[derektrimm]

Codex lets one client write to a conversation at a time. When another client sharing the same CODEX_HOME still has the thread loaded (the Codex app, the CLI, an IDE extension), thread/resume fails with thread <id> already has an active writer. Turn start treats that like any other resume failure: it binds a fresh native thread and hands it a summary of the conversation. The user gets an answer with no sign anything changed, but the thread has left its native history behind for good, and the other client keeps working in the original thread, often in the same workspace.

This is how I hit it: a T3 Codex thread had also been opened in the Codex app, which keeps the writer lock until it quits. T3 could not send to it anymore.

Fix

• CodexAdapterV2.resumeThread maps Codex's active-writer error (-32600, already has an active writer) to a new ProviderAdapterThreadInUseError, which carries a message telling the user which kind of client to quit. Every other resume error is mapped exactly as before.
• ProviderTurnStartService does not take the fresh-session fallback for that error. The run fails through the existing loadFromProvider path, so retries, the failed turn item and the run status all work as they already do for load failures.
• makeProviderFailure shows that error's message when it arrives wrapped in another cause, as it already does for ClaudeBackgroundWorkBlocksQueryReplacementError.

When the other client releases the thread, the next send resumes the same native thread. There is no automatic fork and no change to the native conversation identity. That follows the reason #7414 was closed: recovery should not change the conversation's identity without the user knowing.

Scope and approval

No prior issue or maintainer approval is claimed. I'm submitting this under the exception for a small, focused fix of an obvious bug: a transient lock conflict permanently and silently replaces the conversation, while another agent may still be working in it. The change adds one adapter error type, one branch in turn start and one case in the failure-message mapper. It adds no setting or workflow. A user-facing "fork here instead" choice would be a new feature, so it is left out.

Overlap: #19557 (Codex account switches, #16613) would have stopped this fallback for every Codex resume failure, with a general message; it was closed without merging. This PR handles only the case where another client holds the thread. The cause is known there, the condition clears by itself, and the message can name what to do.

Evidence

Reproduced on main (16918b5) with vp run dev on Linux, a worktree .t3, Codex CLI 0.163.0 and the real ~/.codex home:

1. Send a message in a new Codex thread (native thread 01a14c37…).
2. Stop the dev server so its app-server releases the thread, then hold the writer lock from another process the way a second client does: flock -n ~/.codex/thread-writer-locks/<id>.lock sleep 1800.
3. Restart the dev server and send a follow-up.

On main, the app-server log shows thread 01a14c37-… already has an active writer, followed by Provider resume failed; attempting a fresh native session. The reply arrives and the provider thread now points at a new native thread (01a14c38…). The UI shows nothing:

On this branch, with the lock on 01a14c38… held the same way, the turn fails with the explanation, and the provider thread still points at 01a14c38…. The log has no fallback line:

After the lock is released, the next send resumes 01a14c38… (its rollout contains the new reply) without a handoff:

Tests, each seen failing for the intended reason with its part of the fix removed, then passing:

• CodexAdapterV2.test.ts "reports a thread another Codex client is writing to as in use": a replayed active-writer frame becomes ProviderAdapterThreadInUseError (it was ProviderAdapterResumeThreadError).
• ProviderTurnStartService.test.ts "fails the run instead of replacing a native thread another client holds": no fallback ensureThread, no handoff, native ref unchanged, run failed with the message. Without the guard, the fallback ensureThread runs.
• failure.test.ts "explains a native thread another client is writing to": without the new case, the message falls back to the generic turn-start text.

vp test run for CodexAdapterV2.test.ts, ProviderTurnStartService.test.ts and failure.test.ts (155 passed, rebased on main at a42bdd4). Disabling the new turn-start check makes fails the run instead of replacing a native thread another client holds fail. tsc-rs --noEmit is clean for apps/server and packages/provider-core. vp fmt --check and vp lint are clean on the changed files.

Not checked: the mobile client, which renders the same failed turn item from the projection, and a real second Codex app instead of flock. The lock is the same flock that Codex's WriterLockCoordinator takes, and I saw the Codex app holding it with /proc/<pid>/fd. Rollback's own thread/resume and other providers' equivalents (for example the Claude "Session ID … is already in use" case in #15243) are separate problems and unchanged here.

Made with Claude Opus 5.5 in Claude Code, running in T3 Code.

[macroscopeapp]

Approvability

Verdict: Approved at eb8e447

Macroscope's review found this PR approvable — This is a small, well-tested Codex bug fix that converts one known active-writer conflict into an actionable failed turn instead of silently forking the native conversation. Other resume failures and existing callers retain their prior behavior, with no defaults, infrastructure, security, billing, or static-analysis changes.

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Walkthrough

Codex resume now identifies active-writer conflicts and reports them as provider thread-in-use errors. Turn startup routes these errors through provider loading instead of creating a replacement native session or preparing a context handoff.

Changes: Active-thread conflict handling

Layer / File(s)	Summary
Provider thread-in-use error packages/provider-core/src/server/ProviderAdapter.ts, packages/provider-core/src/server/failure.ts, packages/provider-core/src/server/failure.test.ts	Adds ProviderAdapterThreadInUseError to the provider error union. Failure-message handling returns its message, with coverage for a wrapped error.
Codex active-writer mapping apps/server/src/orchestration-v2/Adapters/CodexAdapterV2.ts, apps/server/src/orchestration-v2/Adapters/CodexAdapterV2.test.ts	Maps matching Codex active-writer resume errors to ProviderAdapterThreadInUseError. The replay test checks the provider thread ID, detail message, and native error message.
Startup handling apps/server/src/orchestration-v2/ProviderTurnStartService.ts, apps/server/src/orchestration-v2/ProviderTurnStartService.test.ts	Routes thread-in-use errors through provider loading. The test checks that startup does not create a fallback thread, start a provider turn, or create a context handoff.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Suggested reviewers: juliusmarminge

Merge Risk: Minimal · previous review 22bf0

The change appears ready to merge after normal checks: a locked Codex thread reports an actionable error instead of being replaced, and no actionable merge-blocking risk remains.

Security architecture risk: Low · up to c5c78

The change prevents silently splitting a conversation when another client is using it. No broader access is apparent, but recovery with simultaneous clients has not been verified end to end.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The demonstrated behavioral scope is startup of the selected native Codex conversation. On a detected writer conflict, the changed path neither overrides the other client's writer ownership nor starts a replacement conversation or provider turn.

Trust Boundaries and Controls

• observed — The classification input is a Codex request failure. Recognition requires the request-error schema, code -32600, and the active-writer substring; user conversation text is not the classification input. The resulting control transition restricts execution by refusing fallback rather than granting additional authority.

Resilience and Maintainability Implications

• inferred — The failure exits before provider-turn startup. The unchanged session wrapper marks a thread loaded only after successful resume, and busy-turn tracking begins at turn startup. Session opening schedules the existing idle-release mechanism, so the inspected conflict path does not introduce an unconditional busy-session retention state. Direct runtime validation of conflict cleanup and races remains unavailable.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main fix: preventing Codex threads opened in another client from being silently replaced.
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 clearly explains the problem, the fix, scope and approval rationale, verification steps, observed results, limitations, and agent details. It uses "Fix" and "Evidence" instead of the t…

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• apps/server/src/orchestration-v2/Adapters/CodexAdapterV2.test.ts
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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
Review comments at @packages/provider-core/src/server/ProviderAdapter.ts:
- Line 232: Update the error schema used by `resumeThread` to require a `cause`
field of type `Schema.Defect()`, and update the construction in `CodexAdapterV2`
to pass the underlying request error as that cause. Keep `detail` as the fixed
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• apps/server/src/orchestration-v2/ProviderTurnStartService.ts
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[maria-rcks]

Note

Written by gpt-6.1-sol on behalf of Maria

Verified in a real dev environment

verified unchanged head 466eab8cdeb93c2a089f818cb9b67caabeda94dc on this linux vm. reproduced first on github/main at e6e2ab05cceb3f8245e87f8499d4891f27de33cb, in a separate worktree. ran one background dev server at a time, with isolated state.

used playwright from the repo and /usr/bin/google-chrome to add a real git repository, select codex gpt-6.1-sol, and send real provider turns through the web client. after stopping the dev server, a separate real codex app-server resumed the native conversation and held its writer lock. restarted the dev server before each contention scenario. no mocked responses or fabricated state.

• main silently replied on a replacement native conversation and created a full_thread_summary recovery handoff.
• this head failed two sends with the explanation telling the user to quit the other client. the native id stayed unchanged; neither failed send created a handoff.
• an unrelated codex conversation completed while the original was locked.
• after quitting the second client, the original conversation resumed and remembered cedar. another normal follow-up completed on that same native id. recovery injected the existing delta history for the two failed runs, without replacing the native conversation.

short runtime transcript, checked against the live server and read-only projection queries:

codex --version: codex-cli 0.164.0
second app-server: initialize -> thread/resume -> original native id
main native: 01a199e6-2024-7de0-9bf8-358e36a97a9b
main follow-up: completed; native -> 01a199e8-16fc-7f72-bb5f-d5898ff9fda3
main log: Recovering provider context in a fresh native session; resume_failed
pr native: 01a199ee-4941-70f3-b05a-9f88804fe6b1
pr runs: completed, failed, failed, completed, completed
pr native after every run: 01a199ee-4941-70f3-b05a-9f88804fe6b1
pr recovery replies: cedar / cedar still here.
pr fallback-recovery log entries: 0

vp run check:changed passed through blacksmith-check against base 8760d063e903406c8e8f695b0852c0a15fc1e00f: format, lint, knip, graphql, affected typechecks, and 181 tests. the three focused files also passed separately, 180 tests. all 30 reported ci checks were successful. no source changes or pushes were needed.

main, same initial prompt and follow-up, with silent replacement:

pr, writer conflict explained:

pr, recovery on the original native conversation:

28-second recording (animated copy for github): return from the independent conversation, resend after release, and receive the remembered word. inspected the exported clip and verified uploaded playback and seeking.

Not verified

native ios/android and electron clients, macos/windows, remote relay/tunnel connections, and the codex desktop app or ide extension ui were not exercised on this linux vm. contention used a real second codex app-server sharing the same codex home. rollback and other providers are outside this change. no performance claim is made.