A standard, language-agnostic interface description for HTTP APIs (formerly Swagger) — JSON or YAML document that describes endpoints, parameters, request/response schemas, and authentication. The contract for modern REST API design.
OpenAPI Specs serve as the single source of truth for an API: code generators produce client SDKs in dozens of languages from the spec, docs tools (Swagger UI, Redoc) render interactive documentation, and validators check requests/responses match the schema at runtime. Modern frameworks generate OpenAPI specs automatically (FastAPI from Python type hints, NestJS from decorators); legacy APIs hand-write or reverse-engineer them.
Generating TypeScript client SDKs for a Python FastAPI backend straight from the auto-generated OpenAPI spec — keeps frontend and backend types in sync.
OpenAPI is the contract that makes large API integrations tractable — without it, every consumer is reading docs and guessing; with it, code generators do the work.
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