fix(chat): add diagnostic logging for Vercel timeout investigation

[fkesheh]

Summary

• Add diagnostic timeout that fires 10s before Vercel hard timeout to capture what phase the request is stuck in
• Add phase tracking throughout request lifecycle (streaming, onFinish steps)
• Add clearPreemptiveOnly() to keep diagnostic timeout alive during onFinish cleanup
• Add immediate log flush after preemptive timeout trigger to ensure capture before process termination

Extracted from #208 to keep the summarization PR focused.

Test plan

• Deploy to staging and trigger a long-running chat request
• Verify diagnostic logs appear in Axiom when request approaches Vercel timeout
• Verify phase tracking correctly reflects the current lifecycle stage

Summary by CodeRabbit

• Bug Fixes

  • Improved error recovery mechanisms in streaming operations.

• Performance & Stability

  • Enhanced timeout management for more reliable chat functionality.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
hackerai	Ready	Preview, Comment	Feb 4, 2026 6:33pm

[coderabbitai]

📝 Walkthrough

Walkthrough

These changes enhance the preemptive timeout system with a diagnostic timeout that logs 10 seconds before the Vercel hard timeout, and add granular phase tracking throughout the stream handling lifecycle. A new clearPreemptiveOnly() method allows selective cleanup of the preemptive portion while preserving diagnostic state during retries and late-stage operations.

Changes

Cohort / File(s)	Summary
Diagnostic Timeout Infrastructure lib/utils/stream-cancellation.ts	Introduces a diagnostic timeout that fires 10 seconds before hard timeout and logs phase/timing information with immediate flush. Adds clearPreemptiveOnly() method to clear only preemptive timeout and setPhase(phase) method for phase tracking. Updates clear() to also clear diagnostic timeout.
Phase Tracking & Selective Cleanup lib/api/chat-handler.ts	Adds phase markers throughout stream lifecycle (streaming, onFinish_started, onFinish_waiting_title, onFinish_updateChat, onFinish_awaitUsage, onFinish_saveMessages, onFinish_stopSubscriber). Replaces full timeout clears with clearPreemptiveOnly() in retry paths to preserve diagnostic timeout state.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• #209: Modifies chat-handler's abort/usage-resolution path and timeout/logging behavior in parallel with this diagnostic enhancement.
• #205: Directly related; modifies the same codepaths and implements the same new methods (setPhase and clearPreemptiveOnly).
• #62: Related; modifies the createPreemptiveTimeout implementation in stream-cancellation.ts.

Poem

🐰 A rabbit hops through phases, marking time with care,
Each timeout logged with wisdom before the hard timeout snare,
Preemptive clears yet diagnostic stays to light the way,
Phase by phase we trace the stream until the final day!

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly and accurately reflects the main change: adding diagnostic logging for Vercel timeout investigation, which is the core objective of both file modifications.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/diagnostic-timeout-logging

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[coderabbitai]

Actionable comments posted: 0

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

lib/api/chat-handler.ts (1)

972-981: ⚠️ Potential issue | 🟡 Minor

Clear diagnostic timeout on the early-return path.
Right now this return can leave the diagnostic timer running, which may emit late logs after a clean completion. Consider clearing after deductAccumulatedUsage() and before returning.

Suggested fix

                 if (
                   isAborted &&
                   !isPreemptiveAbort &&
                   newFileIds.length === 0 &&
                   !hasIncompleteToolCalls &&
                   !hasUsageToRecord
                 ) {
                   await deductAccumulatedUsage();
+                  preemptiveTimeout?.clear();
                   return;
                 }