OpenAPI types: a guide to data types and formats (August 2026)

OpenAPI type declarations function as instructions for code generators as well as documentation, making schema precision central to the quality and safety of generated SDKs. The discussion explains that OpenAPI 3.0 uses a modified JSON Schema subset, while 3.1 adopts JSON Schema 2020-12 and replaces features such as `nullable` with explicit null types, adds `const`, and changes several constraint syntaxes. It emphasizes modeling known object properties, applying recognized formats to strings and numbers, avoiding JSON numeric `int64` identifiers in JavaScript clients, and distinguishing optional, nullable, and defaulted fields, particularly for PATCH operations. Enums provide native typed values but can make newly added server values breaking for older clients, while `oneOf` with a discriminator is presented as the most reliable approach for type-safe polymorphic SDK models; `anyOf` commonly produces loose types and `allOf` represents intersection-based composition. Validation constraints, request/response access controls, named reusable schemas, typed error responses, and careful money modeling can further improve generated clients. Fern is described as supporting OpenAPI 3.0 and 3.1, preserving discriminators, handling PATCH field states, and adding extensions for enum metadata and type naming, with the overall recommendation that API authors evaluate schemas by their generated developer experience rather than validator or linter compliance alone.

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