fix(ai, ai-client, ai-persistence): preserve subagent cancellation

[duohelingdukele]

Stopped subagents currently lose typed cancellation details. This change emits code: 'cancelled' from server and client stop paths, persists the detail with the child card, and restores it after reload.

🎯 Changes

• Emit typed cancellation codes for server-side aborts and client-side stop actions.
• Classify persisted child runs by code, with the old Stopped message as a compatibility fallback.
• Restore cancellation details for child cards with no transcript messages.
• Add package and persistence regression coverage.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested code changes locally with pnpm run test:pr, or these tests do not apply to this pull request.
• I fully understand the code in this pull request, including any code generated with AI assistance.
• Docs: I updated docs/ for this change, or this change is not user-facing.
• Changeset: I added a changeset (pnpm changeset), or this PR does not change a published package.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

Root cause

Issue. Stopped subagents reach the client without a stable cancellation code. A cancelled child with no transcript messages also loses its error when the chat reloads.

Cause. stoppedEvent() and spawnAgentStream() emit only the Stopped message. Persistence classifies cancellation from that text and leaves aborted run records without an error. childCard() then reads only the run record, although child metadata already holds the error.

Fix. Server and client stop paths now emit code: 'cancelled'. Persistence uses the code to classify the run and stores it in child metadata. Reconstruction uses that metadata when the run record has no error.

Possible alternatives

• Store errors on aborted run records. This adds another persisted copy and changes run-record behavior. The child metadata already stores the cancellation detail, so reconstruction reads it there.

Testing

Commands run

• On pinned origin/main, the agent-authored cancellation regressions failed: abort events and client stops had no code, and persistence recorded a cancelled child as failed instead of aborted.
• On this branch, the targeted suites passed: @tanstack/ai 29/29, @tanstack/ai-client 6/6, and @tanstack/ai-persistence 5/5.
• Required workspace builds, formatter check, and git diff --check passed.

Manual test

1. Start a chat with a routed child agent that waits for cancellation.
2. Stop the child before it emits transcript messages.
3. Confirm that the child has status: 'error' and error.code: 'cancelled'.
4. Reload the persisted chat. Confirm that the child card still has the cancellation code.

How this PR makes testing easy

The branch includes regressions for server aborts, client stops, run classification, and child-card reconstruction after reload.

Risk / rollback

Risk is low. Consumers can ignore the optional code and keep their current behavior. Revert this PR to remove the new cancellation code and reconstruction behavior.

Public API change

Before

const wasCancelled = child.error?.message === 'Stopped'

After

const wasCancelled = child.error?.code === 'cancelled'

Summary by CodeRabbit

• Bug Fixes
  • Stopped subagents now retain a typed cancelled error, including when they produce no messages before stopping.
  • Cancellation status and error details are preserved when subagent runs are reloaded.
  • Reloaded subagent cards retain stored error codes, including provider errors.
• Tests
  • Added coverage for stopping subagents and verifying cancellation details in the interface and after reload.

[changeset-bot]

🦋 Changeset detected

Latest commit: 6bbdbf9

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages

Name	Type
@tanstack/ai	Patch
@tanstack/ai-client	Patch
@tanstack/ai-persistence	Patch
@tanstack/ai-octane	Patch
ag-ui	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

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ℹ️ Recent review info

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• Review profile: CHILL
• Plan: Advanced
• Run ID: 299c6080-b9e9-41fb-9de0-d32804f58a76

📥 Commits

Reviewing files that changed from the base of the PR and between 1e57a37 and 6bbdbf9.

📒 Files selected for processing (4)

• packages/ai-persistence/src/reconstruct.ts
• packages/ai-persistence/tests/subagent-persistence-cards.test.ts
• testing/e2e/src/routes/api.persistence-durability.ts
• testing/e2e/tests/persistence-durability.spec.ts

🚧 Files skipped from review as they are similar to previous changes (2)

• packages/ai-persistence/tests/subagent-persistence-cards.test.ts
• packages/ai-persistence/src/reconstruct.ts

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📝 Walkthrough

Walkthrough

Subagent cancellation errors now include the cancelled code. Persistence stores the error and restores it on child-card reconstruction, including when the child produced no transcript messages. Tests and documentation cover cancellation and reload behavior.

Changes

Subagent Cancellation

Layer / File(s)	Summary
Emit typed cancellation errors packages/ai/src/activities/chat/agents/spawn.ts, packages/ai-client/src/chat-client.ts, packages/ai/tests/define-agent.test.ts, packages/ai-client/tests/chat-client-subagents.test.ts	Stopped and aborted subagents emit errors with code cancelled. Client stop handling sets the same code on the event and handle error. Tests check these error details.
Persist and reconstruct child errors packages/ai-persistence/src/subagent-runs.ts, packages/ai-persistence/src/reconstruct.ts, packages/ai-persistence/tests/subagent-persistence-cards.test.ts, testing/e2e/src/routes/api.persistence-durability.ts, testing/e2e/tests/persistence-durability.spec.ts, docs/persistence/chat-persistence.md	Persistence treats an explicit cancelled code as authoritative, stores the child error, and marks the run as aborted. Reconstruction uses stored error details when available. Tests cover cancellation before child output and restoration of failed-child error codes.
Exercise and document cancellation behavior testing/e2e/src/lib/subagents-test.ts, testing/e2e/src/routes/api.subagents-test.ts, testing/e2e/src/routes/subagents-test.tsx, testing/e2e/tests/subagents.spec.ts, docs/chat/subagents.md, .changeset/quiet-wolves-stop.md	The end-to-end test adds a stoppable child scenario and checks for error code cancelled. The chat documentation and changeset describe cancellation behavior.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 6bbdb

No identified cancellation or reload issue blocks merging after normal checks.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 6bbdb

The inspected changes preserve existing cancellation controls and improve recovery of error details. No introduced security issue was established. Some uncertainty remains around custom persistence implementations and deployment exposure of the test endpoint.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• observed — The added attacker-selectable fixture scenario seeds fixed synthetic errors into its separate in-memory store and reconstructs those records. The inspected branch does not add provider calls, credentials, or a production data-store connection; its deployment exposure remains unestablished.

Trust Boundaries and Controls

• observed — The new reconstruction scenario follows the fixture’s existing nonempty-thread-ID authorization pattern. That predicate is not tenant ownership enforcement, but the changed branch uses synthetic fixture state rather than a demonstrated production tenancy boundary.

Resilience and Maintainability Implications

• observed — Client stop still requires a running child handle before aborting, and the server stream retains iterator cleanup in finally. The inspected PR changes annotate cancellation rather than expanding cancellation authority or removing cleanup.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 22.22% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 13 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: preserving subagent cancellation details across the AI packages.
Description check	✅ Passed	The description covers the changes, root cause, fix, alternatives, testing, risks, and release impact. It follows the repository template and provides enough detail to assess the change.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @packages/ai-persistence/src/reconstruct.ts:
- Line 329: Update error selection in the reconstruction flow around child.error
and info?.error to preserve the stored child metadata code when the metadata and
run errors have matching messages, while keeping the run error authoritative
when their messages differ.

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ℹ️ Review info

⚙️ Run configuration

• Configuration used: Repository: TanStack/ai/.coderabbit.yaml
• Review profile: CHILL
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• Run ID: 0555d0bc-1121-4b2d-a785-fc66b543e789

📥 Commits

Reviewing files that changed from the base of the PR and between 7fb4a5f and 5b78162.

📒 Files selected for processing (14)

• .changeset/quiet-wolves-stop.md
• docs/chat/subagents.md
• docs/persistence/chat-persistence.md
• packages/ai-client/src/chat-client.ts
• packages/ai-client/tests/chat-client-subagents.test.ts
• packages/ai-persistence/src/reconstruct.ts
• packages/ai-persistence/src/subagent-runs.ts
• packages/ai-persistence/tests/subagent-persistence-cards.test.ts
• packages/ai/src/activities/chat/agents/spawn.ts
• packages/ai/tests/define-agent.test.ts
• testing/e2e/src/lib/subagents-test.ts
• testing/e2e/src/routes/api.subagents-test.ts
• testing/e2e/src/routes/subagents-test.tsx
• testing/e2e/tests/subagents.spec.ts

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[github-actions]

Thanks for the PR, @duohelingdukele! 🙌 @AlemTuzlak will take a look.

Automated pre-review checks

• ✅ CI passing
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• ✅ Changeset present
• ✅ E2E test changes included

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