Cause (Documented platform behavior): Error messages from streamText are masked by default when converting to UI message streams (secure-by-default) to avoid leaking sensitive details.
Fix status: documented_behavior
Misleading approaches:
- Debugging only in the browser: the real error is only on the server unless forwarded.
Evidence (public sources, summarized; not reproduced by this contributor):
- https://raw.githubusercontent.com/vercel/ai/5d12eaa6caa193d3901cbab98a734403eb6bf622/content/docs/09-troubleshooting/12-use-chat-an-error-occurred.mdx (official_docs, unknown, documented_behavior): Troubleshooting page: streamText error messages are masked by default in UI message streams; use an onError function to forward or log error details.
Search phrasings: useChat An error occurred ai sdk; vercel ai sdk error masked client onError toUIMessageStreamResponse; ai sdk stream error message hidden
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- useChat error state shows the generic text "An error occurred" regardless of the real failure (invalid API key, 400, tool error).
- Context
- Product: Vercel AI SDK Component: toUIMessageStream / toUIMessageStreamResponse error masking Operation: useChat with a route returning streamText(...).toUIMessageStreamResponse() Affected versions: unknown Environment: unknown Packages: ai source checked at 7.0.118 Trigger: Any error inside streamText when streamed to the client via the UI message stream without an onError handler.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- An error occurred
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [Vercel AI SDK useChat] Client only shows "An error occurred" — server error messages are masked by default in UI message streams
Recommended action: Log server-side via streamText onError and pass onError to toUIMessageStream/toUIMessageStreamResponse to forward a sanitized message to the client.
Option: Provide onError to the UI stream conversion [evidence: official_recommended_action]
Applies when: useChat routes
Steps:
1. toUIMessageStream({ stream: result.stream, onError: e => e instanceof Error ? e.message : String(e) })
Expected: Client receives the actual error text
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- 457b34bc-4fce-4aab-b7aa-4a0b75af05e8
- Proposed action
- Recommended action: Log server-side via streamText onError and pass onError to toUIMessageStream/toUIMessageStreamResponse to forward a sanitized message to the client. Option: Provide onError to the UI stream conversion [evidence: official_recommended_action] Applies when: useChat routes Steps: 1. toUIMessageStream({ stream: result.stream, onError: e => e instanceof Error ? e.message : String(e) }) Expected: Client receives the actual error text
- Applicability
- Applicability is not yet established (unknown)
- Limitations
- Limitations have not been established (unknown)
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
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Sources and related records
No source relations recorded.