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 and contract-first development. ## Core Features & Use Cases - Contract-First Design: Define TypeScript interfaces, REST endpoints, and error schemas before implementation, with consistent error semantics across all endpoints. - Boundary Validation Patterns: Enforce validation at system edges (API routes, forms, third-party responses) while trusting internal typed code. - Backward-Compatible Evolution: Apply additive-only changes, pagination standards, discriminated unions, and branded ID types to prevent breaking consumers. - Use Case: When creating a new REST endpoint for a task management feature, use this Skill to define the typed input/output contract, standard error shape, pagination scheme, and naming conventions before writing any handler code. ## Quick Start Use the api-and-interface-design skill to design a versioned REST API contract with typed inputs, outputs, and error responses for a new tasks resource.
