tdd

What problem does it solve? Writing tests after implementation often produces brittle, implementation-coupled tests that break on refactors and fail to verify real behavior. This Skill enforces a disciplined red-green TDD loop so tests act as durable specifications of public 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 only at pre-agreed public boundaries, confirmed 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 Guidelines: Restricts mocks to system boundaries like external APIs, time, and file systems, with dependency-injection design patterns. - Use Case: When adding a checkout feature, use this Skill to agree on the public seams, write one failing behavior test, implement the minimal code to pass, and repeat slice by slice. ## Quick Start Use the tdd skill to build this feature test-first, starting by agreeing on the seams we should test.