Best possible solution:

Land the chunked fallback reader after the maintainer draft is ready and the focused QA OTEL smoke test or equivalent CI/Testbox proof passes.

Do we have a high-confidence way to reproduce the issue?

Yes from source, not from a live failing run: current main reads the full fallback gateway.stdout.log with readFile, so the unbounded artifact-read path is clear. I did not execute a large-artifact runtime repro in this read-only review.

Is this the best way to solve the issue?

Yes, this looks like the narrow best fix for the test harness: it preserves the existing stdout diagnostic capture/parser contract and changes only fallback artifact ingestion. Changing the scenario YAML or child stdout listener would not address the whole-file fallback read.

AGENTS.md: found and applied where relevant.

Codex review notes: model internal, reasoning high; reviewed against 21c966616f61.

Label changes:

• add P3: This is a low-blast-radius internal QA Lab test-runtime reliability fix, not a user-facing runtime regression.
• add rating: 🐚 platinum hermit: Overall readiness is 🐚 platinum hermit; proof is 🌊 off-meta tidepool and patch quality is 🐚 platinum hermit.
• add status: 👀 ready for maintainer look: ClawSweeper has no concrete contributor-facing blocker left for this PR. Not applicable: The external-contributor proof gate is not applicable because this is a maintainer-member PR; the PR body lists syntax checks and a direct import probe, while full focused Vitest proof remains a merge validation item.

Label justifications:

• P3: This is a low-blast-radius internal QA Lab test-runtime reliability fix, not a user-facing runtime regression.
• rating: 🐚 platinum hermit: Overall readiness is 🐚 platinum hermit; proof is 🌊 off-meta tidepool and patch quality is 🐚 platinum hermit.
• status: 👀 ready for maintainer look: ClawSweeper has no concrete contributor-facing blocker left for this PR. Not applicable: The external-contributor proof gate is not applicable because this is a maintainer-member PR; the PR body lists syntax checks and a direct import probe, while full focused Vitest proof remains a merge validation item.

PR surface:

Tests +87. Total +87 across 2 files.

What I checked:

• Current main fallback reads the whole artifact: appendGatewayStdoutArtifactLogs on current main appends await readFile(gatewayStdoutPath, "utf8") before flushing the stdout diagnostic capture, so the fallback path still loads the entire artifact at once. (test/e2e/qa-lab/runtime/qa-otel-smoke-runtime.ts:1078, 21c966616f61)
• Runtime entry point uses the fallback only when child stdout has no records: The smoke runner streams child stdout into the diagnostic capture first, flushes it, and only calls the artifact fallback when no stdout diagnostic records were captured directly. (test/e2e/qa-lab/runtime/qa-otel-smoke-runtime.ts:1760, 21c966616f61)
• PR head changes the fallback to bounded chunks: The PR head calls appendUtf8FileToStdoutDiagnosticCapture from the fallback and reads the file into a reusable buffer before appending each bytesRead slice to the existing capture parser. (test/e2e/qa-lab/runtime/qa-otel-smoke-runtime.ts:1079, 005cc2a018ae)
• PR head adds focused regression tests: The PR adds tests for direct chunked artifact streaming and the real fallback path using a diagnostic JSON line followed by a large trailing stdout body. (test/e2e/qa-lab/runtime/qa-otel-smoke.e2e.test.ts:504, 005cc2a018ae)
• Latest release does not contain the current stdout fallback surface: The latest release tag did not show appendGatewayStdoutArtifactLogs, gateway.stdout.log, or the fallback readFile path; the relevant stdout QA fallback work is current-main-only. (8aa58c5fb0b4)
• Related merged stdout exporter work: Merged pull request Add stdout diagnostics OTEL log exporter #94561 added stdout diagnostics OTEL export, the stdout QA scenarios, and the QA smoke runtime coverage that this PR now tightens. (test/e2e/qa-lab/runtime/qa-otel-smoke-runtime.ts, 5db2f6c1fccb)

Likely related people:

• vincentkoc: git blame shows the current stdout diagnostic capture and fallback artifact reader in the QA OTEL smoke runtime came from commit 8aa58c5. (role: current implementation author and recent area contributor; confidence: high; commits: 8aa58c5fb0b4; files: test/e2e/qa-lab/runtime/qa-otel-smoke-runtime.ts, test/e2e/qa-lab/runtime/qa-otel-smoke.e2e.test.ts)
• jesse-merhi: Merged pull request Add stdout diagnostics OTEL log exporter #94561 introduced the diagnostics-otel stdout exporter and related QA smoke scenarios that this PR tightens. (role: adjacent feature owner; confidence: medium; commits: 5db2f6c1fccb; files: extensions/diagnostics-otel/src/service.ts, qa/scenarios/runtime/otel-stdout-log-smoke.yaml, qa/scenarios/runtime/otel-both-log-smoke.yaml)

• 🦀 challenger crab: rare, exceptional readiness with strong proof, clean implementation, and convincing validation.
• 🦞 diamond lobster: very strong readiness with only minor maintainer review expected.
• 🐚 platinum hermit: good normal PR, likely mergeable with ordinary maintainer review.
• 🦐 gold shrimp: useful signal, but proof or patch confidence is still limited.
• 🦪 silver shellfish: thin signal; proof, validation, or implementation needs work.
• 🧂 unranked krab: not merge-ready because proof is missing/unusable or there are serious correctness or safety concerns.
• 🌊 off-meta tidepool: rating does not apply to this item.

Shiny media proof means a screenshot, video, or linked artifact directly shows the changed behavior. Runtime, network, CSP, and security claims still need visible diagnostics.

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