fix(responses): recover encrypted agent tasks on mid-thread model switches

[lidge-jun]

Summary

Switching a live Codex Desktop thread from a native ChatGPT model to a routed provider model bricked the thread permanently with unreadable_encrypted_agent_task. A thread started on the routed model never hits this; only a thread whose history already contains a backend-minted encrypted_content agent message does, and once it is there every later turn replays it, so the thread can never be continued on that provider. The report's only workaround was to start a new thread.

agentTaskRecovery was enabled and still never ran. The direct (non-combo) recovery block in src/server/responses/core.ts was gated on threadSpawn (isThreadSpawnRequest(req.headers), true only for x-openai-subagent: collab_spawn or turn metadata subagent_kind === "thread_spawn"). A mid-thread model switch is neither, so the request failed closed without any recovery attempt — and, because recovery_reason is attached only when a recovery attempt produced a refusal reason, the error body carried no recovery_reason key at all.

This drops the threadSpawn conjunct from that one gate. Everything else is unchanged:

• recoveryAdmission() in src/server/responses/agent-task-recovery.ts is not widened. The Codex-originator check, the live native-ChatGPT-bearer check (RS256 + kid, OpenAI issuer, https://api.openai.com/v1 audience, Codex OAuth client_id/azp, unexpired, nbf honoured), the requirement that the account id inside the token equals the explicit chatgpt-account-id header, the no-inbound-API-key rule, and the proxy-admission-secret rejection all stay exactly as they are.
• The combo gate keeps its spawn requirement. That path has its own native-target filtering and per-attempt failover, and the reported defect is on the direct path.
• canPassThroughEncryptedV2AgentTask() is unchanged, so an OAuth-mode routed provider still has no ciphertext passthrough.
• restoreCachedEncryptedAgentTasks() lives inside the same if, so it moves with the gate. That fixes the report's third observation: a mid-thread turn can now reuse a plaintext this proxy already paid for instead of paying for it again after a restart.

Why the trust boundary is the same

threadSpawn was never the security boundary; recoveryAdmission() is. The population that gains reachability is a loopback request from a Codex originator, holding a live native ChatGPT bearer for the same account named in chatgpt-account-id, with no inbound API key, on a proxy that does not require inbound API auth — the same user whose session would be spent, on the same machine. threadSpawn narrowed which of that user's own requests could use their own session; it kept nobody else out. The recovery cache is keyed by an HMAC over the token and account id, and restoreCachedEncryptedAgentTasks() re-runs admission per item before touching the cache, so the widened entry point cannot read another caller's recovered plaintext.

src/server/responses/encrypted-payload.ts warns that decrypting a MESSAGE on the parent's behalf would build a plaintext oracle out of a payload the parent's session may not be entitled to read, which is why MESSAGE is matched for the unreadability check while recovery stays NEW_TASK-only. That asymmetry is untouched. Widening the entry gate does not widen what may be decrypted: an unreadable MESSAGE still fails closed with a refusal reason, and the only envelope that reaches an actual decrypt attempt is a NEW_TASK the admitted caller's own session is entitled to read. The unchanged admission checks are what keep the newly reachable callers to the set that could already reach the same decrypt on a spawn turn.

Out of scope and deliberately not grouped in: #2495 (opt-in plaintext V2 rewrite) and #3661 (spawn-path recovery failures). This PR also does not implement the report's alternative suggestion of rejecting the model switch early — that is a product/UX decision for a separate change.

Closes #4089

Verification

Local checks were NOT RUN — no product test suite, no bun run typecheck, no build, no lint, no bun install — per maintainer instruction for this round. The exact-head remote CI on this PR is the only gate.

Regression added in tests/server/agent-task-recovery.test.ts, encoding the reporter's loopback reproduction:

• a mid-thread switch attempts recovery exactly like the spawn it is not — two post() calls with an identical body (one agent_message carrying a routing header plus a structurally valid Fernet-shaped encrypted_content slot) against a routed provider, differing only by the x-openai-subagent: collab_spawn header. recovery_reason is the discriminator, since it is attached only when recovery actually ran. Both arms must now carry recovery_reason: "recovery_invalid_output" and the two error bodies must be equal. Before this change the mid-thread arm had no recovery_reason key at all.
• a recovered mid-thread turn reaches the routed provider as plaintext — the product outcome: the mid-thread turn returns 200, the recovered assignment reaches the provider, and the Fernet ciphertext does not.
• a mid-thread replay reuses the cached plaintext instead of recovering again — covers the cache restore moving with the gate: a second mid-thread turn dispatches to the provider without a second recovery call.
• a mid-thread switch without matching native credentials never spends a session — the negative case for the widened entry point: a mismatched chatgpt-account-id is refused with recovery_reason: "admission_denied", zero upstream calls, and no ciphertext in the response body.

Docs: docs-site framed agentTaskRecovery as spawn-only ("a native ChatGPT parent spawning a routed v2 child"). The reference page and the sub-agent surface guide now name both qualifying request shapes, and the combo paragraph says explicitly that combo recovery is still spawn-only. The same one-clause precision is applied to the seven translated locales so they do not contradict the English source. The docs-site build was not run, per the same instruction; the edits are prose-only inside existing pages, with no frontmatter, component, or navigation changes.

Design and trust-boundary analysis is recorded in devlog/_plan/260909_mid_thread_agent_task_recovery/000_plan.md.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed.
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.

Summary by CodeRabbit

• Bug Fixes

  • Encrypted agent-task recovery now works when switching from a native ChatGPT model to a routed model during an active conversation.
  • Recovered agent messages are correctly delivered as plaintext to routed providers, with replayed turns using cached recovery data.
  • Recovery is refused when native credentials do not match.

• Documentation

  • Updated agent-task recovery and fallback-chain guidance across supported languages, including the distinction between direct routing and combo recovery.

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Reviewing files that changed from the base of the PR and between 6a260ea and 0e95eb7.

📒 Files selected for processing (5)

• docs-site/src/content/docs/ja/reference/configuration/agents.md
• docs-site/src/content/docs/ko/reference/configuration/agents.md
• docs-site/src/content/docs/reference/configuration/agents.md
• docs-site/src/content/docs/ru/reference/configuration/agents.md
• docs-site/src/content/docs/tr/reference/configuration/agents.md

📝 Walkthrough

Walkthrough

The direct Responses recovery gate no longer requires a thread-spawn request. Mid-thread native-to-routed switches can attempt recovery, while admission checks, NEW_TASK-only decryption, and combo-path restrictions remain unchanged. Regression tests cover recovery, plaintext forwarding, replay, and denial.

Changes

Mid-thread encrypted task recovery

Layer / File(s)	Summary
Recovery behavior and security boundaries devlog/_plan/260909_mid_thread_agent_task_recovery/000_plan.md	The plan records the mid-thread failure, unchanged admission checks, the NEW_TASK-only decryption boundary, spawn-only combo recovery, regression expectations, and documentation updates.
Direct Responses recovery gate src/server/responses/core.ts	Lines 3616-3628 remove the threadSpawn requirement from direct recovery while retaining route, configuration, passthrough, and combo restrictions.
Mid-thread regression coverage tests/server/agent-task-recovery.test.ts	Lines 861-982 test recovery metadata, plaintext delivery, cached replay, and admission_denied before any fetch when native credentials do not match.
Recovery documentation docs-site/src/content/docs/reference/configuration/agents.md, docs-site/src/content/docs/guides/sub-agent-surface.md, docs-site/src/content/docs/*/reference/configuration/agents.md	The documentation describes direct recovery for mid-thread model switches and retains spawn-only combo recovery behavior across supported locales.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~15 minutes

Severity of issue fixed: Medium

Merge Risk: 🔵 Low · up to 6a260

This change enables direct recovery for eligible mid-thread model switches, but localized documentation gives inconsistent guidance about when combo recovery begins. The behavior is not shown to be affected, though the documentation should be aligned before or shortly after merge.

Sequence Diagram(s)

sequenceDiagram
  participant ResponsesRequest
  participant ResponsesCore
  participant AgentTaskRecovery
  participant RoutedProvider
  ResponsesRequest->>ResponsesCore: Send routed request with unreadable encrypted agent task
  ResponsesCore->>AgentTaskRecovery: Attempt recovery without thread-spawn marker
  AgentTaskRecovery-->>ResponsesCore: Return plaintext or recovery result
  ResponsesCore->>RoutedProvider: Forward recovered turn as plaintext

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes satisfy issue [#4089]. src/server/responses/core.ts removes the direct-path threadSpawn restriction, while preserving recoveryAdmission(), trust checks, encrypted V2 passthrough behavior, …
Out of Scope Changes check	✅ Passed	The changes are within scope for [#4089]. The implementation, regression tests, plan document, and localized documentation all directly support mid-thread encrypted agent-task recovery and the distinc…
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 2 files. (10 skipped: 1…
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: recovering encrypted agent tasks when a Responses thread switches models mid-thread.

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• Commit unit tests in branch codex/mid-thread-agent-task-recovery

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Actionable comments posted: 2

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Inline comments:
In `@docs-site/src/content/docs/ja/reference/configuration/agents.md`:
- Line 66: Update the Japanese documentation sentence describing combo recovery
to include both triggers: when no native target is selectable and when all
native attempts have been exhausted. Keep the existing behavior and wording for
the remaining recovery conditions unchanged.

In `@docs-site/src/content/docs/zh-cn/reference/configuration/agents.md`:
- Line 65: Update the combo recovery description near the encrypted NEW_TASK
flow to match the canonical English behavior: trigger recovery only when no
selectable canonical native target exists, removing the condition that native
attempts have been exhausted. Keep the surrounding routing and recovery behavior
unchanged.

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• devlog/_plan/260909_mid_thread_agent_task_recovery/000_plan.md
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• docs-site/src/content/docs/ja/reference/configuration/agents.md
• docs-site/src/content/docs/ko/reference/configuration/agents.md
• docs-site/src/content/docs/reference/configuration/agents.md
• docs-site/src/content/docs/ru/reference/configuration/agents.md
• docs-site/src/content/docs/tr/reference/configuration/agents.md
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[lidge-jun]

리뷰 · 우선순위 71 / 80

이 PR은 Codex Desktop에서 이미 시작된 스레드를 네이티브 ChatGPT 모델에서 라우팅 프로바이더로 바꾸면, 히스토리에 남은 encrypted_content agent 메시지 때문에 매 턴이 unreadable_encrypted_agent_task로 영구 실패하던 버그(#4089)를 고칩니다. 처음부터 라우팅 모델로 연 스레드는 암호문이 없어서 재현되지 않고, 한 번 네이티브가 만든 암호문이 들어가면 이후 턴이 계속 재생하므로 “새 스레드 만들기” 말고는 길이 없었습니다.

현재 dev HEAD의 src/server/responses/core.ts 직접(비-combo) 복구 블록은 대략 다음 조건입니다: inboundWire === "responses" && threadSpawn && agentTaskRecovery && !isCanonicalOpenAiForwardProvider(...) && !options.comboAttempt && !canPassThroughEncryptedV2AgentTask(...). 여기서 threadSpawn은 isThreadSpawnRequest(req.headers)로, x-openai-subagent: collab_spawn 또는 turn metadata subagent_kind === "thread_spawn"일 때만 true입니다. 중도 모델 전환은 spawn이 아니므로 복구가 한 번도 시도되지 않고, recovery_reason 필드조차 안 붙습니다(그 필드는 복구 시도가 거절 사유를 만들었을 때만 붙음).

이 PR이 하는 일은 그 한 줄의 threadSpawn 조건을 빼는 것입니다. restoreCachedEncryptedAgentTasks()도 같은 if 안에 있어서 함께 열립니다. 그래서 (1) 중도 전환도 복구를 시도하고, (2) 프록시가 이미 복구한 plaintext를 재시작 후에도 캐시로 재사용할 수 있습니다. combo 쪽 recoverUnreadableEncryptedTask는 여전히 isThreadSpawnRequest를 요구합니다. recoveryAdmission()(Codex originator, 살아 있는 native ChatGPT bearer, account id 일치, inbound API key 없음, proxy-admission secret 거절 등)은 손대지 않습니다. canPassThroughEncryptedV2AgentTask도 그대로라 OAuth 라우팅 프로바이더는 암호문 통과가 없습니다. MESSAGE는 복호화하지 않고 NEW_TASK만 복구하는 비대칭도 유지됩니다.

문서(docs-site 영문 agents.md, sub-agent-surface, 7개 로케일)는 “spawn 전용”처럼 읽히던 문장을 중도 전환까지 포함하도록 고치고, combo는 여전히 spawn-only라고 명시합니다. devlog/_plan/260909_mid_thread_agent_task_recovery/000_plan.md에 C4(신뢰 경계 확장)로 증거와 non-goal(#2495, #3661, 조기 모델전환 거절 UX)을 적어 둔 점도 좋습니다. 회귀 테스트 네 개(중도=spawn과 동일한 recovery_reason, plaintext 전달, 캐시 재사용, account mismatch 시 admission_denied + upstream 0회)가 리포트 재현을 잘 박아 둡니다.

보안상 “입구만 넓히고 문지기는 그대로”라는 설명이 코드와 맞습니다. threadSpawn은 원래 “다른 사람”을 막는 경계가 아니라 “같은 소유자의 요청 중 어떤 형태만” 허용하던 필터였고, 실제 경계는 recoveryAdmission()입니다. 그래도 세션을 쓰는 경로의 도달 범위가 늘어나므로 exact-head CI 그린과 한 번 더 읽기(특히 combo 게이트를 의도적으로 남긴 점)를 거친 뒤 머지하는 편이 맞습니다. 로컬 테스트는 이번 라운드 지침대로 안 돌렸다고 했고, 원격 CI가 게이트입니다.

src/server/responses/core.ts (직접 복구 if) - threadSpawn 제거 + 주석. HEAD에는 아직 threadSpawn이 남아 있어 #4089가 그대로 재현됨. 변경 범위는 의도적으로 한 게이트
src/server/responses/core.ts (combo recoverUnreadableEncryptedTask) - spawn 요구 유지. 리포트 결함은 직접 경로라 타당. 나중에 combo도 넓힐지는 별 이슈
tests/server/agent-task-recovery.test.ts - mid-thread A/B, plaintext 성공, 캐시 재사용, mismatch 거절. recovery_reason을 discriminator로 쓴 점이 좋음
docs-site/.../agents.md 및 로케일 - spawn-only 문구를 바로잡음. combo는 spawn-only로 남김. 영문과 번역이 어긋나지 않게 맞춘 것도 좋음
devlog/_plan/.../000_plan.md - C4·non-goal·신뢰 경계 설명이 PR 본문과 일치. 머지 후 _fin 이동만 남음
#2495 / #3661 / 조기 거절 UX - 이 PR에 안 섞인 것은 맞음. 범위 유지 권장

메인테이너의 판단이 필요한 지점

• C4: 입구 확장에 대한 보안 리뷰 한 번(admission·캐시 HMAC·NEW_TASK-only가 그대로인지). 본문 분석은 설득력 있음
• combo 경로를 이번에 같이 열지 말지 — 지금은 직접 경로만. 동의(한 PR에 두 행동 변화 금지)
• exact-head CI가 아직 rollup BLOCKED/대기인 구간이 있음. 그린 확인 후 머지
• 모델 전환 자체를 클라이언트에서 막는 UX([Bug]: Mid-thread model switch from a native ChatGPT model to a routed provider dies with unreadable_encrypted_agent_task; agentTaskRecovery never runs because it is gated on threadSpawn #4089 대안 2)는 별 유닛으로 둘지 — 이 PR은 복구 쪽만, 그대로 두는 게 맞음

너의 추천
CI exact-head가 그린이면 merge. #4089를 정확히 고치고, 신뢰 경계(recoveryAdmission)는 유지한 채 잘못된 spawn 게이트만 제거한다. combo·passthrough·MESSAGE 복호화는 건드리지 말 것. 머지 후 plan을 _fin으로 옮기고 #4089를 닫으면 된다.

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