fix(ai-client): capture abort signal before await to prevent race condition

[FranDias]

Summary

Fixes a race condition in ChatClient.streamResponse() where this.abortController.signal could reference a stale or null controller by the time it is passed to this.connection.connect().

• Root cause: Between this.abortController = new AbortController() (line 432) and the this.connection.connect(..., this.abortController.signal) call (line 459), there is an await this.callbacksRef.current.onResponse(). During that await, a concurrent stop() call nulls out this.abortController, or a rapid second sendMessage() could replace it with a new controller.
• Fix: Capture const signal = this.abortController.signal immediately after creation, then pass the local signal variable to connect(), ensuring it always receives the signal from the correct AbortController regardless of concurrent mutations.

How to reproduce

1. Create a ChatClient and call sendMessage("hello")
2. Before the stream connects (e.g., during the onResponse callback), call stop() or fire another sendMessage()
3. this.abortController is now null or points to a different controller
4. connect() receives a wrong/null signal, leading to either a runtime error or the inability to abort the correct stream

Changes

• packages/typescript/ai-client/src/chat-client.ts: Capture signal locally right after AbortController creation; pass it to connect() instead of re-reading this.abortController.signal

Test plan

• All 165 existing tests pass (8 test files in @tanstack/ai-client)
• Abort-specific tests in chat-client-abort.test.ts all pass (6 tests)
• Verified the diff is minimal and correct

Summary by CodeRabbit

• Bug Fixes

  • Ensured the correct cancellation signal is captured and used during streaming so stop/send races no longer produce stale or null signals, preventing interrupted or unrecoverable streams.

• Tests

  • Added tests validating cancellation behavior when stop is called during a response and across sequential messages, asserting distinct, valid cancellation signals are passed.

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Reviewing files that changed from the base of the PR and between d70c39f and e6586fb.

📒 Files selected for processing (1)

• packages/typescript/ai-client/src/chat-client.ts

🚧 Files skipped from review as they are similar to previous changes (1)

• packages/typescript/ai-client/src/chat-client.ts

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📝 Walkthrough

Walkthrough

Capture the newly created AbortController's signal into a local variable and use that captured signal when invoking the connection adapter and processing streams to avoid races if stop() or sendMessage() reassigns the controller. Adds tests validating captured-signal behavior during onResponse.

Changes

Cohort / File(s)	Summary
Signal capture change packages/typescript/ai-client/src/chat-client.ts	Capture the newly created AbortController's signal in a local signal constant and pass that captured signal into connection.send(...) and downstream stream handling instead of referencing this.abortController.signal.
Tests for abort behavior packages/typescript/ai-client/tests/chat-client-abort.test.ts	Add two tests: one ensures the captured AbortSignal is passed to the connection when stop() is invoked during onResponse; the other ensures two distinct AbortSignal instances are passed across consecutive connection calls when a second message is queued during onResponse.
Manifest package.json	Minor single-line manifest edit reported.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 I caught a signal snug and tight,
Held it steady through the night.
When stop hops in or messages stack,
Each little signal keeps its track.
Hooray — no races, all intact!

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description provides thorough context including root cause, fix details, reproduction steps, and test plan, but does not follow the repository's required template structure with 🎯 Changes, ✅ Checklist, and 🚀 Release Impact sections.	Restructure the description to follow the required template format with the specified section headings and checklist items, while preserving the detailed technical content.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: capturing the abort signal before an await to prevent a race condition, which is the core fix in this PR.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[FranDias]

Unit tests for the signal capture fix

Added two tests to chat-client-abort.test.ts that exercise the race condition described in this PR:

1. stop() called during onResponse callback

Calls client.stop() inside the onResponse callback, which runs between AbortController creation and connect(). This sets this.abortController = null.

• Without the fix: Fails — this.abortController.signal dereferences null, connect() never runs, and the signal is undefined.
• With the fix: Passes — the locally captured signal is a valid AbortSignal regardless of what happens to this.abortController.

2. sendMessage() called during onResponse callback

Calls client.append() inside onResponse, which queues a second stream that would create a new AbortController.

• Without the fix: Passes — the queued second stream runs after the first completes (via drainPostStreamActions), so the controllers don't actually interleave in this code path.
• With the fix: Passes — both calls receive distinct, valid AbortSignal instances.

Test 1 is the key regression test — it directly reproduces the crash from the original bug report.

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@packages/typescript/ai-client/tests/chat-client-abort.test.ts`:
- Around line 372-398: The test currently fires a nested client.append(...)
inside the onResponse callback without awaiting it and then uses a fixed 50ms
setTimeout to wait, which is non-deterministic and can hide rejections; change
the onResponse callback to capture the returned promise (e.g., assign the result
of client.append(...) to a variable like secondAppendPromise) and then await
that promise after the initial await client.append({...}) instead of using
setTimeout, ensuring any rejection from the second append is surfaced to the
test; reference the onResponse callback and client.append calls to locate and
update the logic accordingly.

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• packages/typescript/ai-client/tests/chat-client-abort.test.ts

[FranDias]

Just a note that this is actively causing a regression for us after upgrading to vite 8 + @tanstack/ai-client@0.7.2. The timing change in vite 8's Rolldown bundler makes this race condition trigger consistently on the first message sent in a new chat thread — the stream is silently aborted.

We're carrying a local patch (patchedDependencies in bun) with the same fix to unblock, but would love to get this merged upstream.