What problem does it solve? Writing tests after implementation often produces brittle tests coupled to internal structure that break during refactors and fail to verify real behavior. This Skill enforces a disciplined red-green TDD loop so tests act as durable specifications of behavior. ## 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: Defines tests at pre-agreed public boundaries confirmed with the user, never against internals. - Anti-Pattern Detection: Flags implementation-coupled, tautological, and horizontally sliced tests with concrete good/bad examples. - Mocking Guidance: 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 behavior test per cycle, mock only the payment API boundary, and keep tests stable through refactors. ## Quick Start Use the tdd skill to build this feature test-first, starting by agreeing on the public seams we should test.
