Test-driven development for Drupal with PHPUnit and Drupal Test Traits (DTT). Use when writing or fixing tests, deciding what is worth a test, reproducing a bug before fixing it, choosing a bootstrap level (Unit vs Kernel vs ExistingSite/functional), testing permission gates, adding Playwright visual verification, or debugging tests that silently run zero assertions. Covers the bug-fix RED-first discipline, the worth-a-test rubric, bootstrap cost tradeoffs, negative tests, the anonymous-403 permission trap, DTT gotchas, and the PHPUnit-version pin that makes Drupal tests pass vacuously.
