Problem

OpenRouter sometimes returns HTTP 404 with a JSON body like:

{"error":{"message":"Healer Alpha was a stealth model revealed on March 18th as an early testing version of MiMo-V2-Omni. Find it here: https://openrouter.ai/xiaomi/mimo-v2-omni","code":404},"user_id":"user_33GTyP8uDSYYbaeBO48AGHXyuMC"}

Even though this means the requested model is unavailable, fallback selection did not treat it as model_not_found.

Repro

1. Call OpenRouter with model: "openrouter/healer-alpha".
2. Receive the JSON 404 payload above.
3. Observe that model fallback stays on the failing candidate instead of continuing to the fallback chain.

Expected

A JSON-wrapped 404 model error should trigger model fallback the same way a plain model not found error does.

Actual

The fallback classifier missed the error because it only recognized 404 when paired with not found text, and OpenRouter's payload uses a different message.

Evidence

• Repro via curl -v against https://openrouter.ai/api/v1/chat/completions
• Local regression tests added for the JSON 404 payload