What problem does it solve? Poorly designed interfaces create breaking changes, inconsistent error handling, and hidden dependencies that break consumers. This Skill guides the design of stable APIs and module boundaries that are hard to misuse, applying principles like Hyrum's Law, contract-first design, and the One-Version Rule. ## Core Features & Use Cases - Contract-First Design: Define typed interfaces before implementation, with input/output separation, discriminated unions, and branded ID types in TypeScript. - REST API Patterns: Resource naming conventions, pagination, filtering, partial updates via PATCH, and consistent error response shapes with proper HTTP status codes. - Idempotency Implementation: Correct handling of Idempotency-Key headers, including atomic key claiming via unique constraints, payload guarding, in-flight duplicate strategies, and retention planning. - Use Case: When designing a new tasks API, use this Skill to define the endpoint contracts, error format, pagination scheme, and idempotency behavior before writing any implementation code. ## Quick Start Ask the agent to design a REST API for a new resource, specifying the endpoints, error format, and pagination approach following the api-and-interface-design guidelines.
