What problem does it solve? Code written before tests produces unverified behavior, biased tests written after the fact, and regressions that slip into production. This Skill enforces a strict test-first discipline so every behavior change is proven by a failing test before implementation exists. ## Core Features & Use Cases - Red-Green-Refactor Cycle: Write one failing test, verify it fails for the right reason, write minimal code to pass, then refactor while staying green. - Test Design Strategies: Apply black-box techniques (boundary value analysis, equivalence partitioning, error guessing) and white-box techniques (branch, path, and condition coverage) to design tests that find errors rather than confirm success. - Rationalization Defense: Built-in counterarguments for common excuses like "too simple to test" or "I'll test after", plus a verification checklist covering mutation score gates, docstring requirements, and integration testing for external dependencies. - Use Case: When fixing a bug where empty emails are accepted, write a failing test asserting the rejection, watch it fail, implement the minimal validation, and confirm the suite passes before committing. ## Quick Start Ask the AI to implement a new feature or bugfix using strict test-driven development with a failing test written and verified first.
