StreamProcessor drops TEXT_MESSAGE_CHUNK events without a warning

[tomer953]

TanStack AI version

v0.61.0

Framework/Library version

React v19.2.0

Describe the bug and the steps to reproduce it

StreamProcessor has no handler for TEXT_MESSAGE_CHUNK. The event reaches
the default branch of processChunk and the processor drops it. There is no
warning.

AG-UI defines TEXT_MESSAGE_CHUNK as a short form of the
TEXT_MESSAGE_START / TEXT_MESSAGE_CONTENT / TEXT_MESSAGE_END triad. A
server can send one chunk event instead of three events. The AG-UI spec gives
the expansion job to the client stream transformer.

Two parts of this repository say that the event is supported:

• packages/ai/src/types.ts declares TextMessageChunkEvent and puts it in
  the AGUIEvent union. That union is the exported StreamChunk type.
• packages/ai/src/utilities/spec-event-keys.ts gives the event an allowlist
  entry, so stripToSpec keeps messageId, role, delta, and metadata
  at the top level.

The runtime does not match this. TOOL_CALL_CHUNK and
REASONING_MESSAGE_CHUNK have the same problem.

Steps to reproduce

live:
https://codesandbox.io/p/sandbox/hvf3mx

Open the sandbox. It needs no API key and makes no network calls. The AG-UI
events are constants in src/App.js.

1. Build a StreamProcessor.
2. Send RUN_STARTED, then one TEXT_MESSAGE_CHUNK with a messageId, a
   delta, and a metadata.error, then RUN_ERROR.
3. Read processor.getMessages().
4. Do the same again with the explicit triad in place of the chunk event.

Result

The chunk event loses three things that the triad keeps:

The server sent this, and the client	TEXT_MESSAGE_CHUNK	START / CONTENT / END
shows the text "the run failed"	lost	yes
keeps the message id "msg-1"	lost	yes
keeps metadata.error	lost	yes

The assistant message is not absent. It is empty, and it has a new id:

[{ "id": "msg-1790507900666-2mxf4p", "role": "assistant", "parts": [] }]

RUN_ERROR created that message. The id msg-1 from the server is gone, so a
later event that uses msg-1 cannot find the message.

Expected result
Both forms describe the same turn. Both must produce the same message.

Notes
MESSAGES_SNAPSHOT is a plain messages[], so a reload of a saved thread
shows the correct text. The fault is visible only during a live run. A unit
test that sends the triad cannot find this fault.

processChunk already has the three handlers that an expansion needs. A
case per chunk event can call them, hold the open message id, and close the
message when the id changes or when finalizeStream runs.

Related: #1465 tried this and closed without a merge. #1460 added the AG-UI 1.0
types and left the runtime out of scope.

Your Minimal, Reproducible Example - (Sandbox Highly Recommended)

https://codesandbox.io/p/sandbox/hvf3mx

Screenshots or Videos (Optional)

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

None

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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.