useChat assistant message.id changes mid-stream for Anthropic tool-first responses

[shaked-frame]

TanStack AI version

latest - 0.10.3

Framework/Library version

19.2.4

Describe the bug and the steps to reproduce it

When using useChat with fetchServerSentEvents and the Anthropic adapter, assistant message IDs can change during a single streamed response when tool calls arrive before text.
This makes UIMessage.id unstable for React list keys and can remount the message subtree mid-stream.
Setup
Client:

const { messages } = useChat({
  connection: fetchServerSentEvents('/api/reports/ask'),
})
return messages.map((msg) => (
  <ReportMessage key={msg.id} message={msg} />
))

Server:
const stream = chat({
  adapter: anthropicText('claude-sonnet-4-6'),
  messages,
  tools: [myServerTool],
})
return toServerSentEventsResponse(stream)

Your Minimal, Reproducible Example - (Sandbox Highly Recommended)

https://stackblitz.com/~/github.com/shaked-frame/ts-ai-reproduction

Screenshots or Videos (Optional)

example logs during streaming -

assistant|id=msg-1776683523350-hhod09           |key=1776683523350     during stream
assistant|id=anthropic-1776683527840-o09nzv     |key=1776683523350   after server confirms

Do you intend to try to help solve this bug with your own PR?

Yes, I think I know how to fix it and will discuss it in the comments of this issue

Terms & Code of Conduct

• I agree to follow this project's Code of Conduct
• I understand that if my bug cannot be reliable reproduced in a debuggable environment, it will probably not be fixed and this issue may even be closed.