API Versioning and Contract Testing: Safe Changes, OpenAPI in CI, Pact, and Retiring Endpoints with Sunset

A practical guide to changing your APIs without breaking the people who consume them: why compatibility rules invert between the request and the response —and why widening a returned enum breaks clients even though you are "only adding"—, the tolerant reader pattern in Pydantic with an escape hatch and a metric, OpenAPI generated from the code and diffed on every pull request with oasdiff (including the git diff --exit-code step without which the whole check is theatre), consumer-driven contract testing with Pact: type matchers instead of literal values, well-designed provider states, version selection with deployed_or_released, and the gate that actually makes it safe, can-i-deploy paired with record-deployment. It also covers rolling this out without stopping the factory using pending and WIP pacts, bi-directional contracts when the provider is a third party, the three versioning strategies with their real operational costs, why versioning the whole API for a single endpoint guarantees nobody migrates, retiring versions with the Deprecation (RFC 9745) and Sunset (RFC 8594) headers plus the migration Link, the per-consumer metric without which no sunset date is ever met, brownouts returning 410 Gone before the final shutdown, and the BACKWARD, FORWARD and FULL compatibility modes for event schemas. With production-ready code in Python, YAML, Bash and PromQL, eight recurring mistakes, a production checklist, FAQ and glossary. It also extends the contract beyond the happy path: errors with RFC 9457 (problem+json), cursor pagination and defaults as part of the contract, the expand/contract pattern for renaming a field across database and API with no maintenance window, governance with Spectral, buf breaking for gRPC and Protobuf, GraphQL schema evolution with @deprecated and real per-field usage, and semantic versioning of generated SDKs.