Why don't two useChat hooks with same ID share state?

two useChat usage, sharing the same id, don't use the same state/store?

Now I have enough understanding. Let me summarize the findings:

##Answer: No, two useChat calls sharing the same id do NOT share state

Looking at the source code, here's why:

###How useChat works:

1. Each useChat creates its own Chat instance via useRef:

   // packages/react/src/use-chat.ts, line 98-100
   const chatRef = useRef<Chat<UI_MESSAGE>>(
     'chat' in options ? options.chat : new Chat(optionsWithCallbacks),
   );
   

2. The id is just an identifier stored on each instance - it's not used for looking up or sharing state:

   // packages/ai/src/ui/chat.ts, line 195
   this.id = id;
   

3. State is local to each Chat instance - each Chat has its own ReactChatState that holds messages, status, and error:

   // packages/react/src/chat.react.ts
   export class Chat<UI_MESSAGE extends UIMessage> extends AbstractChat<UI_MESSAGE> {
     #state: ReactChatState<UI_MESSAGE>;  // <-- private state per instance
   

4. The id is only used for recreation logic: If the id option changes, the useChat hook will create a new Chat instance (line 103-104):

   const shouldRecreateChat =
     ('chat' in options && options.chat !== chatRef.current) ||
     ('id' in options && chatRef.current.id !== options.id);
   

###How to share state between components:

If you want two components to share the same chat state, you have two options:

1. Pass the same Chat instance via the chat option:

   const chat = useMemo(() => new Chat({ id: 'my-chat' }), []);
   
   // Component A
   const chatA = useChat({ chat });
   
   // Component B  
   const chatB = useChat({ chat });
   

2. Lift state up and pass messages/handlers as props to child components.

The id is primarily useful for:

• Identifying which chat a stream belongs to (e.g., for resuming streams)
• Persistence/storage identification
• Not for automatic state sharing between hook instances