After this lesson, you will be able to: Evolve an API without breaking existing clients: additive changes, deprecation strategy, version sunsets.
Real APIs ship for years. Breaking changes lose customers. This lesson is how to make APIs evolve safely.
Renaming or removing a field. Changing a field's type (string → number). Adding a required parameter. Changing default behavior of an endpoint. Removing an endpoint. Tightening validation that previously accepted some inputs.
Adding a new optional field to a response. Adding a new endpoint. Adding a new optional query/body parameter (with a default). Loosening validation. These don't need a version bump. Clients that don't care just ignore them.
Standard headers + docs warning.
// On the response of a deprecated endpoint:Deprecation: true // RFC 9745Sunset: Wed, 31 Dec 2026 23:59:59 GMT // RFC 8594Link: <https://api.example.com/docs/v2>; rel="successor-version"Warning: 299 - "This endpoint is deprecated; use /v2/users"// In your docs: mark deprecated endpoints + give migration steps.// In your changelog: announce deprecation 6-12 months before sunset.
Parallel versions: run /v1 + /v2 simultaneously for 6-12 months. Clients migrate when they can. Feature flags: gate the new behavior behind a header (X-API-Beta: my-feature) for early adopters. GraphQL-style additive evolution: never break, only add. Older clients keep working forever, but you accumulate cruft. Big-bang migration: only if you control all clients (internal API).
Treating ALL renames as 'just a doc update' (it's a breaking change). No deprecation notice (clients break on a Tuesday). Bumping v1 → v2 for an additive change (forces clients to migrate for no reason). Killing v1 without telling anyone (= losing customers).
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