fix(responses): stop combo failover once a request has spent its ambiguous replacement

[FredAmartey]

Summary

• Once a request has spent its retryOnReset replacement, the first send may already have run the turn, so nothing may send it again. fetchWithResetRetry enforces that for non-OK replacement answers: statuses that invite another send settle as the upstream_reset_replay_refused refusal, and the rest are marked non-replayable. A replacement that answers 200 is returned unmarked, because a 200 is an answer.
• The gap is a 200 whose stream then fails with zero output, for example a response.failed with server_is_overloaded. In a failover combo, preflightComboStreamResponse rebuilds that terminal as a fresh 503 that carries no marker, consumeComboFailure finds nothing to stop on and failover sends the turn to the next target: a third send of a turn that may already have run. On the direct path, the streamed encrypted-function-output preflight can hand the same kind of projected failure to the opaque-blob rebuild, which sends again. The pre-header row (feat(retry): opt-in replay of a pre-response reset for self-contained Responses sends #4942) and the SSE row (fix(responses): gate post-header reset recovery on SSE protocol state #4989) have both reached this since they were unified in Rework #4942 and #4989 into one ambiguous-resend gate with one grant per request #5342, and fix(responses): let retryOnReset replace a Codex WebSocket send that died unanswered #5633 adds a Codex WebSocket row that reaches it the same way.
• A marker cannot carry it: between the replacement and the combo classifier the 200 passes through the relay wrapper and the preflight's rebuilt Response, each a new object. The request execution budget is the one thing the parent and every child share, so the fact lives there.
• RequestExecutionBudget now reports ambiguousResendSpent, read from the one grant on the shared ledger, so derived scopes, bridged scopes and the adapter view all see a claim made by any of them. It is optional next to claimAmbiguousResend, for the same reason: a hand-built stub still typechecks and grants nothing.
• Combo failover stops when the grant is spent. The answer follows the rule fetchWithResetRetry already applied, now shared as settleOperatorReplacement: a status the client would resend becomes the replay refusal, and anything else reaches the client with its own status and the marker. A context overflow therefore stays a 400 that says why, instead of a 429 that hides it. A failure that already carries the marker keeps its existing handling.
• On the direct path the streamed opaque-blob rebuild is skipped once the grant is spent, and the preflight's projected failure is settled by the same rule, so the client gets the refusal with x-should-retry: false instead of a bare 502 it would resend. The 4xx rebuild needed no change: a non-OK answer to a spent replacement is already the refusal or marked, and the rebuild's body reader refuses marked responses.
• The routing policy fallback no longer hops on a marked answer. It classified by status and body alone, so once the combo keeps a 400 overflow, a policy-routed request could have sent the turn to another route.
• A claim only succeeds where the spent state can be read back. ledgerFor, createResponsesSendBudget and adapterDispatchBudgetView refuse it unless the budget exposes ambiguousResendSpent, so a hand-built budget cannot grant a replacement that every view then reports as unspent. Production budgets all come from the factory, which implements both.
• reasoningEffortRejectionText skips a non-replayable answer, the same line fix(responses): let retryOnReset replace a Codex WebSocket send that died unanswered #5633 adds, so a spent replacement's effort rejection no longer starts a downgrade send.
• settleOperatorReplacement is the helper fix(responses): let retryOnReset replace a Codex WebSocket send that died unanswered #5633 adds, carried here byte for byte so either PR can land first. git merge-tree merges the two branches cleanly in both orders. That clean merge would also keep the "One case is not covered yet" sentence fix(responses): let retryOnReset replace a Codex WebSocket send that died unanswered #5633 adds to the same doc section, so whichever lands second, I will rebase the other and drop it.
• Docs: a paragraph under the replacement-answer table in structure/transports/responses-failover.md, and the grant paragraph there now says a hand-built parent grants only if it also exposes ambiguousResendSpent.
• The child attempt row keeps the projected status, which is what upstream answered, while the request row records what the client got.

Refs #5633

Verification

On dev at a1dba2ccb, the base of this PR, with the pinned Bun 1.4.0 (node_modules/.bin/bun):

• Driven red first. The new cases in tests/lib/ambiguous-resend-composition.test.ts read an undefined property on dev and fail: a fresh budget reports false, a derived scope sees the parent's claim, the parent sees a derived scope's claim and a bridged scope sees a claim through the view. The combo case in tests/server/replay-refusal-parity.test.ts (first target resets before the head, the granted replacement answers 200 with a zero-output response.failed for server_is_overloaded) hops to the second target on dev; with the fix it makes exactly two sends to the first target and none to the second, and the client gets the replay refusal. That file now clears combo selection state and target cooldowns around each case, so an earlier combo case's cooldown cannot steer the pick.
• The review round adds three cases, each red before this round's fix. A combo whose spent replacement streams a zero-output context_length_exceeded now answers that 400 with its message (it answered the 429 refusal). The direct path case, a spent replacement that streams an encrypted-function-output rejection, now answers the refusal with x-should-retry: false (it answered a bare 502). In tests/routing/routing-policy-fallback.test.ts, a marked context overflow stays on its route (it tried a second one).
• CodeRabbit's round adds four cases, each red before its fix: a hand-built parent that implements the claim but not ambiguousResendSpent now grants nothing through ledgerFor, the Responses send budget or the adapter view (each forwarded the claim), while the same parent with the field still grants and reads back spent; and on the direct path a spent replacement's effort rejection now reaches the client as the 400 after two sends (without the fix a third, downgraded send went out).
• Changed and neighbouring test files on this head, in CI's fresh-process shape: ambiguous-resend-composition, replay-refusal-parity and routing-policy-fallback: 43 pass; responses-reasoning-effort-downgrade, responses-opaque-blob-recovery, upstream-retry and responses-reset-replay: 137 pass; 0 fail.
• Full suite on dba899fc9 (this head adds only a code comment and a doc paragraph on top), sliced the way scripts/ci/run-bun-test-batches.sh shards CI since ci: release preflight, separate release outcomes, duration-balanced shards, narrow scope checks #5653 (duration-balanced batches of at most 12 files, bun test --isolate --timeout 60000, CI=true): every batch of shards 1/4 to 4/4, past failing batches, the four shards in parallel on one macOS machine, with a 300 s kill deadline per batch in place of CI's 120 s because the machine was shared. 30099 pass and 4 fail across 1580 files. Two fail the same way on untouched dev at 052e1d71d when their batch runs alone: codex-runtime.test.ts (treats missing persisted and resolved versions as the same selection) and the provider-option integration spine, whose last assertion hashes the real ~/.claude before and after the test; another process on this machine writes there during the run, so the hash moves. The other two pass when their batch runs alone on this head: the stall observer samples only an identified silent suite case of macos-serial-lanes.test.ts and CI review lanes > release-gates aggregate accepts diagnostic skips but rejects producer failures, which hit its 30 s limit under load.
• bun run typecheck, bun run structure:check, bun run privacy:scan and git diff --check: passed.

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.

Review readiness checklist

This PR stays in draft until every box below is ticked. Tick all four boxes once the requirements are met:

• Required local validation passed; commands, results, and any full-suite exception are documented.

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Summary by CodeRabbit

• Bug Fixes
  • Prevented additional retries and policy fallbacks after a replacement request has already been sent or a response is marked non-replayable.
  • Improved handling of failures before streamed output: responses that invite another resend are returned as replay refusals, while other failures retain their original status.
  • Applied consistent replay-refusal behavior to both combined and direct request paths.
• Documentation
  • Clarified how failures after a replacement request are handled.

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📒 Files selected for processing (5)

• src/lib/request-execution-budget.ts
• src/server/responses/core-opaque-recovery.ts
• src/server/responses/request-send-budget.ts
• tests/lib/ambiguous-resend-composition.test.ts
• tests/server/replay-refusal-parity.test.ts

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📝 Walkthrough

Walkthrough

The request execution budget now exposes whether an ambiguous resend was spent. Combo, passthrough, and policy-fallback paths use this state to settle replacement failures and prevent further sends or fallback hops.

Changes

Replacement failure handling

Layer / File(s)	Summary
Track ambiguous-resend usage src/lib/request-execution-budget.ts, src/server/responses/request-send-budget.ts, tests/lib/ambiguous-resend-composition.test.ts	Execution budgets and send-budget views expose whether any scope claimed the ambiguous-resend grant. Tests cover fresh budgets and visibility between parent and derived scopes.
Settle failures across response paths src/lib/upstream-retry.ts, src/server/responses/core-combo.ts, src/server/responses/passthrough-dispatch.ts, src/server/responses/policy-fallback.ts, tests/routing/routing-policy-fallback.test.ts, tests/server/replay-refusal-parity.test.ts, structure/transports/responses-failover.md	A shared helper settles non-ok replacement responses. Combo and passthrough paths use the spent flag to stop further sends; policy fallback rejects non-replayable responses. Tests and documentation cover these behaviors.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Suggested reviewers: lidge-j

Merge Risk: 🟡 Moderate · up to 69e8c

A request can be sent again after its replacement allowance is spent, either through a compatible budget view or through reasoning-effort recovery. Close both replay paths before merging.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 56.25% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 16 functions across 9 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: preventing combo failover after a request spends its ambiguous replacement.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 56.25% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 16 functions across 9 files. (1 skipped: 1 unsupported.)

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[github-actions]

✅ Deterministic PR hygiene checks passed.

[lidge-jun]

리뷰 · 우선순위 70 / 80

한 번 다시 보낸 요청은 그 턴을 또 보내면 안 됩니다. 첫 전송이 이미 답을 만들기 시작했을 수 있습니다.

다시 보낸 요청이 200으로 오면 금지 표시가 붙지 않습니다. 그 답이 내용을 하나도 쓰기 전에 끊기면, 콤보는 그 실패를 표시 없는 새 오류로 다시 만듭니다. 그래서 다음 서버로 같은 턴을 한 번 더 보냅니다. 콤보가 아닌 직접 길도, 같은 실패를 보고 암호로 된 함수 결과를 고치려고 또 보낼 수 있습니다.

이 수정은 요청 예산에 그 한 번을 이미 썼는지(ambiguousResendSpent)를 적습니다. 부모 예산과 자식 예산이 같은 기록을 봅니다. 콤보는 기록이 켜져 있고 실패에 예전 금지 표시가 없으면, 다음 서버로 넘기지 않고 다시 보내지 말라는 답(429)을 줍니다. 직접 길은 이어지는 답에서 하는 불투명 복구 전송을 건너뜁니다.

src/server/responses/core-combo.ts executeComboResponses - 이 거부는 오류 번호를 보지 않습니다. 내용이 없는 실패가 400이면 클라이언트는 같은 턴을 다시 보내지 않습니다. 그런데도 429로 바뀝니다. 사용자는 요청이 잘못된 이유나 글이 너무 길다는 이유를 못 봅니다. 서버 안 기록에는 원래 오류 번호가 남고, 클라이언트에는 429가 갑니다.

src/server/responses/passthrough-dispatch.ts - 한 번을 이미 썼으면 복구 전송은 막습니다. 막은 뒤에 만든 502는 그대로 나갑니다. 다시 보내지 말라는 헤더(x-should-retry: false)는 없습니다. 클라이언트는 그 502를 보고 턴 전체를 다시 보낼 수 있습니다. 설명문도 이 구멍을 남겨 둔다고 적혀 있습니다. 이 직접 길을 보는 테스트는 이번 변경에 없습니다.

이 PR은 아직 초안입니다. 본문은 전체 테스트 결과를 나중에 적는다고 되어 있고, 그 결과는 아직 없습니다.

메인테이너의 판단이 필요한 지점

콤보에서, 한 번을 이미 쓴 뒤의 표시 없는 실패를 항상 429로 바꿀지 정해야 합니다. 클라이언트가 다시 보내는 번호(401, 402, 408, 409, 429, 307, 308, 413, 500번대)만 429로 막고, 400은 원래 답을 남기는 편이 맞습니다.

직접 길의 502도 같은 거부로 막을지 정해야 합니다. 안 막으면 서버의 세 번째 전송은 멈춰도, 클라이언트가 같은 턴을 다시 보냅니다.

너의 추천

다음 서버로 넘기지 않는 방향은 유지하세요. 429 거부는 클라이언트가 다시 보내는 오류에만 쓰세요. 직접 길도 복구를 건너뛴 뒤 502 대신 같은 거부를 주세요. 초안을 풀기 전에 본문에 적힌 전체 테스트를 끝내고, 직접 길 경우를 테스트에 넣으세요.

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[github-actions]

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[FredAmartey]

Good catches, both of them. New head is 69e8cbb33, on top of fa81e5a2a.

• The combo looks at the status now. A spent grant still stops failover, but the answer goes through settleOperatorReplacement, the same rule fetchWithResetRetry already applied: anything the client would resend (401, 402, 408, 409, 429, 307, 308, 413, any 5xx) turns into the refusal, and everything else reaches the client with its own status plus the marker. There's a new case where the replacement streams a zero-output context_length_exceeded. The client gets that 400 with its message, after two sends to the first target and none to the second. Worth knowing: only hop-classified failures reach this branch at all, and a plain invalid_request_error commits the stream and is relayed untouched.
• The direct path gets the same treatment. Once the grant is spent, the preflight's projected failure is settled by that rule, so the client sees the refusal with x-should-retry: false instead of a 502. In the new case a spent replacement streams an encrypted-function-output rejection: two sends, no rebuild, refusal.
• Keeping the 400 opened one more door. The routing policy fallback only looked at status and body, so a kept overflow could have hopped to another route. shouldHopPolicyCandidate now refuses a marked answer, and routing-policy-fallback.test.ts has a case for it.
• settleOperatorReplacement comes over from fix(responses): let retryOnReset replace a Codex WebSocket send that died unanswered #5633 unchanged, so either PR can go in first and the other still merges cleanly.

Next up is a full-suite run on this head. I'll put the numbers in the body before re-ticking.

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Actionable comments posted: 2

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
In `@src/lib/request-execution-budget.ts`:
- Line 459: Update claim forwarding in ledgerFor and the Responses send-budget
and adapter forwarding views to refuse ambiguous-resend claims unless the parent
exposes ambiguousResendSpent as a boolean; keep the budget shape compatible and
add a regression test using a hand-built parent without that member.

In `@src/server/responses/passthrough-dispatch.ts`:
- Line 1581: Update reasoning-effort rejection handling so marked non-replayable
responses cannot trigger `rebuildAndRefetch`; add the `isNonReplayableResponse`
guard alongside the existing `alreadyAttempted` check in
`reasoningEffortRejectionText`.

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📒 Files selected for processing (10)

• src/lib/request-execution-budget.ts
• src/lib/upstream-retry.ts
• src/server/responses/core-combo.ts
• src/server/responses/passthrough-dispatch.ts
• src/server/responses/policy-fallback.ts
• src/server/responses/request-send-budget.ts
• structure/transports/responses-failover.md
• tests/lib/ambiguous-resend-composition.test.ts
• tests/routing/routing-policy-fallback.test.ts
• tests/server/replay-refusal-parity.test.ts

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[lidge-jun]

Carried onto dev in bundle PR #5675 (squash-merged as aed3bb8), rebuilt on current dev as commit d48eaef on the lane branch with a Co-authored-by trailer for you, so the credit stays on the merged commit. Closing this one as superseded. Thank you for the fix.