
+    setLocalStreaming(streaming);
+  }, []);
+
+  // Poll the latest run of the open chat ONLY when we are a passive observer:
+  // feature on, a chat is open, and we are NOT the local streamer (the streamer
+  // already has the live SSE — polling/merging too would double-render). The
+  // query's own status-keyed refetchInterval stops once the run is terminal.
+  const { data: runData } = useAiChatRunQuery(
+    activeChatId ?? undefined,
+    autonomousRunsEnabled && !localStreaming,
+  );
+  const run = runData?.run ?? null;
+  // The run's incrementally-persisted assistant message to merge into the thread,
+  // but only while we are an observer (never when we are the streamer — guards
+  // against a stale poll fighting the live stream). Includes a terminal run so the
+  // final persisted output is shown on reopen.
+  const observedRow = shouldObserveRun(run, localStreaming)
+    ? (runData?.message ?? null)
+    : null;
+
+  // When the observed run reaches a terminal status, do a final messages refetch
+  // so the persisted final state (token/context badge, export source) is shown,
+  // then the query's refetchInterval has already stopped polling. Deduped per run
+  // id so it fires exactly once per run, not on every subsequent poll-less render.
+  const finalizedRunIdRef = useRef<string | null>(null);
+  useEffect(() => {
+    if (!run || !activeChatId) return;
+    if (run.status === "pending" || run.status === "running") {
+      // Active again (a new run) — re-arm so its terminal transition fires once.
+      finalizedRunIdRef.current = null;
+      return;
+    }
+    if (finalizedRunIdRef.current === run.id) return;
+    finalizedRunIdRef.current = run.id;
+    queryClient.invalidateQueries({
+      queryKey: AI_CHAT_MESSAGES_RQ_KEY(activeChatId),
+    });
+  }, [run, activeChatId, queryClient]);
+
   // The page the user is currently viewing. AiChatWindow lives in a pathless
   // parent layout route, so useParams() can't see :pageSlug. Match the full
   // pathname against the authenticated page route instead so "the current page"
@@ -636,6 +692,12 @@ export default function AiChatWindow() {
               assistantName={currentRole?.name}
               onTurnFinished={onTurnFinished}
               onServerChatId={onServerChatId}
+              // #184: live-follow a still-running run when we reopened the chat as
+              // a passive observer; null when there is nothing to observe or this
+              // tab is the streamer. onStreamingChange lets the window stop polling
+              // while we are the streamer.
+              observedRow={observedRow}
+              onStreamingChange={onStreamingChange}
             />
           )}
         </div>

apps/client/src/features/ai-chat/components/chat-thread.test.tsx

@@ -11,6 +11,7 @@ const h = vi.hoisted(() => ({
     onFinish: null as null | ((arg: Record<string, unknown>) => void),
     sendMessage: vi.fn(),
     stop: vi.fn(),
+    setMessages: vi.fn(),
     transport: null as null | {
       prepareSendMessagesRequest: (arg: {
         messages: unknown[];
@@ -30,6 +31,8 @@ vi.mock("@ai-sdk/react", () => ({
       status: h.state.status,
       stop: h.state.stop,
       error: null,
+      // #184: ChatThread reads setMessages to merge a polled observer run.
+      setMessages: h.state.setMessages,
     };
   },
 }));
@@ -140,3 +143,56 @@ describe("ChatThread — send now (#198)", () => {
     expect(prep({ messages: [], body: {} }).body.interrupted).toBe(false);
   });
 });
+
+// #184 passive-observer merge: when reconnecting to a still-running run, the
+// parent feeds the polled run message via `observedRow`; ChatThread merges it via
+// setMessages — but ONLY when this tab is NOT itself streaming (the streamer's
+// SSE owns the view, so a stale observedRow must never overwrite it).
+describe("ChatThread — observer run merge (#184)", () => {
+  beforeEach(() => {
+    h.state.onFinish = null;
+    h.state.setMessages.mockReset();
+  });
+
+  const observedRow = {
+    id: "a-run",
+    role: "assistant",
+    content: "step 1\nstep 2",
+    metadata: {
+      parts: [{ type: "text", text: "step 1\nstep 2" }],
+    },
+    createdAt: "2026-01-01T00:00:00Z",
+  } as const;
+
+  function renderObserver(status: string) {
+    h.state.status = status;
+    render(
+      <MantineProvider>
+        <ChatThread
+          chatId="c1"
+          initialRows={[]}
+          onTurnFinished={vi.fn()}
+          observedRow={observedRow as never}
+        />
+      </MantineProvider>,
+    );
+  }
+
+  it("merges the polled run message when this tab is a passive observer", () => {
+    renderObserver("ready");
+    expect(h.state.setMessages).toHaveBeenCalledTimes(1);
+    // The updater replaces/append the observed assistant row by id.
+    const updater = h.state.setMessages.mock.calls[0][0] as (
+      prev: { id: string; parts: { text: string }[] }[],
+    ) => { id: string; parts: { text: string }[] }[];
+    const merged = updater([{ id: "u1", parts: [{ text: "hi" }] }]);
+    expect(merged).toHaveLength(2);
+    expect(merged[1].id).toBe("a-run");
+    expect(merged[1].parts[0].text).toBe("step 1\nstep 2");
+  });
+
+  it("does NOT merge while THIS tab is the streamer (no double-render)", () => {
+    renderObserver("streaming");
+    expect(h.state.setMessages).not.toHaveBeenCalled();
+  });
+});

apps/client/src/features/ai-chat/components/chat-thread.tsx

@@ -24,6 +24,7 @@ import {
 } from "@/features/ai-chat/utils/role-launch.ts";
 import { describeChatError } from "@/features/ai-chat/utils/error-message.ts";
 import { extractServerChatId } from "@/features/ai-chat/utils/adopt-chat-id.ts";
+import { mergeObservedMessage } from "@/features/ai-chat/utils/run-polling.ts";
 import {
   dequeue,
   enqueueMessage,
@@ -86,6 +87,19 @@ interface ChatThreadProps {
    *  Copy/export button available mid-stream). Distinct from onTurnFinished,
    *  which fires only at the terminal outcome. */
   onServerChatId?: (serverChatId?: string) => void;
+  /** #184 reconnect-and-live-follow. When THIS tab reopened a chat whose agent
+   *  run is still going (it is a PASSIVE OBSERVER — it did not start the run here),
+   *  the parent polls the reconnect endpoint and feeds the run's incrementally-
+   *  persisted assistant message here; we merge it into the live list so new
+   *  steps/tool-calls appear as they are persisted. Null when there is nothing to
+   *  observe (no run, feature off, or this tab IS the streamer). The merge is
+   *  ADDITIONALLY guarded by our own `isStreaming`, so a stale value can never
+   *  fight the local stream when we are the streamer. */
+  observedRow?: IAiChatMessageRow | null;
+  /** Report this tab's live streaming status up to the parent, so it can stop
+   *  polling the run while WE are the active streamer (the SSE owns the view) and
+   *  resume once we go idle. Called from an effect on every transition. */
+  onStreamingChange?: (streaming: boolean) => void;
 }
 
 /**
@@ -131,6 +145,8 @@ export default function ChatThread({
   assistantName,
   onTurnFinished,
   onServerChatId,
+  observedRow,
+  onStreamingChange,
 }: ChatThreadProps) {
   const { t } = useTranslation();
 
@@ -274,7 +290,7 @@ export default function ChatThread({
     [],
   );
 
-  const { messages, sendMessage, status, stop, error } = useChat({
+  const { messages, sendMessage, status, stop, error, setMessages } = useChat({
     // Stable per-mount key. Existing chats use their real id; new chats use a
     // generated client id (never `undefined`) so the store is NOT re-created on
     // every render mid-stream (see `chatStoreId` above).
@@ -378,6 +394,27 @@ export default function ChatThread({
 
   const isStreaming = status === "submitted" || status === "streaming";
 
+  // #184: report our live streaming status up so the parent stops polling the run
+  // while WE are the streamer (the SSE owns the view) and resumes once we go idle.
+  // Effect (not render) so it never updates parent state during our own render;
+  // fires on mount with `false`, which also re-syncs the parent after a chat
+  // switch remounts this thread (a fresh mount is idle until the user sends).
+  useEffect(() => {
+    onStreamingChange?.(isStreaming);
+  }, [isStreaming, onStreamingChange]);
+
+  // #184 passive-observer merge: when the parent feeds a polled run message (we
+  // reopened a chat whose run is still going and did NOT start it here), merge it
+  // into the live list so new steps/tool-calls appear as they are persisted. Hard-
+  // gated by `!isStreaming`: if THIS tab is actually the streamer, the local SSE
+  // owns the view and a stale observedRow must never overwrite it. `observedRow`
+  // is a stable per-poll object, so this runs once per poll, not per render.
+  useEffect(() => {
+    if (isStreaming || !observedRow) return;
+    const observed = rowToUiMessage(observedRow);
+    setMessages((prev) => mergeObservedMessage(prev, observed));
+  }, [observedRow, isStreaming, setMessages]);
+
   // "Send now" on a queued message: interrupt the current turn and immediately
   // send THIS message, keeping the agent's partial output. Other queued messages
   // stay queued and flush normally after the new turn. Reuses the existing

apps/client/src/features/ai-chat/queries/ai-chat-query.ts

@@ -12,6 +12,7 @@ import {
   deleteAiChat,
   deleteAiRole,
   getAiChatMessages,
+  getAiChatRun,
   getAiChats,
   getAiRoleCatalog,
   getAiRoleCatalogBundle,
@@ -24,6 +25,7 @@ import {
 import {
   IAiChat,
   IAiChatMessageRow,
+  IAiChatRunResponse,
   IAiRole,
   IAiRoleCatalog,
   IAiRoleCatalogBundle,
@@ -34,6 +36,7 @@ import {
   IAiRoleUpdateFromCatalogResult,
 } from "@/features/ai-chat/types/ai-chat.types.ts";
 import { IPagination } from "@/lib/types.ts";
+import { runPollInterval } from "@/features/ai-chat/utils/run-polling.ts";
 
 export const AI_CHATS_RQ_KEY = ["ai-chats"];
 export const AI_ROLES_RQ_KEY = ["ai-roles"];
@@ -51,6 +54,7 @@ export const AI_CHAT_MESSAGES_RQ_KEY = (chatId: string) => [
   "ai-chat-messages",
   chatId,
 ];
+export const AI_CHAT_RUN_RQ_KEY = (chatId: string) => ["ai-chat-run", chatId];
 
 /** Paginated list of the current user's chats (auto-loads further pages). */
 export function useAiChatsQuery() {
@@ -131,6 +135,33 @@ export function useAiChatMessagesQuery(chatId: string | undefined) {
   };
 }
 
+/**
+ * Reconnect to a chat's latest agent run and LIVE-FOLLOW it (#184). While the run
+ * is active the query re-polls every {@link runPollInterval} ms (driven off the
+ * fetched `run.status`, the same status-keyed refetchInterval pattern as the
+ * embeddings reindex polling); once the run reaches a terminal status — or there
+ * is no run — the interval returns `false` and polling stops on its own. Polling
+ * is thus naturally bounded by the run terminating; no separate timeout cap.
+ *
+ * `enabled` gates the whole thing: callers pass `false` when the autonomous-runs
+ * feature is off (the endpoint is flag-gated server-side and would 403) OR when
+ * THIS tab is the one actively streaming the run (the live SSE owns the view, so
+ * we must not also poll/merge). The global `retry: false` means a 403/anything
+ * leaves `data` undefined, so refetchInterval(undefined run) returns false — a
+ * failed fetch can never spin a tight loop.
+ */
+export function useAiChatRunQuery(
+  chatId: string | undefined,
+  enabled: boolean,
+) {
+  return useQuery<IAiChatRunResponse, Error>({
+    queryKey: AI_CHAT_RUN_RQ_KEY(chatId ?? ""),
+    queryFn: () => getAiChatRun(chatId as string),
+    enabled: !!chatId && enabled,
+    refetchInterval: (query) => runPollInterval(query.state.data?.run),
+  });
+}
+
 export function useRenameAiChatMutation() {
   const queryClient = useQueryClient();
   const { t } = useTranslation();

apps/client/src/features/ai-chat/queries/ai-chat-run-query.test.tsx

@@ -0,0 +1,92 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import React from "react";
+import { renderHook, waitFor } from "@testing-library/react";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import type { IAiChatRunResponse } from "@/features/ai-chat/types/ai-chat.types.ts";
+
+// react-i18next is pulled in transitively by ai-chat-query.ts (the mutation hooks
+// use it); stub it so the module imports cleanly in this hook test.
+vi.mock("react-i18next", () => ({
+  useTranslation: () => ({ t: (key: string) => key }),
+}));
+
+vi.mock("@mantine/notifications", () => ({
+  notifications: { show: vi.fn() },
+}));
+
+// Mock the whole service module; only getAiChatRun is exercised here, but the
+// other named exports must exist so ai-chat-query.ts imports resolve.
+vi.mock("@/features/ai-chat/services/ai-chat-service.ts", () => ({
+  getAiChatRun: vi.fn(),
+  getAiChatMessages: vi.fn(),
+  getAiChats: vi.fn(),
+  getAiRoleCatalog: vi.fn(),
+  getAiRoleCatalogBundle: vi.fn(),
+  getAiRoles: vi.fn(),
+  importAiRolesFromCatalog: vi.fn(),
+  createAiRole: vi.fn(),
+  deleteAiChat: vi.fn(),
+  deleteAiRole: vi.fn(),
+  renameAiChat: vi.fn(),
+  updateAiRole: vi.fn(),
+  updateAiRoleFromCatalog: vi.fn(),
+}));
+
+import { getAiChatRun } from "@/features/ai-chat/services/ai-chat-service.ts";
+import { useAiChatRunQuery } from "@/features/ai-chat/queries/ai-chat-query.ts";
+
+function createWrapper() {
+  const queryClient = new QueryClient({
+    defaultOptions: { queries: { retry: false } },
+  });
+  return function Wrapper({ children }: { children: React.ReactNode }) {
+    return (
+      <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+    );
+  };
+}
+
+const runningResponse: IAiChatRunResponse = {
+  run: { id: "run-1", chatId: "c1", status: "running" },
+  message: {
+    id: "a1",
+    role: "assistant",
+    content: "working...",
+    createdAt: "2026-01-01T00:00:00Z",
+  },
+};
+
+describe("useAiChatRunQuery — enable gating", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("fetches the run when enabled (passive observer, feature on)", async () => {
+    vi.mocked(getAiChatRun).mockResolvedValue(runningResponse);
+    const { result } = renderHook(() => useAiChatRunQuery("c1", true), {
+      wrapper: createWrapper(),
+    });
+    await waitFor(() => expect(result.current.isSuccess).toBe(true));
+    expect(getAiChatRun).toHaveBeenCalledWith("c1");
+    expect(result.current.data?.run?.status).toBe("running");
+  });
+
+  it("does NOT fetch when disabled (this tab is the streamer / feature off)", async () => {
+    vi.mocked(getAiChatRun).mockResolvedValue(runningResponse);
+    renderHook(() => useAiChatRunQuery("c1", false), {
+      wrapper: createWrapper(),
+    });
+    // Give any errant fetch a chance to fire, then assert none did.
+    await new Promise((r) => setTimeout(r, 20));
+    expect(getAiChatRun).not.toHaveBeenCalled();
+  });
+
+  it("does NOT fetch when there is no chat id", async () => {
+    vi.mocked(getAiChatRun).mockResolvedValue(runningResponse);
+    renderHook(() => useAiChatRunQuery(undefined, true), {
+      wrapper: createWrapper(),
+    });
+    await new Promise((r) => setTimeout(r, 20));
+    expect(getAiChatRun).not.toHaveBeenCalled();
+  });
+});

apps/client/src/features/ai-chat/services/ai-chat-service.ts

@@ -5,6 +5,7 @@ import {
   IAiChatListParams,
   IAiChatMessageRow,
   IAiChatMessagesParams,
+  IAiChatRunResponse,
   IAiRole,
   IAiRoleCatalog,
   IAiRoleCatalogBundle,
@@ -42,6 +43,21 @@ export async function getAiChatMessages(
   return req.data;
 }
 
+/**
+ * Reconnect to the latest agent run of a chat (#184). Returns the run's
+ * persisted lifecycle state and the assistant message it materializes (the
+ * partial output while the run is in-flight, the final output once it finished).
+ * The DB is the source of truth, so this works for an in-flight run (the browser
+ * dropped, the run kept going) and a finished one alike; `{ run: null }` when the
+ * chat has never had a run. Owner-gated and flag-gated server-side.
+ */
+export async function getAiChatRun(
+  chatId: string,
+): Promise<IAiChatRunResponse> {
+  const req = await api.post<IAiChatRunResponse>("/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.

apps/client/src/features/ai-chat/types/ai-chat.types.ts

@@ -200,6 +200,38 @@ export interface IAiChatMessageRow {
   createdAt: string;
 }
 
+/**
+ * A persisted agent-run row (#184), mirroring the `ai_chat_runs` fields the
+ * client reads from `POST /ai-chat/run`. Only `status` is load-bearing for the
+ * reconnect-and-live-update UX (it drives the poll cadence); the rest are carried
+ * for display/diagnostics. The DB is the source of truth, so this resolves for an
+ * in-flight run (the browser dropped, the run kept going) and a finished one.
+ */
+export interface IAiChatRun {
+  id: string;
+  chatId: string;
+  // 'pending' | 'running' | 'succeeded' | 'failed' | 'aborted'. The first two are
+  // ACTIVE (keep polling); the rest are TERMINAL (stop polling).
+  status: "pending" | "running" | "succeeded" | "failed" | "aborted" | string;
+  error?: string | null;
+  stepCount?: number;
+  assistantMessageId?: string | null;
+  startedAt?: string | null;
+  finishedAt?: string | null;
+  createdAt?: string;
+  updatedAt?: string;
+}
+
+/**
+ * Response of `POST /ai-chat/run` (#184): the latest run of a chat and the
+ * assistant message it materializes (the partial/final output, projected from the
+ * persisted rows). Both are `null` when the chat has never had a run.
+ */
+export interface IAiChatRunResponse {
+  run: IAiChatRun | null;
+  message: IAiChatMessageRow | null;
+}
+
 export interface IAiChatListParams extends QueryParams {}
 
 export interface IAiChatMessagesParams {

apps/client/src/features/ai-chat/utils/run-polling.test.ts

@@ -0,0 +1,104 @@
+import { describe, it, expect } from "vitest";
+import type { UIMessage } from "@ai-sdk/react";
+import type { IAiChatRun } from "@/features/ai-chat/types/ai-chat.types.ts";
+import {
+  RUN_POLL_INTERVAL_MS,
+  isRunActive,
+  runPollInterval,
+  shouldObserveRun,
+  mergeObservedMessage,
+} from "./run-polling.ts";
+
+function makeRun(status: string): IAiChatRun {
+  return { id: "run-1", chatId: "c1", status };
+}
+
+function makeMsg(id: string, text: string): UIMessage {
+  return {
+    id,
+    role: "assistant",
+    parts: [{ type: "text", text }],
+  } as UIMessage;
+}
+
+describe("isRunActive", () => {
+  it("treats pending and running as active", () => {
+    expect(isRunActive(makeRun("pending"))).toBe(true);
+    expect(isRunActive(makeRun("running"))).toBe(true);
+  });
+
+  it("treats terminal / unknown / nullish as not active", () => {
+    expect(isRunActive(makeRun("succeeded"))).toBe(false);
+    expect(isRunActive(makeRun("failed"))).toBe(false);
+    expect(isRunActive(makeRun("aborted"))).toBe(false);
+    expect(isRunActive(makeRun("weird-future-status"))).toBe(false);
+    expect(isRunActive(null)).toBe(false);
+    expect(isRunActive(undefined)).toBe(false);
+  });
+});
+
+describe("runPollInterval (the refetchInterval helper)", () => {
+  it("returns 2000ms while the run is pending/running", () => {
+    expect(runPollInterval(makeRun("pending"))).toBe(RUN_POLL_INTERVAL_MS);
+    expect(runPollInterval(makeRun("running"))).toBe(RUN_POLL_INTERVAL_MS);
+    expect(RUN_POLL_INTERVAL_MS).toBe(2000);
+  });
+
+  it("returns false (stop polling) once the run is terminal", () => {
+    expect(runPollInterval(makeRun("succeeded"))).toBe(false);
+    expect(runPollInterval(makeRun("failed"))).toBe(false);
+    expect(runPollInterval(makeRun("aborted"))).toBe(false);
+  });
+
+  it("returns false (no polling) when there is no run", () => {
+    expect(runPollInterval(null)).toBe(false);
+    expect(runPollInterval(undefined)).toBe(false);
+  });
+});
+
+describe("shouldObserveRun (observer-vs-streamer decision)", () => {
+  it("observes an active run when this tab is NOT the local streamer", () => {
+    expect(shouldObserveRun(makeRun("running"), false)).toBe(true);
+    expect(shouldObserveRun(makeRun("pending"), false)).toBe(true);
+  });
+
+  it("observes a terminal run too (so the final output shows on reopen)", () => {
+    expect(shouldObserveRun(makeRun("succeeded"), false)).toBe(true);
+  });
+
+  it("does NOT observe when this tab IS the streamer (no double-render)", () => {
+    expect(shouldObserveRun(makeRun("running"), true)).toBe(false);
+    expect(shouldObserveRun(makeRun("succeeded"), true)).toBe(false);
+  });
+
+  it("does NOT observe when there is no run", () => {
+    expect(shouldObserveRun(null, false)).toBe(false);
+    expect(shouldObserveRun(undefined, false)).toBe(false);
+  });
+});
+
+describe("mergeObservedMessage", () => {
+  it("replaces the message with the same id in place (per-step growth)", () => {
+    const prev = [makeMsg("u1", "hi"), makeMsg("a1", "step 1")];
+    const observed = makeMsg("a1", "step 1\nstep 2");
+    const next = mergeObservedMessage(prev, observed);
+    expect(next).toHaveLength(2);
+    expect(next[1]).toBe(observed);
+    expect(next[0]).toBe(prev[0]); // untouched
+    expect(next).not.toBe(prev); // new array (never mutates input)
+  });
+
+  it("appends when the observed message is not yet present", () => {
+    const prev = [makeMsg("u1", "hi")];
+    const observed = makeMsg("a1", "first token");
+    const next = mergeObservedMessage(prev, observed);
+    expect(next).toHaveLength(2);
+    expect(next[1]).toBe(observed);
+  });
+
+  it("returns the original list unchanged when there is nothing to merge", () => {
+    const prev = [makeMsg("u1", "hi")];
+    expect(mergeObservedMessage(prev, null)).toBe(prev);
+    expect(mergeObservedMessage(prev, undefined)).toBe(prev);
+  });
+});

apps/client/src/features/ai-chat/utils/run-polling.ts

@@ -0,0 +1,71 @@
+import type { UIMessage } from "@ai-sdk/react";
+import type { IAiChatRun } from "@/features/ai-chat/types/ai-chat.types.ts";
+
+/**
+ * Reconnect-and-live-follow helpers (#184). When a chat is reopened while its
+ * agent run is STILL going, this tab is a PASSIVE OBSERVER: it did not start the
+ * run here (no local SSE stream), so it catches up by POLLING the reconnect
+ * endpoint (`POST /ai-chat/run`) and merging the run's incrementally-persisted
+ * assistant message into the rendered thread. These are the small pure decisions
+ * that machinery hangs off, extracted so they can be unit-tested in isolation
+ * (mirrors how reindex polling / editor-sync-state are tested).
+ */
+
+/** How often to re-poll the reconnect endpoint while a run is ACTIVE. */
+export const RUN_POLL_INTERVAL_MS = 2000;
+
+// 'pending' and 'running' are the two ACTIVE statuses; 'succeeded' | 'failed' |
+// 'aborted' are TERMINAL (and any unknown future status is treated as terminal,
+// so a stale/odd value never polls forever).
+const ACTIVE_STATUSES = new Set(["pending", "running"]);
+
+/** Whether a run is still going (worth polling / merging live updates from). */
+export function isRunActive(run: IAiChatRun | null | undefined): boolean {
+  return !!run && ACTIVE_STATUSES.has(run.status);
+}
+
+/**
+ * The TanStack Query `refetchInterval` value for the run query: poll every
+ * {@link RUN_POLL_INTERVAL_MS} while the run is active, and `false` (stop) once
+ * it is terminal or there is no run. Polling is thus naturally bounded by the run
+ * reaching a terminal status — no separate timeout cap is needed.
+ */
+export function runPollInterval(
+  run: IAiChatRun | null | undefined,
+): number | false {
+  return isRunActive(run) ? RUN_POLL_INTERVAL_MS : false;
+}
+
+/**
+ * Observer-vs-streamer decision. We render the polled run message (catch up +
+ * keep advancing) ONLY when this tab is a passive observer: there IS a run AND
+ * this tab is NOT the one locally streaming it (we reconnected, we didn't start
+ * it here). When this tab is the streamer, the live SSE stream owns the view, so
+ * we neither poll nor merge — avoiding a double-render fight. Terminal runs still
+ * merge (so the final persisted output is shown on reopen); the poll itself is
+ * stopped separately by {@link runPollInterval}.
+ */
+export function shouldObserveRun(
+  run: IAiChatRun | null | undefined,
+  localStreaming: boolean,
+): boolean {
+  return !!run && !localStreaming;
+}
+
+/**
+ * Merge an observed assistant message into the rendered list: replace the message
+ * with the same id in place (the in-progress assistant row is already seeded from
+ * history, so per-step growth replaces it), or append it when absent. Returns a
+ * new array; the input is never mutated.
+ */
+export function mergeObservedMessage(
+  messages: UIMessage[],
+  observed: UIMessage | null | undefined,
+): UIMessage[] {
+  if (!observed) return messages;
+  const idx = messages.findIndex((m) => m.id === observed.id);
+  if (idx === -1) return [...messages, observed];
+  const next = messages.slice();
+  next[idx] = observed;
+  return next;
+}

apps/client/src/features/workspace/types/workspace.types.ts

@@ -65,6 +65,9 @@ export interface IWorkspaceAiSettings {
   dictation?: boolean;
   dictationStreaming?: boolean;
   publicShareAssistant?: boolean;
+  // #184: detached agent runs (a run survives a browser disconnect and can be
+  // reconnected to / live-followed on reopen). Gates the run-reconnect polling.
+  autonomousRuns?: boolean;
 }
 
 export interface IWorkspaceSharingSettings {