Hydration failure with persistence: true is silent, apps can't show a load error

TanStack AI version

@tanstack/ai-react v0.23.0 (@tanstack/ai-client v0.30.0; verified unchanged in v0.31.0 and main)

Framework/Library version

React v19

Describe the bug and the steps to reproduce it

What happens

I'm using persistence: true, so the client loads an existing thread from my server when the chat mounts. When that load fails (my server returned a 500), the failure is invisible: onError doesn't fire, chat.error stays undefined, and the client reports status: 'ready' with zero messages, exactly what it reports for a thread that's genuinely empty. So my app can't tell "this thread failed to load" from "this thread has no messages", and the user is stuck on an empty/loading screen. There's no retry either; the load happens once on mount.

Steps to reproduce

No provider/API key needed, the failure happens before any send:

const chat = useChat({
	connection: fetchServerSentEvents('/nonexistent-endpoint'), // hydrate GET fails
	persistence: true,
	threadId: 'thread-1',
	onError: (e) => console.log('onError', e), // never logs
})
// Render chat.status + chat.error: status stays 'ready', error stays undefined,
// while the Network tab shows the failed GET.

What I expected

An error signal. GenerationClient.failHydration handles this exact case (status: 'error' + onError) with the comment "so a broken server is distinguishable from an empty one and the app can retry."

Cause

hydrateFromServer in chat-client.ts:

try {
	result = await hydrate(this.threadId)
} catch {
	return
}

No callback, state change, event, or retry is reachable from that catch. Same in 0.30.0, 0.31.0, and main.

Related breakage

For apps where users supply their own provider key (BYOK), a 401 during hydrate throws ByokMissingError into this same catch, so the key-request flow never triggers on thread load. The send path special-cases that error; the hydrate path eats it.

Fix

Minimal: catch (error) { this.callbacksRef.current.onError(error) }.
Fuller: port the failHydration pattern so state reflects it too.
(#1120 asks for hydration state generally, this may want to fold into that.)

Happy to PR either. Which shape do you prefer?

Your Minimal, Reproducible Example - (Sandbox Highly Recommended)

https://codesandbox.io/p/sandbox/pvgmmh?file=%2Findex.js

Screenshots or Videos (Optional)

No response

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

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• 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.