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-first workflow so tests verify real behavior through public interfaces and survive code changes. ## Core Features & Use Cases - Red-Green Loop Discipline: Enforces writing one failing test before each minimal implementation, one vertical slice at a time. - Seam Identification: Helps you agree on public interface boundaries (seams) 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 Guidance: Defines when to mock (system boundaries only) and how to design mockable interfaces via dependency injection and SDK-style APIs. - Use Case: When adding a checkout feature, use this Skill to define the seams, write a failing test like "user can checkout with valid cart", implement just enough code to pass, and repeat for each behavior. ## Quick Start Use the tdd skill to build this feature test-first, starting by agreeing on the seams we should test.