API Versioning & Deprecation Strategy

Every growing API eventually needs breaking changes. The question is whether you plan for them or scramble when clients break.

This toolkit provides complete versioning strategies for REST and GraphQL APIs: URL path versioning, header-based versioning, query parameter versioning, and content negotiation — with trade-offs for each approach and when to use which.

Covers the full lifecycle: introducing new versions, running parallel versions, deprecation notices, sunset headers, migration guides for consumers, SDK version matrices, changelog communication, and finally sunsetting old versions safely.

Includes decision frameworks for when to version (breaking vs non-breaking changes), how to version (semver for APIs, date-based, custom schemes), deprecation timeline templates, client migration playbooks, and monitoring dashboards to track version adoption before sunset.

Practical patterns for: backward-compatible field additions, additive-only changes, API gateway routing by version, feature flags for gradual rollouts, and automated compatibility testing between versions.

Built from real-world patterns managing APIs serving thousands of clients. Every template and checklist has been battle-tested.