fix(streaming): stop writing workflow stream frames after the client cancels

[waleedlatif1]

Summary

• After a client disconnects, the ReadableStream's cancel() closes the controller, but start() kept enqueueing frames: the "Client cancelled request" error, final, [DONE], chunk/thinking/tool frames, and selected-output frames from onBlockComplete
• Each of those enqueues threw Invalid state: Controller is already closed. The terminal path logged a spurious "Stream error" (the catch's own enqueue then threw out of start()), and the block-completion path logged "Failed to materialize selected output" and rejected the executor callback. It also finished selected-output materialization and base64 hydration for a client that was already gone
• cancel() now sets a streamCancelled flag first. Every enqueue in start() goes through one helper that does nothing once the stream is cancelled, controller.close() is skipped after cancel, and onBlockComplete returns early (and stops between descriptors) before any materialization work
• Everything else cancel() does is unchanged: abort propagation, keepalive stop, base64 cache cleanup, and marking the logging session failed with "Client cancelled request". A client that stays connected gets byte-identical output

Type of Change

• Bug fix

Testing

• New regression test in streaming.test.ts: cancel the reader mid-execution, run the captured onBlockComplete with a selected output, then resolve the execution as cancelled. It asserts the callback resolves and no "Stream error" is logged
• Red on the pre-fix code: the callback rejects with Controller is already closed. With only that assertion relaxed, the "Stream error" assertion fails on its own
• streaming.test.ts: 32/32 passing. check:comment-hygiene and check:test-patterns clean

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing (new tests pass the test-audit authoring gate)
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

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1 Skipped Deployment

Project	Deployment	Actions	Updated
docs	Skipped		Oct 18, 2026 9:05pm UTC

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Turn on auto-fix | Re-trigger cubic}

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge.

Summary

Stops workflow response writes after the client cancels.

• Canceled workflow streams stop writing frames and selected outputs.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Client cancels response] --> B[Set streamCancelled]
  B --> C[Stop keepalive and abort execution]
  C --> D[Clean up request hooks and cache]
  B --> E[Later frame writes do nothing]
  B --> F[Skip new selected-output work]
  E --> G[Skip final controller close]

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_{Reviews (1) · Last reviewed commit: "fix(streaming): stop writing workflow st..." · Reviewed by Greptile}