diff --git a/apps/client/src/features/ai-chat/hooks/use-chat-session.test.tsx b/apps/client/src/features/ai-chat/hooks/use-chat-session.test.tsx
index fdd13f91..8104d1e6 100644
--- a/apps/client/src/features/ai-chat/hooks/use-chat-session.test.tsx
+++ b/apps/client/src/features/ai-chat/hooks/use-chat-session.test.tsx
@@ -1,25 +1,25 @@
 import { describe, it, expect, vi, beforeEach } from "vitest";
 import { renderHook } from "@testing-library/react";
 import { useChatSession } from "./use-chat-session";
+import type { UseChatSessionOptions } from "./use-chat-session";
+
+// The props the test drives: the parent-owned subset of UseChatSessionOptions
+// (the spies are injected by setup, not per-render). messagesLoading is optional
+// here (defaulted to false in setup) for terser test call sites.
+type DriverProps = Pick<UseChatSessionOptions, "activeChatId" | "chats"> & {
+  messagesLoading?: boolean;
+};
 
 // Drive the hook the way the window does: the parent owns `activeChatId` and
 // passes it back in. `setActiveChatId` is a spy so we can assert the EXACT id the
 // hook adopts (the #137 regression: it must be the authoritative streamed id, not
 // the newest chat in the list).
-function setup(initial: {
-  activeChatId: string | null;
-  chats: { items?: { id: string }[] } | undefined;
-  messagesLoading?: boolean;
-}) {
+function setup(initial: DriverProps) {
   const setActiveChatId = vi.fn();
   const onInvalidateChatList = vi.fn();
   const onInvalidateChatMessages = vi.fn();
   const { result, rerender } = renderHook(
-    (props: {
-      activeChatId: string | null;
-      chats: { items?: { id: string }[] } | undefined;
-      messagesLoading?: boolean;
-    }) =>
+    (props: DriverProps) =>
       useChatSession({
         activeChatId: props.activeChatId,
         setActiveChatId,
@@ -145,6 +145,32 @@ describe("useChatSession", () => {
     expect(onInvalidateChatMessages).toHaveBeenCalledWith("chat-1");
   });
 
+  it("double onTurnFinished on a failed-after-start turn: primary adopt, 2nd no-id call does NOT re-arm the fallback", () => {
+    // ai@6 fires onFinish AND onError on a failed turn. If the failure happened
+    // AFTER the `start` chunk, onFinish carries the streamed id and onError does
+    // not — so onTurnFinished runs twice in one turn (id, then no-id) before any
+    // re-render. The 2nd call must NOT re-arm the fallback off the still-null
+    // closure; otherwise a late refetch (parent hasn't reflected the adoption yet)
+    // would wrongly adopt a sibling row.
+    const { result, rerender, setActiveChatId } = setup({
+      activeChatId: null,
+      chats: { items: [{ id: "x" }] },
+    });
+    result.current.onTurnFinished("A"); // onFinish: primary adoption
+    expect(setActiveChatId).toHaveBeenCalledWith("A");
+    result.current.onTurnFinished(undefined); // onError: same turn, no id
+    // Even in the worst case (the parent has NOT yet reflected activeChatId="A"
+    // and a late refetch lands a new row), the just-failed sibling must NOT be
+    // adopted. Two layers guarantee this: the ref guard keeps the 2nd call from
+    // re-arming at the source, and the render-phase reconciler disarms anything
+    // stale once thread.chatId ("A") diverges from the still-null activeChatId.
+    rerender({
+      activeChatId: null,
+      chats: { items: [{ id: "x" }, { id: "late" }] },
+    });
+    expect(setActiveChatId).not.toHaveBeenCalledWith("late");
+  });
+
   it("in-place adopt keeps threadKey stable; an external switch remounts", () => {
     const chats = { items: [{ id: "B" }] };
     const { result, rerender } = setup({ activeChatId: null, chats });
diff --git a/apps/client/src/features/ai-chat/hooks/use-chat-session.ts b/apps/client/src/features/ai-chat/hooks/use-chat-session.ts
index 46283ee6..998f2631 100644
--- a/apps/client/src/features/ai-chat/hooks/use-chat-session.ts
+++ b/apps/client/src/features/ai-chat/hooks/use-chat-session.ts
@@ -10,9 +10,23 @@ import {
   threadSessionReducer,
 } from "@/features/ai-chat/utils/thread-identity.ts";
 
+/** Inputs to {@link useChatSession}. `activeChatId`/`setActiveChatId` are the
+ *  public selection atom (also written from outside the window, e.g. page
+ *  history); the rest is read-only context the hook needs. */
+export interface UseChatSessionOptions {
+  activeChatId: string | null;
+  setActiveChatId: (id: string | null) => void;
+  chats: { items?: { id: string }[] } | undefined;
+  messagesLoading: boolean;
+  /** Wraps queryClient.invalidateQueries(AI_CHATS_RQ_KEY). */
+  onInvalidateChatList: () => void;
+  /** Wraps the per-chat messages invalidation. */
+  onInvalidateChatMessages: (chatId: string) => void;
+}
+
 /** What the window needs from a chat session: the ChatThread mount key, the
  *  history-loader gate, and the turn-finished callback. */
-export interface ChatSession {
+export interface UseChatSessionResult {
   /** ChatThread mount key (was `thread.key`). */
   threadKey: string;
   /** Show the history loader instead of the live thread. */
@@ -26,6 +40,14 @@ export interface ChatSession {
   cancelPendingAdoption: () => void;
 }
 
+/** Project a chat list to its id array (the before/after snapshot for the
+ *  error-path fallback). */
+function chatIdSnapshot(
+  chats: { items?: { id: string }[] } | undefined,
+): string[] {
+  return chats?.items?.map((c) => c.id) ?? [];
+}
+
 /**
  * Owns the AI-chat thread-identity lifecycle: the single atomic thread identity,
  * both new-chat id adoption paths (primary streamed-metadata + bounded error-path
@@ -35,16 +57,9 @@ export interface ChatSession {
  * This is the twice-bugged area for the #137 two-tab adoption race; the canonical
  * explanation of the adoption design lives in adopt-chat-id.ts.
  */
-export function useChatSession(params: {
-  activeChatId: string | null;
-  setActiveChatId: (id: string | null) => void;
-  chats: { items?: { id: string }[] } | undefined;
-  messagesLoading: boolean;
-  /** Wraps queryClient.invalidateQueries(AI_CHATS_RQ_KEY). */
-  onInvalidateChatList: () => void;
-  /** Wraps the per-chat messages invalidation. */
-  onInvalidateChatMessages: (chatId: string) => void;
-}): ChatSession {
+export function useChatSession(
+  params: UseChatSessionOptions,
+): UseChatSessionResult {
   const {
     activeChatId,
     setActiveChatId,
@@ -54,6 +69,15 @@ export function useChatSession(params: {
     onInvalidateChatMessages,
   } = params;
 
+  // Live mirror of `activeChatId`, read by onTurnFinished. ai@6 fires both
+  // onFinish AND onError on a failed turn, so onTurnFinished can run twice in one
+  // turn (once with the streamed id, once without) BEFORE a re-render. Reading
+  // the ref — which the primary-adoption branch updates imperatively — makes that
+  // second call see the just-adopted id, so it cannot re-arm the fallback. (A
+  // plain closure over `activeChatId` would still read null on the second call.)
+  const activeChatIdRef = useRef(activeChatId);
+  activeChatIdRef.current = activeChatId;
+
   // 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
@@ -88,7 +112,11 @@ export function useChatSession(params: {
   // list, which races a second tab — #137; see adopt-chat-id.ts).
   const onTurnFinished = useCallback(
     (serverChatId?: string) => {
-      const adopted = resolveAdoptedChatId(activeChatId, serverChatId);
+      // 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.
+      const current = activeChatIdRef.current;
+      const adopted = resolveAdoptedChatId(current, serverChatId);
       if (adopted) {
         // PRIMARY path. In-place adoption: set the public selection and the
         // thread identity to the real id together. `adopt` keeps the SAME mount
@@ -96,35 +124,30 @@ export function useChatSession(params: {
         // and keeps the SAME mounted thread (its useChat already holds the
         // just-finished turn) instead of remounting + re-seeding from
         // not-yet-persisted history.
+        activeChatIdRef.current = adopted; // a same-turn 2nd call now sees the id
         setActiveChatId(adopted);
         dispatch({ type: "adopt", chatId: adopted });
         // Primary adoption won — disarm any previously-armed fallback.
         pendingNewChatRef.current = null;
-      } else if (activeChatId === null) {
+      } else if (current === null) {
         // FALLBACK path: a brand-new chat finished with NO server id (the first
         // turn errored before the `start` chunk). Arm the bounded list-refetch
         // fallback by snapshotting the currently-known chat ids. `chats` is still
         // the pre-refetch list here, so the just-created row is NOT yet in it; the
         // effect below adopts the single id that newly appears after the refetch.
-        pendingNewChatRef.current = chats?.items?.map((c) => c.id) ?? [];
+        pendingNewChatRef.current = chatIdSnapshot(chats);
       }
       onInvalidateChatList();
       // Re-sync the persisted message rows for the active chat so the Markdown
       // export and token counters reflect the just-finished turn. The live thread
       // renders from its own useChat store (stable thread.key), so this never
-      // re-seeds or tears down the open thread. For a brand-new chat activeChatId
-      // is still null here; later turns hit this with the adopted id.
-      if (activeChatId) {
-        onInvalidateChatMessages(activeChatId);
+      // re-seeds or tears down the open thread. For a brand-new chat `current` is
+      // still null here; later turns hit this with the adopted id.
+      if (current) {
+        onInvalidateChatMessages(current);
       }
     },
-    [
-      activeChatId,
-      chats,
-      setActiveChatId,
-      onInvalidateChatList,
-      onInvalidateChatMessages,
-    ],
+    [chats, setActiveChatId, onInvalidateChatList, onInvalidateChatMessages],
   );
 
   // FALLBACK resolver. Armed only by onTurnFinished when a brand-new chat's first
@@ -136,7 +159,7 @@ export function useChatSession(params: {
   useEffect(() => {
     const before = pendingNewChatRef.current;
     if (before === null || activeChatId !== null) return; // not armed / already adopted
-    const after = chats?.items?.map((c) => c.id) ?? [];
+    const after = chatIdSnapshot(chats);
     const added = newlyAddedChatIds(before, after);
     // Keep waiting until a genuinely-new id appears. Set-based, so it is robust
     // to an add+delete in the same window (a length compare would miss it), and