test-the-testlisted

# Test the Test ## Purpose Use this skill to write, evaluate, and verify tests that prove behavior rather than implementation details. ## Core Rule Tests prove behavior through public seams, not implementation internals. A test that breaks when you rename an internal variable without changing behavior is testing the wrong thing. A test that passes when the behavior is broken is worse than no test at all. ## Vertical Slice Discipline Write one test. Implement to pass it. Repeat. For every behavior change with testable logic, write the smallest targeted test and observe it fail for the missing behavior before changing the implementation. Do not write a broad suite for imagined internals before learning from one vertical slice. ```text WRONG (horizontal): RED: write all tests GREEN: write all implementation RIGHT (vertical): RED → GREEN: one test → its implementation → repeat ``` For a behavior change in existing code: ```text diagnose → write targeted behavior test → observe RED → implement narrow behavior → observe GREEN ``` For an extraction or refactor intended to preserve current behavior: ```text diagnose current behavior → add focused characterization test → observe PASS → move logic without changing behavior → observe PASS ``` A characterization test is not speculative: it pins behavior already present at a durable seam before logic moves. If the slice also changes behavior, characterize the current behavior first, then use a separate observed RED