OpenAPI Specification (OAS 3.x) is a standard for describing REST APIs in JSON/YAML. Benefits: interactive documentation (Swagger UI, Redoc), client SDK generation (openapi-generator), server stub generation, request/response validation (middleware), and contract testing. Schema-first approach: define the OpenAPI spec before implementing. This enables frontend teams to work against mock servers while the backend is being built.
Each stage in order — click any step to read what it does.
What you can generate/do from one OpenAPI spec file.
The trade-offs worth knowing before you build this.
API design decisions are cheapest to change in a YAML file. Once endpoints are in production with clients depending on them, changes are expensive. Spec-first enables parallel frontend/backend with mock servers.
Hand-written API clients go stale immediately. openapi-generator generates fully typed clients in any language from the live spec. Regenerate on every spec change — always in sync.
Documentation says field X is required but code makes it optional. Schemathesis automatically tests your API against the spec and catches these inconsistencies. Run in CI on every PR.
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