fix(combos): say a turn was refused locally when no combo target can fit it

[vadymhimself]

Summary

When every target of a combo is refused by the local input-admission gate (checkComboTargetInputAdmission, #1524), nothing is sent upstream. Today the client still gets the provider wording, "The provider rejected this turn because its input exceeds the provider size or context limit…", and the usage ledger logs every attempt as http_413. Neither one mentions that the proxy refused the turn locally, or what the estimate and window were.

Incident behind this: one Claude Code conversation picked up a single ~4.9M-character message, so its estimated input (~1.79M tokens) went past every target window in a two-target failover combo (1M and 272k). Each turn was refused locally for both targets with send count 0. The client error and the ledger both read like an upstream rejection. Operators blamed a recently upgraded gateway build and rolled it back. The refusal was correct, and the rollback changed nothing.

This change:

• src/server/responses/core-combo.ts: an attempt whose failure is input_admission_refused with sendCount === 0 now keeps that code as its ledger errorCode (not http_413). When an exhausted combo is mapped to the overflow envelope and every failed target was such a refusal, the final row also gets input_admission_refused. The message is proxy-owned and includes each target's estimate and window:
  opencodex refused this turn locally; nothing was sent upstream. No target of combo <id> can fit its input: Estimated input (~5000 tokens) plus 500 tokens of requested output headroom cannot fit the context window of kimi-k3 (1000 tokens). … Compact the conversation before retrying.
• src/server/responses/context-overflow.ts: jsonContextOverflowResponse / streamingContextOverflowResponse take an optional message. It defaults to PROVIDER_INPUT_TOO_LARGE_MESSAGE, so every other caller is unchanged.
• structure/transports/responses-failover.md (Combo output headroom): documents the rule. The edit fits the 600-line budget.

Unchanged:

• The wire shape: HTTP 413 JSON, or response.failed for streams, still uses type: invalid_request_error and code: context_length_exceeded.
• Status codes, retry behavior, and the [Bug]: preflight fallback candidates for context-window and input-modality compatibility #1524 hop. A local refusal still lets the ladder continue.
• Claude Code compaction. The Messages lane still turns context_length_exceeded into a prompt is too long: … envelope (src/claude/outbound.ts, not touched), so reactive compaction triggers as before.
• Mixed combos. If any target reached a provider, the combo keeps today's provider wording.

Verification

• Red before the fix: new tests/responses/responses-context-overflow.test.ts cases failed 3/3. The all-local combo returned PROVIDER_INPUT_TOO_LARGE_MESSAGE, and attempt codes were ["http_413", "http_413"].
• Green after: bun test tests/responses/responses-context-overflow.test.ts passed 22/22.
  • The all-local case (stream and non-stream) checks the local wording, ~N tokens, both windows (1000, 2000), 0 upstream hits, attempts [413, sendCount 0, input_admission_refused] ×2, and final logCtx.errorCode === "input_admission_refused".
  • The mixed case checks that one local refusal plus one target reaching a provider keeps PROVIDER_INPUT_TOO_LARGE_MESSAGE, gives attempt codes ["input_admission_refused", "http_413"], and sets no final override.
  • The existing "a combo stops on 413" and "an exhausted combo classifies a hopping 413" cases still assert the exact provider wording for genuine upstream 413s.
• Focused, related files: 373 pass / 0 fail across responses-context-overflow, responses-combo-exhausted-error, server-combo-failover-e2e, input-admission, claude-prompt-too-long, router-combo-failover-classification, and routing-policy-fallback.
• bun run typecheck: clean.
• bun run structure:check: passed.
• bun run privacy:scan: passed.
• file-size-ratchet, structure-ssot, test-layout: 62 pass.
• bun run test:changed (Bun 1.4.3): 14636 pass / 35 skip / 1 fail. The failure is connected-client runtime probe scope > a rejected preferred runtime falls back without rewriting the saved selection (tests/cli/cli-connect-readiness.test.ts). It also fails on a clean upstream/dev checkout and is unrelated to this change.
• Full bun run test was not run locally. CI covers it.

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.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Combo requests refused locally by every target now return a clear context-limit message with each target’s estimated input size and context window, for both streaming and standard responses.
  • When a target reaches a provider, the response continues to show the provider’s context-limit message.

Review readiness checklist

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

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[vadymhimself]

@coderabbitai review

[coderabbitai]

📝 Walkthrough

Combo responses now distinguish targets refused locally before sending from targets that reach a provider. When every target is refused locally, the exhausted response uses proxy-owned wording and retains input-admission refusal details. Otherwise, it preserves provider overflow wording.

Changes: Combo overflow handling

Layer / File(s)	Summary
Custom context-overflow responses src/server/responses/context-overflow.ts	The JSON and streaming response functions accept an optional message. They use the existing provider-size message when no message is supplied.
Combo refusal tracking and response selection src/server/responses/core-combo.ts, structure/transports/responses-failover.md, tests/responses/responses-context-overflow.test.ts	Combo execution records zero-send input-admission refusals and selects a local message when all targets are refused. Tests cover local-only refusals and a later provider attempt. The transport documentation describes the response behavior.

Priority: ⬇️ Low

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

Change: Bug fix

Suggested reviewers: lidge-jun, ingwannu

Merge Risk: 🔵 Low · up to 5b387

Some exhausted combos may incorrectly say that no target can fit the input. Narrow the wording before merge, or accept this limited messaging risk.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 3 files. (1 skipped: 1…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
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		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: reporting that combo requests were refused locally when no target can accept the turn.

Full details: Docstring Coverage

Explanation

Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 3 files. (1 skipped: 1 unsupported.)

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Actionable comments posted: 1

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Inline comments:
Review comments at @src/server/responses/core-combo.ts:
- Line 1239: Update the refusal message in the localAdmissionRefusals response
path to say that none of the attempted targets fit, rather than claiming no
target in the combo can fit when cooldown-unavailable targets were not checked.
Add a regression case with one target refused at local admission and another
unavailable because of cooldown.

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📥 Commits

Reviewing files that changed from the base of the PR and between 0b13396 and 5b387c5.

📒 Files selected for processing: 4

• src/server/responses/context-overflow.ts
• src/server/responses/core-combo.ts
• structure/transports/responses-failover.md
• tests/responses/responses-context-overflow.test.ts

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Review readiness checklist

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• ⬜ 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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[lidge-jun]

Being carried into #7224 with your authorship preserved; this PR will be closed when the bundle lands.

[lidge-jun]

Landed on dev in 6ccb47a via #7224 (your change is its own commit in that bundle, and the squash keeps your authorship as Co-authored-by). Thank you!