OpenAPI
Standard för att beskriva REST-API:er. Forken från Swagger 2.0 → donated till Linux Foundation 2015 → OpenAPI Specification (OAS). v3.0 (2017), v3.1 (2021, JSON Schema-compatible).
YAML eller JSON. Beskriver: paths, methods, params, request/response schemas, authentication, examples. Drives auto-genererade: docs (Swagger UI, Redoc), klient-SDKs (openapi-generator), server-stubs, mock servers (Prism), testing (Schemathesis). Spring (springdoc-openapi), FastAPI (Python, native), NestJS, .NET, Go-frameworks alla auto-genererar OAS från koden. Konkurrenter: gRPC + Protobuf (binary, RPC-style), GraphQL (single endpoint), AsyncAPI (för messaging/event-driven).