test(ai-chat): cover the #161 New-chat-during-stream reset + changelog

Address PR #162 review must-fixes:

- CHANGELOG: add an [Unreleased] > Fixed bullet for #161 (New chat during
  the first turn's stream now resets the thread; a late refetch/onFinish
  from the abandoned thread no longer pulls the user back in).
- Re-anchor the stale startNewChat/cancelPendingAdoption test to selectChat:
  startNewChat now calls startFreshThread, but cancelPendingAdoption still
  backs selectChat, so the disarm guard is decoupled and kept valid.
- Add a test pinning that startFreshThread() disarms the armed error-path
  fallback (the justification for dropping cancelPendingAdoption from
  startNewChat).
- Add a ChatThread render test (mocked useChat) asserting the onError branch
  forwards onTurnFinished(undefined, threadKey).

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

CHANGELOG.md

@@ -92,6 +92,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   no longer froze on the previous step's authoritative usage; the current step's
   estimate is combined per-component with `max`, so the count rises smoothly and
   never jumps backwards. (#163)
+- **AI chat: "New chat" pressed during the first turn's stream now resets the
+  thread instead of leaving the old turn streaming.** While a brand-new,
+  not-yet-adopted chat streamed its first turn, hitting "New chat" left
+  `activeChatId === null` (a no-op for the atom), so the reconciler never
+  remounted and the in-flight thread kept streaming behind the fresh one — and a
+  late refetch / late `onFinish` from that abandoned thread could yank the user
+  back into the chat they just left. "New chat" now forces a fresh empty thread
+  unconditionally and the finished thread's mount key is checked so a late
+  callback from an abandoned thread no longer adopts or re-arms the fallback.
+  (#161)
 
 ## [0.93.0] - 2026-06-21
 

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

@@ -195,6 +195,7 @@ export default function AiChatWindow() {
     waitingForHistory,
     onTurnFinished,
     onServerChatId,
+    startFreshThread,
     cancelPendingAdoption,
   } = useChatSession({
     activeChatId,
@@ -209,18 +210,26 @@ export default function AiChatWindow() {
 
   // startNewChat/selectChat set the public atom; the hook's render-phase
   // reconciler handles the remount when activeChatId actually CHANGES. But
-  // pressing "New chat" while already in a new chat leaves activeChatId === null
-  // (a no-op for the atom), so the reconciler never fires — explicitly disarm any
-  // armed error-path fallback here so a late refetch can't yank the user into a
-  // just-failed chat after they chose a fresh one.
+  // pressing "New chat" while already in a not-yet-adopted new chat leaves
+  // activeChatId === null (a no-op for the atom), so the reconciler never fires —
+  // startFreshThread forces the remount unconditionally (and disarms any armed
+  // error-path fallback) so a late refetch can't yank the user into a just-failed
+  // chat after they chose a fresh one (#161).
   const startNewChat = useCallback((): void => {
-    cancelPendingAdoption();
+    // Force a fresh thread UNCONDITIONALLY. On a brand-new, not-yet-adopted chat
+    // (activeChatId === null) whose first turn is still streaming, setActiveChatId
+    // (null) is a no-op and the render-phase reconciler would not remount — leaving
+    // the streaming thread in place (#161). startFreshThread guarantees a clean
+    // remount and already disarms any armed error-path fallback (so a separate
+    // cancelPendingAdoption() call here is redundant); the abandoned thread's late
+    // finish is rejected by the threadKey guard in the session hook.
+    startFreshThread();
     setActiveChatId(null);
     setHistoryOpen(false);
     setDraft("");
     // Default the picker back to "Universal assistant" for the fresh chat.
     setSelectedRoleId(null);
-  }, [cancelPendingAdoption, setActiveChatId, setDraft, setSelectedRoleId]);
+  }, [startFreshThread, setActiveChatId, setDraft, setSelectedRoleId]);
 
   const selectChat = useCallback(
     (chatId: string): void => {
@@ -633,6 +642,7 @@ export default function AiChatWindow() {
               onRolePicked={(role) => setSelectedRoleId(role.id)}
               assistantName={currentRole?.name}
               onTurnFinished={onTurnFinished}
+              threadKey={threadKey}
               onServerChatId={onServerChatId}
               onLiveTurnTokens={setLiveTurnTokens}
             />

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

@@ -0,0 +1,94 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { render, act } from "@testing-library/react";
+import { MantineProvider } from "@mantine/core";
+
+// Capture the options ChatThread passes to useChat so the test can drive the
+// hook's terminal callbacks (here: onError) directly, without a real stream. The
+// box is created via vi.hoisted so the hoisted vi.mock factory below can close
+// over it.
+const { useChatBox } = vi.hoisted(() => ({
+  useChatBox: { options: null as unknown as Record<string, unknown> | null },
+}));
+
+// Mock the AI SDK hook: record the options and return an inert, ready store so
+// ChatThread renders without any network/streaming machinery.
+vi.mock("@ai-sdk/react", () => ({
+  useChat: (options: Record<string, unknown>) => {
+    useChatBox.options = options;
+    return {
+      messages: [],
+      sendMessage: vi.fn(),
+      status: "ready",
+      stop: vi.fn(),
+      error: null,
+    };
+  },
+}));
+
+// Stub react-i18next so `t` returns the key (other component tests use the same
+// pattern); ChatThread's rendered chrome is irrelevant to this wiring test.
+vi.mock("react-i18next", () => ({
+  useTranslation: () => ({ t: (key: string) => key }),
+}));
+
+// Mock the heavy presentational children to trivial stubs — this test only
+// exercises the onError → onTurnFinished wiring, not their rendering.
+vi.mock("@/features/ai-chat/components/message-list.tsx", () => ({
+  default: () => null,
+}));
+vi.mock("@/features/ai-chat/components/chat-input.tsx", () => ({
+  default: () => null,
+}));
+vi.mock("@/features/ai-chat/components/role-cards.tsx", () => ({
+  default: () => null,
+}));
+vi.mock("@/features/ai-chat/components/chat-error-alert.tsx", () => ({
+  default: () => null,
+}));
+vi.mock("@/features/ai-chat/components/chat-stopped-notice.tsx", () => ({
+  default: () => null,
+}));
+
+import ChatThread from "./chat-thread";
+
+// matchMedia (read by MantineProvider) is stubbed globally in vitest.setup.ts.
+
+describe("ChatThread onError wiring (#161)", () => {
+  beforeEach(() => {
+    useChatBox.options = null;
+    vi.clearAllMocks();
+  });
+
+  it("onError calls onTurnFinished with (undefined, threadKey) so a late error from an abandoned thread is rejected", () => {
+    const onTurnFinished = vi.fn();
+    // Silence the deliberate console.error ChatThread logs for devtools.
+    const consoleError = vi
+      .spyOn(console, "error")
+      .mockImplementation(() => {});
+
+    render(
+      <MantineProvider>
+        <ChatThread
+          chatId="c1"
+          onTurnFinished={onTurnFinished}
+          threadKey="thread-key-1"
+        />
+      </MantineProvider>,
+    );
+
+    const options = useChatBox.options;
+    expect(options).not.toBeNull();
+    expect(typeof options?.onError).toBe("function");
+
+    // Drive the captured onError exactly as the AI SDK would on a stream error.
+    act(() => {
+      (options!.onError as (e: Error) => void)(new Error("stream blew up"));
+    });
+
+    // The thread's own mount key must be forwarded with NO server id, so the
+    // session hook can reject this finish if the thread has been abandoned.
+    expect(onTurnFinished).toHaveBeenCalledWith(undefined, "thread-key-1");
+
+    consoleError.mockRestore();
+  });
+});

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

@@ -60,7 +60,12 @@ interface ChatThreadProps {
    *  new chat, adopts the freshly created chat id. `serverChatId` is the
    *  authoritative id the server streamed on the assistant message metadata, or
    *  undefined on a failed turn — see adopt-chat-id.ts for the full #137 design. */
-  onTurnFinished: (serverChatId?: string) => void;
+  onTurnFinished: (serverChatId?: string, finishingThreadKey?: string) => void;
+  /** This thread's mount key (the same value the parent uses as React `key`).
+   *  Forwarded back through onTurnFinished so the session hook can tell a finish
+   *  from THIS still-mounted thread from a late finish of an abandoned thread the
+   *  user already left via New chat / switch (#161). */
+  threadKey?: string;
   /** Called EARLY (at the stream's `start` chunk) with the authoritative server
    *  chat id streamed on the assistant message metadata, so a brand-new chat
    *  adopts its real id WHILE the first turn is still streaming (#174 — makes the
@@ -116,6 +121,7 @@ export default function ChatThread({
   onRolePicked,
   assistantName,
   onTurnFinished,
+  threadKey,
   onServerChatId,
   onLiveTurnTokens,
 }: ChatThreadProps) {
@@ -258,7 +264,7 @@ export default function ChatThread({
       // Forward the authoritative server chatId (streamed on the assistant
       // message metadata) so the parent adopts the REAL created chat id for a new
       // chat — see adopt-chat-id.ts for the full #137 design.
-      onTurnFinished(extractServerChatId(message));
+      onTurnFinished(extractServerChatId(message), threadKey);
       // Show a neutral "stopped" marker for an aborted turn; the red error banner
       // (via `error`) already covers isError, and a clean finish clears any marker.
       if (isError) setStopNotice(null);
@@ -279,7 +285,7 @@ export default function ChatThread({
       // Surface the raw failure in the browser console (devtools) for debugging;
       // the UI separately shows a friendly classified banner (see errorView).
       console.error("AI chat stream error:", streamError);
-      onTurnFinished();
+      onTurnFinished(undefined, threadKey);
     },
   });
 

apps/client/src/features/ai-chat/hooks/use-chat-session.test.tsx

@@ -1,5 +1,5 @@
 import { describe, it, expect, vi, beforeEach } from "vitest";
-import { renderHook } from "@testing-library/react";
+import { renderHook, act } from "@testing-library/react";
 import { useChatSession } from "./use-chat-session";
 import type { UseChatSessionOptions } from "./use-chat-session";
 
@@ -120,16 +120,40 @@ describe("useChatSession", () => {
     expect(setActiveChatId).not.toHaveBeenCalledWith("new");
   });
 
-  it("startNewChat while already in a new chat: cancelPendingAdoption stops a late refetch adopting the failed chat", () => {
-    // The Warning path the render-phase reconciler can't catch: pressing "New
-    // chat" while already in a new chat keeps activeChatId === null (a no-op for
-    // the atom), so only the explicit cancelPendingAdoption() disarms.
+  it("cancelPendingAdoption (selectChat) disarms a late refetch from adopting the just-failed chat", () => {
+    // cancelPendingAdoption is the explicit disarm the window calls from
+    // selectChat: switching to a chat whose id == null is a no-op for the atom, so
+    // the render-phase reconciler never fires and only this call disarms an armed
+    // error-path fallback. (startNewChat no longer routes through here — it calls
+    // startFreshThread, covered by the next test — but cancelPendingAdoption still
+    // backs selectChat, so this guard must hold.)
     const { result, rerender, setActiveChatId } = setup({
       activeChatId: null,
       chats: { items: [{ id: "x" }] },
     });
     result.current.onTurnFinished(undefined); // first turn failed → arm (before=["x"])
-    result.current.cancelPendingAdoption(); // window calls this from startNewChat
+    result.current.cancelPendingAdoption(); // window calls this from selectChat
+    // The just-failed row lands in a late refetch; it must NOT be adopted.
+    rerender({
+      activeChatId: null,
+      chats: { items: [{ id: "x" }, { id: "failed" }] },
+    });
+    expect(setActiveChatId).not.toHaveBeenCalledWith("failed");
+  });
+
+  it("#161: startFreshThread disarms the armed error-path fallback (New chat during the first turn)", () => {
+    // Pressing "New chat" while already in a not-yet-adopted new chat keeps
+    // activeChatId === null, so the render-phase reconciler never fires. The
+    // window now calls startFreshThread() (NOT cancelPendingAdoption) to force a
+    // fresh thread; this test pins the load-bearing fact that startFreshThread
+    // ALSO nulls pendingNewChatRef, so a late refetch of the just-failed row can't
+    // yank the user back into the abandoned chat.
+    const { result, rerender, setActiveChatId } = setup({
+      activeChatId: null,
+      chats: { items: [{ id: "x" }] },
+    });
+    result.current.onTurnFinished(undefined); // first turn failed → arm (before=["x"])
+    act(() => result.current.startFreshThread()); // "New chat" → fresh thread + disarm
     // The just-failed row lands in a late refetch; it must NOT be adopted.
     rerender({
       activeChatId: null,
@@ -227,6 +251,48 @@ describe("useChatSession", () => {
     expect(result.current.threadKey).toBe("C");
   });
 
+  it("#161: startFreshThread remounts even when activeChatId stays null (New chat mid-stream)", () => {
+    // The bug: pressing New chat while still on a brand-new, not-yet-adopted chat
+    // leaves activeChatId === null, so the render-phase reconciler never fires.
+    // startFreshThread must remount UNCONDITIONALLY (a new mount key).
+    const { result } = setup({ activeChatId: null, chats: { items: [] } });
+    const keyBefore = result.current.threadKey;
+    act(() => result.current.startFreshThread());
+    expect(result.current.threadKey).not.toBe(keyBefore);
+  });
+
+  it("#161: a late finish from an ABANDONED thread does not adopt or invalidate messages", () => {
+    const {
+      result,
+      setActiveChatId,
+      onInvalidateChatList,
+      onInvalidateChatMessages,
+    } = setup({ activeChatId: null, chats: { items: [{ id: "x" }] } });
+    const abandonedKey = result.current.threadKey;
+    // User pressed New chat mid-stream → fresh thread (new mount key).
+    act(() => result.current.startFreshThread());
+    expect(result.current.threadKey).not.toBe(abandonedKey);
+    // The left-behind thread's onFinish fires late, carrying ITS (now stale) key
+    // and the server id of the chat the user just left. It must NOT be adopted.
+    act(() => result.current.onTurnFinished("A", abandonedKey));
+    expect(setActiveChatId).not.toHaveBeenCalled();
+    expect(onInvalidateChatMessages).not.toHaveBeenCalled();
+    // The abandoned chat should still surface in the history list.
+    expect(onInvalidateChatList).toHaveBeenCalled();
+  });
+
+  it("#161: a finish from the CURRENT thread (matching key) still adopts", () => {
+    const { result, setActiveChatId } = setup({
+      activeChatId: null,
+      chats: { items: [{ id: "x" }] },
+    });
+    // Same thread that is mounted reports its finish with the matching key.
+    act(() =>
+      result.current.onTurnFinished("A", result.current.threadKey),
+    );
+    expect(setActiveChatId).toHaveBeenCalledWith("A");
+  });
+
   it("waitingForHistory gates the loader only while opening an unloaded existing chat", () => {
     // Open an existing chat whose history is still loading => loader on.
     const { result, rerender } = setup({

apps/client/src/features/ai-chat/hooks/use-chat-session.ts

@@ -32,8 +32,13 @@ export interface UseChatSessionResult {
   /** Show the history loader instead of the live thread. */
   waitingForHistory: boolean;
   /** Call when a turn finishes; `serverChatId` is the authoritative streamed id
-   *  (undefined on a failed turn). Handles new-chat id adoption + invalidations. */
-  onTurnFinished: (serverChatId?: string) => void;
+   *  (undefined on a failed turn). `finishingThreadKey` is the mount key of the
+   *  thread that produced this callback — when it no longer matches the mounted
+   *  thread (the user pressed New chat / switched mid-stream), the call is from an
+   *  abandoned thread and must NOT adopt or re-arm the fallback. Omitting it (old
+   *  callers / tests) treats the call as belonging to the current thread. Handles
+   *  new-chat id adoption + invalidations. */
+  onTurnFinished: (serverChatId?: string, finishingThreadKey?: string) => void;
   /** Call EARLY (at the stream's `start` chunk) with the authoritative streamed
    *  chat id so a brand-new chat adopts its real id WHILE its first turn is still
    *  streaming — making `activeChatId`-gated affordances (e.g. the Copy/export
@@ -41,6 +46,13 @@ export interface UseChatSessionResult {
    *  no list/messages invalidation — that is left to onTurnFinished at the end).
    *  Idempotent and a no-op once the chat already has an id. */
   onServerChatId: (serverChatId?: string) => void;
+  /** Force a brand-new, empty thread (new mount key, no chat id) UNCONDITIONALLY.
+   *  The render-phase reconciler only remounts when `activeChatId` actually
+   *  changes; pressing "New chat" while already in a not-yet-adopted new chat
+   *  leaves `activeChatId === null` (a no-op for the atom), so the reconciler
+   *  never fires and the stale streaming thread stays mounted (#161). The window
+   *  calls this from startNewChat to guarantee a fresh thread regardless. */
+  startFreshThread: () => void;
   /** Disarm any pending error-path new-chat fallback. The window calls this from
    *  startNewChat/selectChat so a late refetch can't yank the user back into a
    *  just-failed chat after they explicitly moved on. */
@@ -85,6 +97,14 @@ export function useChatSession(
   const activeChatIdRef = useRef(activeChatId);
   activeChatIdRef.current = activeChatId;
 
+  // Live mirror of the mounted thread's key, read by onTurnFinished to tell a
+  // current-thread finish from an ABANDONED one. ai@6's useChat does not abort
+  // its request on unmount, and its callbacks are proxied so onFinish/onError of
+  // a thread the user already left (via New chat / switch) still fire AFTER that
+  // thread unmounts. By then this ref holds the NEW thread's key, so comparing it
+  // to the key the finishing thread reports rejects the abandoned turn (#161).
+  const threadKeyRef = useRef<string>("");
+
   // The mounted thread's identity: ONE atomic value tying ChatThread's mount key
   // (`thread.key`) to the chat id that mounted thread holds (`thread.chatId`).
   // Consolidating these makes the "key vs chat id diverged" state unrepresentable
@@ -98,6 +118,10 @@ export function useChatSession(
       : switchThread(activeChatId),
   );
 
+  // Keep the live mirror pointed at the currently-mounted thread's key so a late
+  // onTurnFinished can be matched against it (see threadKeyRef above).
+  threadKeyRef.current = thread.key;
+
   // Error-path fallback for new-chat id adoption. When a brand-new chat's first
   // turn errors BEFORE the server's `start` chunk, no authoritative chatId ever
   // reaches the client, so the primary metadata adoption cannot run. We then ARM
@@ -115,7 +139,21 @@ export function useChatSession(
   // yet) we adopt the server's AUTHORITATIVE streamed id (never the newest in the
   // list, which races a second tab — #137; see adopt-chat-id.ts).
   const onTurnFinished = useCallback(
-    (serverChatId?: string) => {
+    (serverChatId?: string, finishingThreadKey?: string) => {
+      // Reject a finish from an ABANDONED thread. After the user pressed New chat
+      // (or switched chats) mid-stream, the left-behind thread's onFinish/onError
+      // still fire (ai@6 does not abort on unmount). Adopting/arming off that late
+      // callback would yank the user back into the chat they just left (#161).
+      // `undefined` (legacy callers/tests) is treated as the current thread.
+      const isCurrentThread =
+        finishingThreadKey === undefined ||
+        finishingThreadKey === threadKeyRef.current;
+      if (!isCurrentThread) {
+        // Still surface the abandoned chat in the history list, but do NOT adopt,
+        // arm the fallback, or invalidate per-chat messages (no thread shows it).
+        onInvalidateChatList();
+        return;
+      }
       // Read the live id from the ref, not the closure: on a failed turn this can
       // run twice in one turn (onFinish + onError) before any re-render, and the
       // primary branch below updates the ref so the second call sees the adopted id.
@@ -258,11 +296,29 @@ export function useChatSession(
     pendingNewChatRef.current = null;
   }, []);
 
+  // Force a fresh, empty thread regardless of the current `activeChatId`. The
+  // render-phase reconciler only remounts on an activeChatId CHANGE, so "New chat"
+  // pressed while already in a not-yet-adopted new chat (activeChatId stays null)
+  // would otherwise leave the in-flight streaming thread mounted (#161). Dispatch
+  // `reconcile` to chatId:null with a brand-new key so React remounts ChatThread
+  // (a fresh useChat store). Disarm any armed fallback too. After this dispatch
+  // thread.chatId is null; the window also sets activeChatId to null, so the
+  // render-phase reconciler then finds them equal and does not double-remount.
+  const startFreshThread = useCallback(() => {
+    pendingNewChatRef.current = null;
+    dispatch({
+      type: "reconcile",
+      chatId: null,
+      newKey: `new-${generateId()}`,
+    });
+  }, []);
+
   return {
     threadKey: thread.key,
     waitingForHistory,
     onTurnFinished,
     onServerChatId,
+    startFreshThread,
     cancelPendingAdoption,
   };
 }