awb-tddlisted

# Test-Driven Development (vertical slice) > **Announce on activation:** "Using awb-tdd — red-green-refactor in vertical slices." ## Scope - **Does:** turn a behavior list into a sequence of `failing test → minimal implementation` cycles, one behavior at a time, through the public interface. - **Does NOT:** prove correctness (a green suite only proves what its assertions check), choose *which* behaviors matter, or run your test infrastructure. It is a discipline, not a test runner. ## Slice vertically, not horizontally Writing *all* the tests then *all* the code ("horizontal") breeds tests that assert the *shape* you imagined, not the behavior a user sees — they pass when behavior breaks. Instead take one thin slice end-to-end at a time, so each test responds to what the previous cycle taught you: ``` horizontal (wrong): test1 test2 test3 → impl1 impl2 impl3 vertical (right): test1→impl1 test2→impl2 test3→impl3 ``` ## Process 1. **List behaviors, not steps.** Name the observable behaviors, pick the few that matter, confirm them. You cannot test everything — choose. 2. **Tracer bullet.** Write ONE test for the first behavior, watch it fail for the right reason (RED), then write the minimal code to pass it (GREEN). This proves the path is wired end to end. 3. **HARD GATE: prove the test actually runs.** A test file the runner doesn't recognize collects **zero** tests and still exits **green** — it looks like a passing cycle but asserts nothing. A