api-and-interface-design

What problem does it solve? Poorly designed interfaces create breaking changes, inconsistent error handling, and hidden dependencies that frustrate consumers and multiply maintenance cost. This Skill guides the design of stable, hard-to-misuse APIs and module boundaries before implementation begins. ## Core Features & Use Cases - Contract-First Design: Define typed interfaces (REST endpoints, TypeScript contracts, component props) before writing implementation code. - Consistent Error Semantics: Establish a single error format with structured codes, HTTP status mapping, and boundary-only validation. - Backward-Compatible Evolution: Apply additive-only changes, pagination from day one, discriminated unions, and branded ID types to prevent breaking consumers. - Use Case: When creating a new /api/tasks endpoint, use this Skill to define the input/output schemas, pagination shape, error format, and naming conventions so frontend and backend teams can build in parallel against a stable contract. ## Quick Start Ask the AI to design a REST API contract for a new resource, including typed inputs, outputs, error responses, and pagination, following the api-and-interface-design guidelines.