fix(react): prevent stale transport in useChat when React state changes

[DukeDeSouth]

Human View

Summary

Fixes #7819

The transport option (including body, headers) passed to useChat was captured once when the Chat instance was created and never updated on subsequent renders. This caused transport.body values referencing React state to always send the initial (stale) value, even when the state changed.

Problem

const [subagent, setSubagent] = useState('todos-agent');

useChat({
  transport: new DefaultChatTransport({
    body: { subagent }, // ← always sends initial value 'todos-agent'
  }),
});

Workarounds like passing body as a function also didn't help because the entire transport was stale.

Fix

Applied the same ref-based indirection pattern already used for callbacks (onToolCall, onFinish, onData, onError, sendAutomaticallyWhen) to the transport option:

1. A transportRef is created and updated on every render with the latest transport
2. The Chat instance receives stable wrapper functions that always delegate to transportRef.current
3. When sendMessages or reconnectToStream is called, it executes on the latest transport with fresh body/headers values

This is a minimal, non-breaking change — only packages/react/src/use-chat.ts is modified, following the existing pattern exactly.

Verification

• Existing test suite passes (28 pre-existing failures on Node 25, identical on main)
• No new test failures introduced
• Pattern is identical to the existing callbacks ref pattern (lines 64-86) that already prevents stale closures for onToolCall etc.

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AI View (DCCE Protocol v1.0)

Metadata

• Generator: Claude (Anthropic) via Cursor IDE
• Methodology: AI-assisted development with human oversight and review

AI Contribution Summary

• Solution design and implementation

Verification Steps Performed

1. Reproduced the reported issue
2. Analyzed source code to identify root cause
3. Implemented and tested the fix

Human Review Guidance

• Core changes are in: transport.body, transportRef.current, packages/react/src/use-chat.ts
• Review the breaking change implications

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