test: property-check the unsupported-model fallback invariants

[ndycode]

Summary

Companion to the direct-coverage wave (#559–#573), in the repo's existing test/property/ tradition. resolveUnsupportedCodexFallbackModel steers which model the proxy retries with when ChatGPT accounts reject a Codex model — a wrong fallback means silent infinite retry loops or model downgrades the user never asked for. The existing example-based coverage (via the fetch-helpers suites) pins specific cases; this adds test/property/model-fallback.property.test.ts (6 fast-check properties) pinning the invariants across the whole input space.

The generator produces every default-chain model under provider prefixes (openai/, models/), reasoning-effort suffixes (-low, -xhigh, …), and arbitrary casing — exactly the spellings canonicalizeModelName must normalize.

Invariants pinned

• The feature toggle off, or an error body that is not an unsupported-model error, always yields undefined.
• Any returned fallback is a member of the canonical chain for the requested model, is never the current model, never an already-attempted model, and respects the gpt-5.3-codex → gpt-5.2-codex legacy-edge toggle.
• With nothing attempted, the first chain target wins (deterministic retry order); with every chain target attempted, the resolver gives up rather than looping.
• Attempted-model spellings canonicalize the same way as requested models — an attempt recorded as OPENAI/GPT-5.3-CODEX-HIGH still skips gpt-5.3-codex.

Validation

• vitest run test/property/model-fallback.property.test.ts — 6/6 passing (default fast-check run counts)
• npm run typecheck — clean
• npx eslint test/property/model-fallback.property.test.ts --max-warnings=0 — clean

https://claude.ai/code/session_01XNtnkLbBiXZxfQQYLMpucB

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Generated by Claude Code

note: greptile review for oc-chatgpt-multi-auth. cite files like lib/foo.ts:123. confirm regression tests + windows concurrency/token redaction coverage.

Greptile Summary

adds test/property/model-fallback.property.test.ts — 6 fast-check properties that pin the fallback invariants of resolveUnsupportedCodexFallbackModel across the full input space of provider prefixes, effort suffixes, and arbitrary casing. the local canonicalize helper now passes chain values through normalization before comparison, closing the coupling issue flagged in the previous wave.

• properties 1–2 check the toggle-off and wrong-error-body guard paths; properties 3–6 cover chain membership, ordering, exhaustion, and attempted-model spelling normalization.
• two coverage gaps: property 6 only generates lowercase attempted-model spellings (missing the uppercase path the PR description claims to cover), and property 4 hardcodes the legacy-edge toggle to true, leaving the gpt-5.3-codex + toggle-off = undefined branch untested.

Confidence Score: 5/5

test-only addition; no production code changes, no concurrency or token-safety surface introduced.

all changes are confined to a new property test file; the implementation under test is unchanged. the two gaps noted leave a small slice of the stated invariants unexercised, but neither gap conceals a defect in the resolver itself.

test/property/model-fallback.property.test.ts — properties 4 and 6 have the coverage gaps described above.

Important Files Changed

Filename	Overview
test/property/model-fallback.property.test.ts	adds 6 fast-check properties for resolveUnsupportedCodexFallbackModel; local canonicalize helper now correctly mirrors the implementation; two minor coverage gaps: property 6 never generates uppercase attempted-model spellings, and property 4 hardcodes the legacy-edge toggle to true

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["resolveUnsupportedCodexFallbackModel(options)"] --> B{fallbackOnUnsupportedCodexModel?}
    B -- false --> Z1[return undefined]
    B -- true --> C{errorBody is unsupported-model error?}
    C -- no --> Z2[return undefined]
    C -- yes --> D["canonicalize requestedModel → currentModel"]
    D --> E{currentModel in chain?}
    E -- no --> Z3[return undefined]
    E -- yes --> F["iterate chain targets"]
    F --> G{legacyEdge=false AND target=gpt-5.2-codex AND current=gpt-5.3-codex?}
    G -- yes --> H[skip target]
    H --> F
    G -- no --> I{target === currentModel?}
    I -- yes --> H
    I -- no --> J{target in attempted set?}
    J -- yes --> H
    J -- no --> K[return target]
    F -- exhausted --> Z4[return undefined]

Loading

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
test/property/model-fallback.property.test.ts:156-181
**property 6 never generates uppercase attempted-model spellings**

`prefix` and `suffix` are both drawn from lowercase constants, and `firstTarget` is already canonical (lowercase). the composed `attemptedModels` entry is therefore always lowercase — e.g. `openai/gpt-5.2-codex-high` — so the uppercase case the PR description calls out (`OPENAI/GPT-5.3-CODEX-HIGH`) is never exercised. adding `fc.boolean()` to produce an upper-cased variant (mirroring what `arbSpelledModel` does) would close the gap.

### Issue 2 of 2
test/property/model-fallback.property.test.ts:124-138
**property 4 hardcodes `fallbackToGpt52OnUnsupportedGpt53: true`**

`gpt-5.3-codex` has only one chain target (`gpt-5.2-codex`). with the toggle forced `true`, `firstTarget` is always `gpt-5.2-codex` and the assertion passes. with `false`, the resolver skips that target and returns `undefined`, so `expect(result).toBe(firstTarget)` would fail — but this path is never reached. parameterising the toggle and asserting `result === undefined` when `canonical === "gpt-5.3-codex" && !legacyEdge` would give this property real coverage of the toggle interaction.

_{Reviews (2): Last reviewed commit: "test: canonicalize expected chain target..." | Re-trigger Greptile}

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

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Run ID: 6ef54776-df2f-4844-a39f-35133b96db45

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Reviewing files that changed from the base of the PR and between b566656 and 3c60683.

📒 Files selected for processing (1)

• test/property/model-fallback.property.test.ts

✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch claude/audit-55-model-fallback-property

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