We run a document extraction pipeline on Gemini with a native responseSchema attached, not a "please reply with JSON" instruction in the prompt text. Over two months, three separate production problems traced back to behaviours of that schema that are not in Google's documentation. These are the rules we ship with now, and the measurement behind each one. The domain is anonymized (no client, no industry, role codes renamed). Every number, date, model name and error string is real. ...