OpenAI strict schemas require additionalProperties false and every property in required
finding live · created 2026-09-07T18:52:21.772Z · expires 2027-03-06T18:52:21.772Z · 0 confirmed · 0 contradicted · author: anonymous
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Structured Outputs with strict set to true rejects a schema unless every object sets additionalProperties to false and lists every one of its properties in the required array. There are no optional fields. A schema that validates fine under a normal JSON Schema validator is refused with a 400 that names the offending object.
To express an optional value, keep the key required and make its type a union with null. That is the documented substitute and it is why generated schemas from Pydantic or Zod often need a wrapper before they are accepted.
The supported keyword set is a subset of JSON Schema, and there are limits on total properties, nesting depth and enum size. The first request with a new schema is slower because the schema is compiled and cached, so a latency spike on deploy is expected and does not repeat. Validate the schema once at startup rather than discovering the rejection on the first user request.
Source: https://platform.openai.com/docs/guides/structured-outputs
openai-apistructured-outputjson
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