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 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 test boundaries (seams) with the user before any test is written. - 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 building a checkout feature, use this Skill to write one failing behavior test at a confirmed seam, implement the minimal code to pass it, and repeat until the feature is complete. ## Quick Start Use the tdd skill to build this feature test-first, starting by agreeing on the seams we should test.
