StreamProcessor discards metadata from REASONING_MESSAGE_START
维护者通常 1 天内回复
@tombeckenham 已经在做这个了。
开始于 2026年10月9日。
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描述
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
- Download the files from the reproduction gist.
- Run
npm install --ignore-scripts --no-audit --no-fund. - 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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