fix(ai-react): avoid random IDs during SSR render

packages/ai-client/src/devtools.ts

@@ -742,23 +742,11 @@ export class ClientDevtoolsBridge<TSnapshot extends object> {
   }
 }
 
-let bridgeIdSequence = 0
-
 function createBridgeId(hookId: string): string {
-  const cryptoLike = (
-    globalThis as {
-      crypto?: {
-        randomUUID?: () => string
-      }
-    }
-  ).crypto
-
-  if (cryptoLike?.randomUUID) {
-    return `bridge:${hookId}:${cryptoLike.randomUUID()}`
-  }
-
-  bridgeIdSequence += 1
-  return `bridge:${hookId}:${bridgeIdSequence}`
+  // hookId comes from React's useId and is stable across SSR and hydration.
+  // Do not generate randomness during render: crypto.randomUUID is unavailable
+  // or restricted in some server runtimes, and discarded renders must be pure.
+  return `bridge:${hookId}`
 }
 
 // Owns the chat-client devtools surface so the chat client itself stays a

packages/ai-react/src/use-chat.ts

@@ -41,10 +41,10 @@ export function useChat<
 ): UseChatReturn<TTools, TSchema> {
   // The hook's identity is its `threadId` — also the persistence key, so a
   // reload with the same `threadId` restores the same conversation. `hookId` is
-  // only a stable fallback for React's client-recreation keying when no
-  // `threadId` is given (an ephemeral chat), never a persistence key.
+  // a stable fallback when an empty or missing `threadId` creates an ephemeral
+  // chat, never a persistence key.
   const hookId = useId()
-  const clientId = options.threadId ?? hookId
+  const clientId = options.threadId || hookId
 
   const [messages, setMessages] = useState<Array<UIMessage<TTools>>>(
     options.initialMessages || [],
@@ -128,9 +128,7 @@ export function useChat<
       ...transport,
       initialMessages: messagesToUse,
       ...(initialOptions.body !== undefined && { body: initialOptions.body }),
-      ...(initialOptions.threadId !== undefined && {
-        threadId: initialOptions.threadId,
-      }),
+      threadId: clientId,
       ...(initialOptions.forwardedProps !== undefined && {
         forwardedProps: initialOptions.forwardedProps,
       }),

packages/ai-react/tests/use-chat.test.ts

@@ -290,6 +290,23 @@ describe('useChat', () => {
       expect(typeof messageId).toBe('string')
     })
 
+    it.each(['', undefined])(
+      'uses the hook ID instead of generating a random ID for threadId=%s',
+      (threadId) => {
+      const chatClientPrototype = ChatClient.prototype as unknown as {
+        generateUniqueId: () => string
+      }
+      const generateUniqueId = vi.spyOn(chatClientPrototype, 'generateUniqueId')
+
+      renderUseChat({
+        connection: createMockConnectionAdapter(),
+        ...(threadId === undefined ? {} : { threadId }),
+      })
+
+        expect(generateUniqueId).not.toHaveBeenCalled()
+      },
+    })
+
     it('should generate id if not provided', async () => {
       const chunks = createTextChunks('Response')
       const adapter = createMockConnectionAdapter({ chunks })