fix(combo): refuse first dispatch when send budget is exhausted

[oocheol]

Summary

When the shared request-send budget denied a combo's first target reservation, the combo continued into the child dispatcher anyway. An exhausted request could still reach an upstream model. The combo now returns a typed local 429 request_send_budget_exhausted before any first-target send. Denial of a later target still returns the last real upstream failure without contacting the denied target. Closes #5688.

Regression tests assert zero upstream hits on first-target denial and preserve the prior response on later denial, including a classified 413 that would otherwise be remapped after the denied hop. The English/Korean combo guides and Responses failover contract describe both outcomes.

Verification

Windows, Bun 1.4.0, isolated test homes:

$bun = '.\node_modules\@oven\bun-windows-x64\bin\bun.exe'
& $bun scripts/test.ts --parallel=1 ./tests/responses/responses-send-budget-counts.test.ts ./tests/server/server-combo-failover-e2e.test.ts
& $bun node_modules/typescript/bin/tsc --noEmit
& $bun scripts/structure-ssot.ts
& $bun scripts/privacy-scan.ts
git diff --check

Results: 196 pass, 0 fail on the final rebased code (base 0996ecb59); typecheck, structure, privacy, and diff checks passed. The docs-site build passed (505 pages).

Full-suite exception: a prior --changed=dev run on this shared Windows host selected a large import-connected set and was stopped after more than four minutes without a result. It is not passing evidence. Focused send-budget and combo end-to-end suites cover this control-flow change; the full suite and cross-platform validation remain for CI.

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.

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• src/server/responses/core-combo.ts
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📝 Walkthrough

Walkthrough

Combo execution now returns a local HTTP 429 when the shared send budget refuses the first target's reservation. When the budget refuses a later target's reservation, execution preserves the last upstream failure. Tests and documentation describe both cases.

Changes

Combo send-budget admission

Layer / File(s)	Summary
Handle refused combo reservations src/server/responses/core-combo.ts, tests/responses/responses-send-budget-counts.test.ts, docs-site/src/content/docs/guides/combos.md, docs-site/src/content/docs/ko/guides/combos.md, structure/transports/responses-failover.md	A refused first-target reservation now returns HTTP 429 with request_send_budget_exhausted before dispatch. Tests verify zero upstream requests and zero logged sends. When a later reservation is refused, execution preserves the prior upstream failure and does not contact the denied target. The documentation describes both outcomes.

Priority: ➖ Normal

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

Change: Bug fix

Suggested reviewers: lidge-jun

Merge Risk: ⚪ Minimal · up to c50c4

The combo now refuses an unbudgeted first send and preserves the upstream failure when a later target is denied. No identified issue remains that should block merging after normal checks.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The PR meets the coding requirements in issue #5688. In src/server/responses/core-combo.ts, executeComboResponses rejects a denied first-target reservation with HTTP 429 and `request_send_budget_e…
Out of Scope Changes check	✅ Passed	The changed files remain within issue #5688. The source change fixes combo send-budget admission. The regression tests verify the reported first-target and later-target failures. The documentation cha…
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 2 functions across 2 files.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: preventing the first combo dispatch when the send budget is exhausted.

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[oocheol]

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Review readiness checklist

• ✅ Required local validation passed; commands, results, and any full-suite exception are documented.
• ✅ I pushed my PR to a recent dev commit (at most 10 behind; a maintainer may still ask for the exact tip before merge).
• ✅ I resolved all correct Codex and CodeRabbit findings.
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Maintainers notified: @lidge-jun @Ingwannu

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
In `@src/server/responses/core-combo.ts`:
- Line 458: When a later reservation is denied after a classified 413 permits a
combo hop, the `hopDecision` branch falls through to proxy-owned error mapping
and loses the original response. In that branch, preserve the
`lastFailedChildLog` adoption and return `lastFailure`; add a regression case
covering a classified 413 followed by a denied second reservation.

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📥 Commits

Reviewing files that changed from the base of the PR and between 782bfb8 and 455f02f.

📒 Files selected for processing (5)

• docs-site/src/content/docs/guides/combos.md
• docs-site/src/content/docs/ko/guides/combos.md
• src/server/responses/core-combo.ts
• structure/transports/responses-failover.md
• tests/responses/responses-send-budget-counts.test.ts

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[lidge-jun]

리뷰 · 우선순위 58 / 80

이 PR은 콤보의 첫 모델이, 전송 예산을 거절당한 뒤에도 바깥으로 나가던 구멍을 막습니다. 전송 예산은 요청 하나가 모델에게 보낼 수 있는 횟수입니다. 횟수를 다 쓰면 그 요청은 더 보내면 안 됩니다.

콤보는 모델 목록을 순서대로 시도합니다. 예전 코드는 거절이 첫 대상일 때 그 거절을 건너뛰고 자식 요청을 만들었습니다. 예산이 바닥난 요청이 공급자에 닿을 수 있었습니다. 이슈 #5688이 그 버그입니다.

지금은 첫 예약이 거절되면 공급자에 요청하지 않고, 이 프로세스가 만든 429를 돌려줍니다. 코드는 request_send_budget_exhausted입니다. 두 번째 이후가 거절되면 그 대상에는 보내지 않고, 이미 받은 업스트림 실패를 그대로 돌려줍니다. 가짜 오류로 바꾸지 않습니다.

수정은 src/server/responses/core-combo.ts 한 분기입니다. 허용되면 예약을 쓰고, 첫 대상 거절이면 429로 끝나고, 그 뒤의 거절이면 마지막 실패에서 멈춥니다. 영어 안내, 한국어 안내, structure/transports/responses-failover.md가 같은 규칙을 적습니다. 테스트는 첫 거절의 업스트림 횟수가 0인지, 나중 거절이 첫 실패 문장 first target busy를 남기는지 봅니다.

기준 브랜치는 dev입니다. 커밋은 dev 끝 782bfb8 위에 하나 있고, 뒤처진 커밋은 없습니다. types.ts와 config.ts 분할과 겹치지 않습니다. #5688을 연 다른 PR은 없습니다.

이 PR은 초안입니다. 준비 체크 4칸은 모두 비어 있습니다. 본문은 Windows에서 예산 테스트와 콤보 e2e가 195건 통과했다고 적습니다. 타입 검사, 구조 검사, privacy 검사도 통과했다고 적습니다. 전체 스위트는 4분을 넘기고 끊겼고, 작성자도 그것을 통과로 치지 않습니다. 이 초안에 붙어 있는 GitHub 검사는 위생, 라벨, 대상 브랜치, CodeRabbit입니다. 테스트 작업은 없습니다.

PR 본문 준비 체크 - 4칸이 비어 있습니다. 브랜치는 이미 dev보다 뒤처지지 않습니다. 칸을 비워 두면 게이트가 초안을 유지하고, 테스트 CI는 돌지 않습니다.

src/server/responses/core-combo.ts 예약 분기 - 거절 이유는 가리지 않고 첫 대상이면 같은 429입니다. #5688이 요구한 모양과 같습니다. 나중 대상 테스트는 첫 fetch 안에서 used를 콤보 상한과 같게 올려, 총량이 바닥난 경우만 만듭니다. 전환 횟수나 관측 예산이 거절하는 길은 이 테스트에 없습니다. 코드는 그 거절도 같은 else if로 보냅니다.

메인테이너의 판단이 필요한 지점

분기 방향은 #5688과 맞습니다. 합치기 전에 준비 체크를 채우고 초안을 풀어, CI 테스트가 끝날 때까지 둘지 정하면 됩니다. 로컬 195건은 이 분기와 기존 콤보 e2e입니다. 끊긴 전체 스위트 대신으로 쓰기는 어렵습니다.

너의 추천

코드는 이 상태로 두세요. 같은 이슈의 다른 PR은 없어서 닫을 것이 없습니다. 준비 체크 4칸을 채우고 초안을 푼 다음, CI 테스트가 통과하면 합치세요. 전체 스위트를 통과로 적지는 마세요.

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[lidge-jun]

Carried into bundle #5741, which is now on dev (squash-merged as 893c81c) with a Co-authored-by trailer for you, so this PR is closing as landed. Thank you for the fix. If something from this branch did not make it in, the bundle description lists what was changed during the carry.