An empty AI completion was returned as a successful answer

[catomean]

completeWithOpenAICompat ended with:

return body.choices?.[0]?.message?.content ?? ''

So a response carrying content: null came back as the empty string — a success. Three things followed, none of them visible:

• withProviderFallback called recordAIHealthSuccess(), so /api/health reported the AI chain ok on a request that produced nothing.
• The chain never fell through, so the second provider was never asked. The fallback existed and did not engage on the one failure it couldn't see.
• The caller received '' and had to invent its own meaning for it.

How it happens

Not exotic: a reasoning model spends the entire max_tokens budget on reasoning tokens and stops with finish_reason: "length" before emitting any content. Observed on an OpenRouter auto-routed model at a small budget.

The app's real budgets (2000 for form fill) are far from that today — but which model sits behind a provider is not ours to pin forever, and this repo has already been bitten by model ids rotating underneath it.

Fix

An empty completion is now an AIProviderError with status 502, chosen deliberately: shouldTryNextProvider falls through on >= 500, so an unusable response from one vendor gets retried at the next — which is the entire point of having a chain.

finish_reason rides on the error body so the log says why it was empty, following the existing rule that the body is for logs and never for a browser.

Why it matters beyond this line

This is the shape this codebase keeps meeting: absence read as an answer. A health signal reporting ok while returning nothing is the same failure as a green gate that tests nothing — the instrument agrees with you and measures the wrong thing.

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Mutation-proven: deleting the guard fails six tests. npm run verify 200 suites / 3524 passed, and verified with a real next build (exit 0) — the gate verify explicitly does not cover, now runnable per #146.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Cd183M6472xBgTKWA2is6h