API design: the contract that outlives the code

Designing the API contract for system design: resources and standard methods, safe and idempotent promises, ETags against lost updates, status codes, offset vs keyset pagination with opaque tokens, sizing a list endpoint, backward-compatible changes and versioning, problem-details errors, idempotency keys, rate-limit headers, 202 and long-running operations, and choosing REST, gRPC or GraphQL by caller.