OpenAI Structured Outputs metaschema

OpenAI Structured Outputs ensure that model outputs conform to a JSON schema provided by the client, but it supports only a subset of JSON schemas. This repo offers a metaschema that validates a schema's conformance to many of OpenAI's constraints.

Usage

Important

The metaschema is built on top of the 2019-09 JSON Schema spec. You may need to set up your validator appropriately, for example import Ajv from "ajv/dist/2019"; rather than import Ajv from "ajv";

Depending on your validator of choice, you will probably need to do the following:

• Set your JSON schema's $schema to "https://pdavies.co.uk/openai-structured-outputs-schema/2019-09/schema"
• Add the https://pdavies.co.uk/openai-structured-outputs-schema/2019-09/schema metaschema to your validator, e.g. ajv.addMetaSchema(...)
• Also add https://pdavies.co.uk/openai-structured-outputs-schema/2019-09/innerSchema to your validator

Validated Constraints

Constraint	Validated	Notes
Root objects must not be anyOf	✅
All fields must be required	⚠️	Only validates that required exists and is non-empty
Capped object count, nesting depth, total string size, enum size	❌
additionalProperties must be set and false	✅
String constraints not supported (minLength, maxLength, pattern, format)	✅	Rejected by metaschema. (OpenAI rejects when strict: true.)
Number constraints not supported (minimum, maximum, multipleOf)	✅	As above
Object constraints not supported (patternProperties, unevaluatedProperties, propertyNames, minProperties, maxProperties)	✅	As above
Array constraints not supported (unevaluatedItems, contains, minContains, maxContains, minItems, maxItems, uniqueItems)	✅	As above