Implement or extend production behavior test-first, or diagnose focused unit-test quality and flakiness, using red-green-refactor, ZOMBIES, Tidy First?, FIRST microtests, and Save Your Game checkpoints. Use for TDD, microtests, test-first bug fixes, flaky tests, and refactoring within an active green cycle. Do not use to choose delivery slices, format user stories, conduct a broad representation review, or perform a focused identifier-naming analysis; those belong to the corresponding sibling skills.
