tdd

What problem does it solve? Writing tests after implementation often produces brittle, implementation-coupled tests that break during refactors and fail to verify real behavior. This Skill enforces a disciplined red-green loop so every test verifies observable behavior through public interfaces. ## 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-Based Test Design: Defines tests only at pre-agreed public boundaries, confirmed with the user before any test is written. - Anti-Pattern Detection: Identifies 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 the public checkout interface, implement just enough code to pass, and repeat per slice. ## Quick Start Use the tdd skill to build this feature test-first, starting by agreeing on the seams we should test.