I face this error too. This happens frequently for Cloud-hosted servers with auto-scaling.

I've traced the root cause to this:

• When server restarts, frontend requests a new websocket session with X-Chainlit-Thread-Id header (code below). But this header is empty because idToResume is never set for new threads.
  

  chainlit/libs/react-client/src/useChatSession.ts

  Line 90 in d9fa5ba

  'X-Chainlit-Thread-Id': idToResume || '',

  
  idToResume is only set when a thread is resumed:
  

  chainlit/frontend/src/pages/ResumeButton.tsx

  Line 29 in d9fa5ba

  setIdToResume(threadId!);

• Due to this, backend creates a new thread and ignore the existing messages completely (because the old thread is not loaded).

Possible solutions:

• In order to fix this I think we need to set idToResume for new threads after first user interaction.
  Building on this PR Update thread list on first interaction #923, idToResume should be set here along with currentThreadId:
  

  chainlit/libs/react-client/src/useChatSession.ts

  Line 156 in d9fa5ba

  setCurrentThreadId(event.thread_id);

• Alternatively, the code in useChatSession.ts can be updated to:
  'X-Chainlit-Thread-Id': idToResume || currentThreadId || '',

@tpatel, what do you think? I can open a new PR for this fix.

@qtangs, my analysis of the problem found the same root cause. However, I don't think your suggested solutions will work.
The problem is that the X-Chainlit-Thread-Id value is given to the socket.io session when the session is created and cannot be updated later. So even if you use setIdToResume(threadId!); the session will still have the same values for the headers.
When the server restarts, socket.io will retry the connection using the existing headers.

A possible solution would be to listen on the reconnect or reconnect_attempt event of socket.io and create a new session, if idToResume is not set.

@qvalentin you're right. Using currentThreadId like that won't work as the initial value is undefined and the connection headers are not updated after the initial socket initialization.

I've tested this addition of the update after useChatSession.ts#L68 and verified that it works:

  const idToResume = useRecoilValue(threadIdToResumeState);
  const setCurrentThreadId = useSetRecoilState(currentThreadIdState);

  // Use currentThreadId as thread id in websocket header if it's set
  const currentThreadId = useRecoilValue(currentThreadIdState);

  useEffect(() => {
    if (session?.socket && currentThreadId) {
        session.socket.io.opts.extraHeaders!['X-Chainlit-Thread-Id'] = currentThreadId;
    }
  }, [currentThreadId]);

Great, will you open a PR?

Yeah, I'm planning to do that when time permits. Will need to add some test cases too

Please submit a PR!

Please submit a PR!

PR is created, pls review.