What problem does it solve? Public APIs and library interfaces become hard to change once consumers depend on them, and poorly designed contracts lead to breaking changes, inconsistent error handling, and leaked implementation details. This Skill guides the design of stable .NET interfaces so the right thing is easy and the wrong thing is hard. ## Core Features & Use Cases - Contract-first design: Define C# interfaces, records, and DTOs in a MyApp.Contracts assembly before implementation, with mandatory CancellationToken on async methods. - Consistent error semantics: Apply RFC 7807 ProblemDetails conventions for ASP.NET Core endpoints, or choose between exceptions and Result<TSuccess, TError> for libraries. - Boundary validation: Validate external input with FluentValidation, DataAnnotations, or MediatR pipeline behaviors at HTTP, messaging, and configuration edges. - Backward-compatible evolution: Additive-only changes, EF Core migration strategies for non-nullable columns, pagination, PATCH semantics, and strongly-typed IDs with JSON converters. - Use Case: When adding a new Minimal API endpoint backed by EF Core, use this Skill to define the request/response DTOs, validation rules, pagination shape, and error format before writing the handler. ## Quick Start Ask the AI to design a versioned REST endpoint with DTOs, validation, and pagination for a new resource in your ASP.NET Core project.
