What problem does it solve? Poorly designed interfaces create breaking changes, inconsistent error handling, and hidden dependencies that break consumers. This Skill provides concrete principles and patterns for designing APIs and module boundaries that remain stable as systems evolve. ## Core Features & Use Cases - Contract-First Design: Define typed interfaces before implementation, with consistent error semantics, boundary validation, and predictable naming conventions. - Idempotency Implementation: Detailed guidance on honouring idempotency keys, including atomic key claiming, payload guards, in-flight duplicate handling, and retention policies. - REST and TypeScript Patterns: Covers resource design, pagination, filtering, PATCH semantics, discriminated unions, input/output separation, and branded ID types. - Use Case: When designing a new tasks API, apply this Skill to define the endpoint contracts, error response shape, pagination scheme, and idempotency strategy before writing any implementation code. ## Quick Start Ask the AI to design a REST API for a new resource following the api-and-interface-design principles, including typed contracts and error handling.
