tdd

What problem does it solve? Writing tests after implementation often produces brittle tests coupled to internal structure that break on every refactor. This Skill enforces a disciplined test-driven development workflow so tests verify behavior through public interfaces and survive code changes. ## Core Features & Use Cases - Red-Green Loop Rules: Enforces writing a failing test first, then only enough code to pass it, one vertical slice at a time. - Seam Identification: Requires agreeing on public interface seams with the user before any test is written, focusing effort on critical paths. - Anti-Pattern Detection: Flags implementation-coupled, tautological, and horizontally-sliced tests with concrete good and bad examples. - Mocking Guidelines: Provides rules for mocking only at system boundaries like external APIs, time, and file systems, with dependency injection patterns. - Use Case: When adding a checkout feature, use this Skill to write a failing integration test like "user can checkout with valid cart", implement the minimal code to pass, and repeat slice by slice. ## Quick Start Ask the assistant to build the next feature or bug fix using test-driven development with the red-green loop.