fix(acp): retain errors reported by idle work state

[juliusmarminge]

Problem

ACP v2 reports structured failures on terminal idle updates.

Change

Preserve that error in the existing schema and normal decoder so a failed Claude reply retains its explanation.

Scope and approval

This is the maintainer-directed replacement for #19770, recorded in the takeover comparison and handoff. The five dependent layers fix the same native Claude background-work lifecycle defect. This is not presented as an approval exemption: human approval remains required for the ownership and default changes before merging.

Verification

Verification: 31 client tests and 30 protocol tests pass, including failed-idle error preservation. Affected typecheck and scoped lint/format checks pass.

This layer replaces part of #19770, originally by @StiensWout. Substantial implementation commits retain Wout Stiens’s co-author credit.

Current cumulative head: 49928218bd. Authenticated adapter proof: baseline foreground completion 20.956s; replacement 5.290s while the child continued until 19.285s. Autonomous continuation cleared pending work at 21.058s. Active foreground Stop reported interrupted before session-close confirmation, which returned after 27ms.

Detailed current-head verification and limits.

Authenticated verification uses official claude-agent-acp 0.93.0 on headless Linux with disposable state. macOS, Windows, iOS, Android, Electron desktop and relay/tunnel connections were not verified. Background permissions/questions, nested resume, transport errors, refused close and detached-request cancellation use deterministic stdio replays. A live forced-close timeout was not exercised.

Final independent full-diff round 8 Opus and Sol reviews both have no confirmed findings on this cumulative head. Current-head CI remains blocked after one failed-job rerun per updated workflow: unchanged browser tests fail on layers 3–5: highlighting, attachment preview recovery, and keyboard focus, respectively (each run has 8047 web tests passing and one failing). All other code checks pass; the aggregate Check fails because Test Web fails. This stack changes no web source or web tests. The CI failure remains a merge blocker. CodeRabbit automated reviews are paused on layers 3–5; layers 4–5 report that their included review quota is exhausted. Independent Opus/Sol reviews cover the exact cumulative head. Human review remains required for the default and ownership changes and remaining review gates. No merge is authorized.

Tests: GPT-6 Astra, Codex harness. Implementation: GPT-6.1 Sol, Codex harness. Independent reviews: Claude Opus 5.5, Claude Code harness, and GPT-6 Sol, Codex harness.

[macroscopeapp]

Approvability

Verdict: Approved at 3f33373

Macroscope's review found this PR approvable — This is a small, backwards-compatible ACP protocol fix that preserves structured errors on failed idle updates through the existing notification path. Production behavior changes only for previously rejected error-bearing idle notifications, with focused regression coverage and no default, infrastructure, or static-analysis changes.

Notes:

• No code objects were reviewed. Approvability was decided on eligibility alone.

No code changes detected at 32156bf. Prior analysis still applies.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	5.0 KiB	5.0 KiB	0 B (0.0%)	6.8 KiB	✅
Codex	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Codex	Live turn WebSocket wire	1.2 KiB	1.2 KiB	0 B (0.0%)	2.0 KiB	✅
Codex	Live turn WebSocket decoded	20.9 KiB	20.9 KiB	0 B (0.0%)	29.3 KiB	✅
Codex	Live turn messages	2	2	0 (0.0%)	8	✅
Claude	Total thread wire	5.0 KiB	5.0 KiB	−7 B (−0.1%)	6.8 KiB	✅
Claude	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Claude	Live turn WebSocket wire	1.2 KiB	1.2 KiB	−7 B (−0.6%)	2.0 KiB	✅
Claude	Live turn WebSocket decoded	21.2 KiB	21.2 KiB	0 B (0.0%)	29.3 KiB	✅
Claude	Live turn messages	2	2	0 (0.0%)	8	✅

Baseline: 673c5e3 · PR result: 32156bf · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 108.5 KiB
• Claude decoded thread snapshot: 108.8 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

[coderabbitai]

Important

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• packages/effect-acp/src/protocol.test.ts

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📝 Walkthrough

The protocol now accepts idle state updates with optional nullable error fields. The compatibility notification type and transform input include these updates. Tests check that the client receives the JSON-RPC error fields and that decode-failure reporting remains valid.

Changes: Idle Update Error Compatibility

Layer / File(s)	Summary
Accept and preserve idle update errors packages/effect-acp/src/compat.ts, packages/effect-acp/src/protocol.ts, packages/effect-acp/src/client.test.ts, packages/effect-acp/src/protocol.test.ts	The idle update type and decoder accept optional nullable error fields. The transform accepts compatibility notifications. Client and protocol tests check error preservation and decode-failure reporting.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: maria-rcks

Merge Risk: ⚪ Minimal · up to 3f333

The test continues to check for the expected missing-field diagnostic while allowing additional details. No material merge risk is identified in the supplied change.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: preserving errors reported by idle work states.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description check		The description includes all required sections: Problem, Change, Scope and approval, and Verification. It explains the defect, the implementation, approval context, focused test results, verification …

Full details: Description check

Explanation

The description includes all required sections: Problem, Change, Scope and approval, and Verification. It explains the defect, the implementation, approval context, focused test results, verification limits, known CI failures, and agent details. It also clearly states that merging is not authorized while required gates remain unresolved.

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