An API is a contract—not only with compilers, but with every client in the wild you cannot force-update instantly. Full stack engineers feel this acutely when mobile users linger on old binaries or when third parties ingest your endpoints.
Prefer additive change
Breaking changes should be rare, announced, and measurable. Additive fields, parallel endpoints during migration windows, and explicit deprecation headers reduce drama. Document error envelopes so clients can distinguish retryable failures from programmer mistakes.
Operational guarantees are part of the contract
Rate limits, payload caps, and authentication lifetimes belong in the same mental model as schema. Clients that cache aggressively need clarity on consistency. When infrastructure throttles traffic, the API layer should degrade gracefully with honest status codes.