What problem does it solve? Poorly designed interfaces create breaking changes, inconsistent error handling, and duplicate side effects that erode consumer trust. 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 TypeScript interfaces, discriminated unions, and branded ID types before writing implementation code. - REST API Patterns: Apply conventions for resource naming, pagination, filtering, PATCH partial updates, and consistent structured error responses. - Idempotency Implementation: Honour Idempotency-Key headers with atomic claims via unique constraints, payload hash guards, and deliberate in-flight duplicate strategies. - Use Case: When adding a payments endpoint, use this Skill to derive a stable idempotency key from the order ID, claim it atomically against a unique constraint, and reject reused keys with mismatched payloads. ## Quick Start Use the api-and-interface-design skill to review my new REST endpoint design for consistency, pagination, and idempotency handling.
