fix(ai-chat): New chat during first-turn stream resets the chat, not just the role badge (#161)

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

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

@@ -193,6 +193,7 @@ export default function AiChatWindow() {
   const {
     threadKey,
     waitingForHistory,
+    startFreshThread,
     onTurnFinished,
     onServerChatId,
     cancelPendingAdoption,
@@ -215,12 +216,25 @@ export default function AiChatWindow() {
   // just-failed chat after they chose a fresh one.
   const startNewChat = useCallback((): void => {
     cancelPendingAdoption();
+    // Force a fresh, empty thread UNCONDITIONALLY (#161). Pressing "New chat"
+    // while a brand-new chat's first turn is still streaming leaves activeChatId
+    // null (the real id is adopted only at turn end), so setActiveChatId(null)
+    // alone is a no-op and the reconciler never remounts — the chat/stream/history
+    // would persist and only the role badge would drop. This always remounts the
+    // thread into a clean new chat.
+    startFreshThread();
     setActiveChatId(null);
     setHistoryOpen(false);
     setDraft("");
     // Default the picker back to "Universal assistant" for the fresh chat.
     setSelectedRoleId(null);
-  }, [cancelPendingAdoption, setActiveChatId, setDraft, setSelectedRoleId]);
+  }, [
+    cancelPendingAdoption,
+    startFreshThread,
+    setActiveChatId,
+    setDraft,
+    setSelectedRoleId,
+  ]);
 
   const selectChat = useCallback(
     (chatId: string): void => {
@@ -622,6 +636,7 @@ export default function AiChatWindow() {
           ) : (
             <ChatThread
               key={threadKey}
+              threadKey={threadKey}
               chatId={activeChatId}
               initialRows={activeChatId ? messageRows : []}
               openPage={openPage}

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

@@ -38,6 +38,11 @@ export interface OpenPageContext {
 interface ChatThreadProps {
   /** The open chat id, or null for a brand-new (not-yet-created) chat. */
   chatId: string | null;
+  /** This thread's mount key (the same value the parent uses as React `key`).
+   *  Forwarded to onTurnFinished so the session can tell a turn finishing on the
+   *  CURRENT thread from one ABANDONED by New chat mid-stream — whose onFinish/
+   *  onError still fire after unmount and must not adopt the abandoned chat (#161). */
+  threadKey?: string;
   /** Persisted rows to seed initial messages (existing chats only). */
   initialRows?: IAiChatMessageRow[];
   /** The page currently open in the workspace, or null on a non-page route.
@@ -59,8 +64,10 @@ interface ChatThreadProps {
   /** Called when a turn finishes; the parent refreshes the chat list and, for a
    *  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;
+   *  undefined on a failed turn — see adopt-chat-id.ts for the full #137 design.
+   *  `finishingThreadKey` (this thread's mount key) lets the session ignore a turn
+   *  finishing on a thread already abandoned by New chat mid-stream (#161). */
+  onTurnFinished: (serverChatId?: string, finishingThreadKey?: string) => void;
   /** 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
@@ -109,6 +116,7 @@ function rowToUiMessage(row: IAiChatMessageRow): UIMessage {
  */
 export default function ChatThread({
   chatId,
+  threadKey,
   initialRows,
   openPage,
   roleId,
@@ -257,8 +265,10 @@ export default function ChatThread({
     onFinish: ({ message, isAbort, isDisconnect, isError }) => {
       // 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));
+      // chat — see adopt-chat-id.ts for the full #137 design. `threadKey` lets the
+      // session ignore this finish if it belongs to a thread abandoned by New chat
+      // mid-stream (#161).
+      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 +289,7 @@ 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";
 
@@ -227,6 +227,50 @@ describe("useChatSession", () => {
     expect(result.current.threadKey).toBe("C");
   });
 
+  it("#161: New chat during a streaming first turn forces a fresh thread (remount), not just a no-op", () => {
+    // Brand-new chat whose first turn is still streaming: the id is adopted only
+    // at turn end, so activeChatId AND thread.chatId are both null. Pressing "New
+    // chat" must still remount to a clean thread even though the atom is unchanged
+    // — the render-phase reconciler (null === null) would otherwise do nothing,
+    // leaving the old chat/stream/history in place (the bug: only the role badge
+    // dropped).
+    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: an abandoned thread's late onTurnFinished does NOT adopt its chat (thread-aware guard)", () => {
+    // New chat mid-stream remounts to a fresh thread, but @ai-sdk/react does not
+    // abort the abandoned stream on unmount: its onFinish still fires later with
+    // the real server id, tagged with the OLD (abandoned) mount key. That must not
+    // adopt — it would yank the user back into the chat they just left.
+    const { result, setActiveChatId, onInvalidateChatList } = setup({
+      activeChatId: null,
+      chats: { items: [] },
+    });
+    const abandonedKey = result.current.threadKey;
+    act(() => result.current.startFreshThread());
+    expect(result.current.threadKey).not.toBe(abandonedKey);
+    // The abandoned turn finishes in the background, streaming its real id "A".
+    result.current.onTurnFinished("A", abandonedKey);
+    expect(setActiveChatId).not.toHaveBeenCalledWith("A");
+    // It still refreshes the chat list so the left-behind chat shows in history.
+    expect(onInvalidateChatList).toHaveBeenCalled();
+  });
+
+  it("#161: a turn finishing on the CURRENT thread still adopts (guard is key-scoped, not blanket)", () => {
+    // The happy path must keep working: onTurnFinished tagged with the mounted
+    // thread's own key adopts in place as before.
+    const { result, setActiveChatId } = setup({
+      activeChatId: null,
+      chats: { items: [] },
+    });
+    const currentKey = result.current.threadKey;
+    result.current.onTurnFinished("A", currentKey);
+    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

@@ -31,9 +31,19 @@ export interface UseChatSessionResult {
   threadKey: string;
   /** Show the history loader instead of the live thread. */
   waitingForHistory: boolean;
+  /** Force a brand-new, empty thread (new mount key, no chat id) UNCONDITIONALLY,
+   *  even when `activeChatId` is unchanged. The window calls this from
+   *  startNewChat so "New chat" pressed WHILE a brand-new chat's first turn is
+   *  still streaming (activeChatId still null, nothing to diverge) actually
+   *  resets the chat instead of only dropping the role badge (#161). */
+  startFreshThread: () => void;
   /** 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 the turn (omit => "current thread", back-compatible):
+   *  a turn ABANDONED by New chat mid-stream still fires this after its thread
+   *  unmounted, so adoption is gated to the still-mounted thread (#161). 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
@@ -98,6 +108,15 @@ export function useChatSession(
       : switchThread(activeChatId),
   );
 
+  // Live mirror of the mounted thread's mount key, read by onTurnFinished to tell
+  // the CURRENT thread from one ABANDONED by New chat mid-stream. @ai-sdk/react
+  // does not abort a stream on unmount and proxies callbacks through a ref, so an
+  // abandoned turn's onFinish/onError still fires AFTER its ChatThread unmounted;
+  // matching its key against this ref keeps that late finish from adopting the
+  // abandoned chat and yanking the user out of the fresh chat they opened (#161).
+  const threadKeyRef = useRef(thread.key);
+  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 +134,23 @@ 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) => {
+      // Thread-aware guard (#161). A turn ABANDONED by "New chat" mid-stream still
+      // fires onFinish/onError after its ChatThread unmounted (@ai-sdk/react does
+      // not abort on unmount and proxies callbacks through a ref). If that late
+      // finish ran the adoption path it would set activeChatId to the abandoned
+      // chat's real id and yank the user out of the fresh chat they just opened.
+      // So adopt / arm the fallback ONLY for the still-mounted thread; an
+      // abandoned one merely refreshes the chat list (so the left-behind chat
+      // surfaces in history) and does nothing else. A missing key (undefined)
+      // means "current thread" — keeps old call sites/tests working.
+      if (
+        finishingThreadKey !== undefined &&
+        finishingThreadKey !== threadKeyRef.current
+      ) {
+        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.
@@ -258,9 +293,28 @@ export function useChatSession(
     pendingNewChatRef.current = null;
   }, []);
 
+  // Force a fresh, empty thread regardless of `activeChatId` (#161). The render-
+  // phase reconciler only remounts when activeChatId diverges from thread.chatId,
+  // so "New chat" pressed while a brand-new chat's first turn is still streaming
+  // (activeChatId AND thread.chatId both null — the real id is adopted only at the
+  // end of the turn) is a no-op for it and the abandoned thread/stream/history
+  // would persist. Dispatching reconcile with a fresh key and chatId:null here
+  // always produces a new mount key, so React remounts ChatThread (a clean useChat
+  // store) and the post-dispatch state (activeChatId null === thread.chatId null)
+  // keeps the reconciler from interfering. Also disarms any pending fallback.
+  const startFreshThread = useCallback(() => {
+    pendingNewChatRef.current = null;
+    dispatch({
+      type: "reconcile",
+      chatId: null,
+      newKey: `new-${generateId()}`,
+    });
+  }, []);
+
   return {
     threadKey: thread.key,
     waitingForHistory,
+    startFreshThread,
     onTurnFinished,
     onServerChatId,
     cancelPendingAdoption,