Api Compat

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name: api-compat description: "Change public APIs without breaking clients—versioning schemes, additive vs breaking changes, deprecation windows, and comms. Use when shipping breaking changes, sunsetting fields, or coordinating mobile/web SDK consumers."

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API Compatibility

Own the lifecycle of a public API: who breaks when you ship, how long old behavior lives, and how clients discover what’s next. Pair with http-api for how requests look today; this skill is about time and promises.

When to use

Adding/removing fields, routes, or semantics that existing clients rely on.

Choosing URL vs header vs package versioning—or when no formal version and only additive JSON.

Deprecation: timelines, metrics (who still calls old paths?), and removal gates.

Core ideas

Additive first — nullable new fields beat silent behavior changes.

Explicit contracts — integration tests or consumer-driven checks for critical partners.

Communicate — changelog, developer email, in-response Sunset / warnings where standards allow.

Avoid

“We’ll just bump the version” without a migration story for slow-moving apps.

Breaking auth or pagination with no coordination window.

Deprecating without usage data—you’ll cut traffic you didn’t know existed.

Checklist before breaking

[ ] Who is affected (internal only vs third parties)?

[ ] Minimum notice period and rollback if telemetry spikes errors?

[ ] Docs + examples updated before the flag day?

Done when

Old and new behaviors are measurable; removal is gated on evidence, not hope.