OpenAI 'Invalid schema for response_format' with strict structured outputs: how do I fix additionalProperties false, required fields and unsupported keywords?

Cause: With Structured Outputs (text.format / response_format of type json_schema with strict: true) the schema must follow a restricted subset of JSON Schema. A schema that breaks a rule is rejected with an invalid-schema error when strict is on. The rules in OpenAI's docs: the root must be an object (a top-level anyOf, as Zod produces for discriminated unions, is invalid); every object needs additionalProperties: false; every property must be listed in required; allOf, not, if/then/else, dependentRequired and dependentSchemas are unsupported; and there are size limits (up to 5,000 properties in total, 10 levels of nesting, 1,000 enum values in total). Fix (best first): 1. On every object in the schema, including nested ones and those inside $defs and array items, set "additionalProperties": false. 2. List every property of each object in "required". To make a field optional, keep it required but allow null, for example "type": ["string", "null"]. 3. Wrap a top-level union in an object with one property that holds the anyOf. 4. Replace unsupported keywords with supported ones, or validate those constraints in your own code after parsing. Supported types include string, number, integer, boolean, object, array, enum and anyOf; $defs/$ref and recursion are supported. 5. Prefer generating the schema from Pydantic or Zod via the official SDK helper, which applies these rules, and then inspect the output. 6. Handle non-schema outcomes at runtime: a refusal arrives as a refusal content part on a completed response, and status "incomplete" with reason max_output_tokens can cut the JSON off. Check both before parsing. Verify: The request with strict: true succeeds, and the parsed output validates against your model; a refusal or incomplete response is handled without a parse exception. Sources: https://developers.openai.com/api/docs/guides/structured-outputs https://developers.openai.com/api/docs/guides/function-calling