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StreamProcessor discards metadata from REASONING_MESSAGE_START

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I maintainer di solito rispondono entro 1 giorno

@tombeckenham ci sta già lavorando.

Dal 9/10/2026.

Valutazione

Questa issue non è ancora stata valutata.

Descrizione

waiting-on: maintainer
TanStack AI version

@tanstack/ai 0.66.0, installed from npm in an isolated directory.
The same behavior also occurs on main commit 96191eafee12040c78f849fe7d47220f64591684.

Framework/Library version

Node.js 26.6.0, with a bare StreamProcessor.
The reproduction does not need React, a provider, or an API key.

Describe the bug and the steps to reproduce it

Custom metadata on REASONING_MESSAGE_START does not survive conversion into a UIMessage.
The thinking text appears, but the resulting message has no metadata.

The reproduction uses one root run.
It compares the same metadata on two events:

  • On REASONING_MESSAGE_START: the metadata is lost.
  • On REASONING_MESSAGE_CONTENT: the metadata is preserved.

Expected

The metadata remains available after processing the reasoning stream, consistently with the content-event case.

Actual

UIMessage.metadata is undefined when only the start event carries the metadata.

Steps

  1. Download the files from the reproduction gist.
  2. Run npm install --ignore-scripts --no-audit --no-fund.
  3. Run npm run metadata.

The script first checks the content-event control.
That check passes.
The assertion for the start event then fails:

Metadata on START: undefined
Metadata on CONTENT: { source: 'reasoning', traceId: 'trace-1' }
AssertionError [ERR_ASSERTION]: Metadata on reasoning start is lost

Source

The dispatcher ignores REASONING_MESSAGE_START.
The content handler only merges metadata from the content event.

Please clarify the intended destination for reasoning-start metadata: the owning UIMessage or its ThinkingPart.
At present, neither exposes it.

Related issue

#1533 concerns reasoning ownership across different messages.
This report isolates metadata loss in one run.
Our local workaround copies the metadata onto reasoning content events.

Your Minimal, Reproducible Example - (Sandbox Highly Recommended)

Complete runnable reproduction: metadata.mjs, package.json, and instructions

The package version is pinned to 0.66.0.
The second script in the gist provides an additional reproduction for #1533.

Screenshots or Videos (Optional)

No screenshot is needed for this framework-independent reproduction.

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

Maybe, I'll investigate and start debugging.

No PR accompanies this report.

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