Designing APIs - REST resource modeling, contract-first workflow, versioning, pagination, error format, idempotency, and internal service interfaces. Use whenever creating or changing any endpoint, webhook, or service-to-service interface, reviewing an OpenAPI spec, or when the user asks about REST/GraphQL/gRPC shape, status codes, or breaking changes. Trigger for every new endpoint, not just "design the API" requests - a single endpoint added carelessly is how APIs rot.
