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 red-green TDD loop so tests verify observable behavior through public interfaces and survive code changes. ## Core Features & Use Cases - Red-Green Loop Rules: Enforces writing a failing test first, then only the minimal 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/bad examples. - Mocking Guidelines: Restricts mocks to system boundaries like external APIs, time, and file systems, with dependency injection patterns. - Use Case: When building a checkout feature, use this Skill to write one failing test for cart checkout behavior, implement the minimal code to pass it, and repeat slice by slice. ## Quick Start Use the tdd skill to build the new payment feature test-first, starting by agreeing on the seams we should test.
