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: Standard conventions for resource naming, pagination, filtering, partial updates (PATCH), and consistent error response shapes with proper HTTP status codes. - Idempotency Implementation: Detailed guidance on idempotency keys, atomic claiming via unique constraints, payload guarding, in-flight duplicate handling, and retention windows. - Use Case: When creating a new REST endpoint for a task management service, use this Skill to define the typed contract, error format, pagination scheme, and idempotency behavior before writing any handler code. ## Quick Start Use the api-and-interface-design skill to review my new REST endpoint design for consistency, pagination, and idempotency.
