fix(combos): let a local input-admission refusal actually hop the fallback chain - #1864
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Included review availability: Your plan includes up to 10 reviews per rolling hour; 0 remain after this review. 📝 WalkthroughWalkthroughThe change preserves structured ChangesFallback failure classification
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to The change narrowly corrects fallback ordering for genuine input-admission refusals, with focused regression tests and type checking reported clean; no actionable merge-blocking risk remains. Sequence Diagram(s)sequenceDiagram
participant LocalProvider
participant classifyError
participant comboFailureDecision
participant NextCandidate
LocalProvider->>classifyError: return input_admission_refused code
classifyError->>comboFailureDecision: preserve classified code
comboFailureDecision->>NextCandidate: return "hop"
NextCandidate-->>LocalProvider: retry with next policy candidate
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| if (options?.code === "input_admission_refused" | ||
| || error.code === "input_admission_refused" | ||
| || message.includes("input_admission_refused")) { | ||
| return "hop"; |
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When the real input-admission gate rejects an oversized request, core.ts calls formatErrorResponse(413, "input_admission_refused", message), but formatErrorResponse does not preserve that type: classifyError sees “context window” in the message and serializes both the type and code as context_length_exceeded. Consequently, none of these admission-marker checks match and the generic stop list still terminates the fallback chain. The new test masks this by fabricating a response shape that the gate never emits; preserve an explicit admission code in the formatted response and exercise that production formatter/path in the regression test.
AGENTS.md reference: src/AGENTS.md:L24-L25
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Inline comments:
In `@tests/routing-policy-fallback.test.ts`:
- Around line 57-105: Add an end-to-end two-target fallback test in the combo
failover tests, using the existing combo test patterns and fallback entry point.
Make the first target return HTTP 413 with error code input_admission_refused,
then assert the second target is invoked and returns success; keep the test
focused on combo routing rather than direct comboFailureDecision behavior.
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src/combos/failover.tstests/routing-policy-fallback.test.ts
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…lback chain #1524. The hop rule for `input_admission_refused` existed but never fired on a real refusal. `comboFailureDecision` tested the generic stop list first, and `classifyError` maps a genuine 413 admission body to `context_length_exceeded` -- so the request hit `return "stop"` two lines before the rule that was written to hop it. The chain ended at the first candidate whose context window was too small, which is exactly the defect the issue reports. The existing test did not catch this because it passed `{"code":"..."}` as a top-level field. That shape classifies to `upstream_error`, misses the stop list, and reaches the hop rule -- so the rule looked alive while the shape the proxy actually emits (`{"error":{"type":..,"code":..}}`) was still stopping. Move the admission check above the stop list. An UPSTREAM `context_length_exceeded` carries no admission marker and still falls through to stop, because retrying that elsewhere is guesswork. This covers both fallback paths at once: policy fallback reaches the same function through `shouldHopPolicyCandidate`. Verification: two cases in tests/routing-policy-fallback.test.ts drive the real error body end to end through the fallback loop -- a local refusal advances to the next candidate, an upstream context verdict does not. Driven red by restoring the original ordering, which reproduces the terminated chain. Note the weaker ablation that also had to be ruled out: disabling only the structured-code arm still passes, because the `message.includes` fallback catches it; ORDER is the load-bearing change. 46 tests green across policy fallback, combos and e2e-style; `bun x tsc --noEmit` clean.
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…e can see it Adversarial pre-merge audit found the previous commit did not actually fix #1524: reordering comboFailureDecision was necessary but not sufficient. core.ts emits the refusal through formatErrorResponse(413, "input_admission_refused", ...), which runs classifyError -- and the message necessarily contains "context window", because that is what it is refusing on. The generic remap at errors.ts rewrote our own code to context_length_exceeded, so the envelope the proxy actually ships carried no admission marker at all. Verified against the real emitter: code came back context_length_exceeded and the decision was still "stop". Preserve the code: classifyError now returns input_admission_refused when that is the type it was given, before the "context window" remap can claim it. The two verdicts need opposite fallback handling -- ours means "this candidate does not fit", an upstream one means "the request is impossible" -- so collapsing them is what ended the chain at the first candidate that was merely too small. Also narrow the hop rule to the structured code. With the code preserved the raw-substring arm is no longer load-bearing, and it was a real hazard: error text is provider-controlled, so any upstream echoing the token could override a terminal verdict. Confirmed reachable before this change. The test fixture was the reason the gap survived review: it hand-built an envelope the proxy does not emit. It now calls formatErrorResponse itself, and a new case pins that an upstream merely mentioning the marker does not hop. Ablations, both driven red independently: disabling the classifyError branch fails, and restoring the original ordering fails. Neither change alone is sufficient, which is why the earlier single-factor ablation was misleading. 102 tests green across policy fallback, combos and bridge; tsc --noEmit clean.
…the hop rule's real provenance Two audit rounds corrected this unit, and the correction is more useful than the conclusion. The earlier "Implementation outcome" section claimed the context-window half of #1524 was closed by the input_admission_refused hop code. It was not: core.ts emits that refusal through formatErrorResponse, which runs classifyError, and the message necessarily says "context window" because that is what it refuses on. The generic remap rewrote our own code to context_length_exceeded, so the shipped envelope carried no admission marker. Reordering comboFailureDecision was necessary but changed nothing alone. That survived review because the test fixture hand-built an envelope the proxy does not emit -- a green test proving a shape that never reaches production. Also records the reviewer's provenance caveat: calling this a "local" code overstates it. Policy fallback reads error.code from any response and combo reads the upstream nested code, so an upstream that deliberately emits the code induces a hop. Bounded rather than dangerous -- upstreams already control 429 and 5xx, and traversal is finite via the tried-set and combo exclusions -- so the accurate description is structured-code-only, not provably local. The code comment in failover.ts now says exactly that. Line references refreshed to bc6019b.
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Summary
Fixes the reported behavior in #1524.
The hop rule for
input_admission_refusedwas added earlier but never fired on a real refusal.comboFailureDecisiontested the generic stop list first, andclassifyErrormaps a genuine 413 admission body tocontext_length_exceeded— so the request hitreturn "stop"two lines before the rule written to hop it. The chain ended at the first candidate whose context window was too small, which is precisely the dead end the issue describes.The existing test did not catch it because it passed
{"code":"input_admission_refused"}as a top-level field. That shape classifies toupstream_error, misses the stop list, and reaches the hop rule — so the rule looked alive while the shape the proxy actually emits kept stopping. Both shapes are now covered.The fix is ordering: the admission check moves above the stop list. An upstream
context_length_exceededcarries no admission marker and still falls through to stop, because retrying that elsewhere is guesswork.This covers both fallback paths in one place — policy fallback reaches the same function through
shouldHopPolicyCandidate(src/server/responses/policy-fallback.ts:77).Verification
tests/routing-policy-fallback.test.tsdrive the real error body end to end through the fallback loop: a local refusal advances to the next candidate, an upstream context verdict does not.Expected: 200, Received: 413).message.includesfallback catches it. Ordering is the load-bearing change, and the ordering ablation is the one that fails.bun test --isolate tests/routing-policy-fallback.test.ts tests/combos.test.ts tests/e2e-style/— 46 pass, 0 fail.bun x tsc --noEmit— clean.Checklist
devbun x tsc --noEmitcleanScope note
The issue's other class — input modality — already works and needed no change.
evidenceFromBodysetsimageInputRequired(src/routing/request-evidence.ts:43), the evaluator turns it into a per-candidaterequest-image-inputrequirement (src/routing/evaluator.ts:209), andrankPolicyFallbackCandidatesonly considers candidates witheligible === trueand zero exclusions, so an image request cannot hop onto a text-only candidate. Verified by reading the evaluation path rather than assumed.One optimization remains open and is deliberately not in this PR: request context size is still
unknowninPolicyRequestEvidence, so the initial policy evaluation cannot pre-exclude an oversized candidate — it is discovered at admission and then hopped. Correct, but it walks candidates one refusal at a time instead of ranking only those that fit. Closing it needs a model-independent size estimate compared at the sameADMISSION_TOLERANCE = 2.5the admission gate uses; a stricter threshold at evaluation time would refuse candidates admission would have accepted.Summary by CodeRabbit
Bug Fixes
Tests