test-verification

test-verification/SKILL.md

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.

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:

diagnose → write targeted behavior test → observe RED → implement narrow behavior → observe GREEN

For an extraction or refactor intended to preserve current behavior:

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 for the behavior change.

Test Quality Checklist

Before accepting a test as complete:

[ ] Test describes behavior, not implementation details
[ ] Test uses a public seam: route, server action, query helper, or pure helper interface
[ ] Test would survive an internal refactor without changing
[ ] Code written is the minimum to pass the current test
[ ] No broad test suite speculates about unverified internal structure
[ ] Test covers at least one failure/rejection case, not just the happy path
[ ] For high-risk seams: failure path, wrong input, unauthorized access are covered