Test-Driven Development

Philosophy

Tests verify behavior through public interfaces, not implementation details. Good tests read like a spec and survive refactors; bad tests mock internals and break without a behavior change. See tests.md and mocking.md.

Anti-Pattern: Horizontal Slices

DO NOT write all tests first, then all implementation ("horizontal slicing") -- it produces tests that check imagined shape, not real behavior. Correct approach: vertical slices via tracer bullets -- one test, one implementation, repeat.

WRONG (horizontal):
  RED:   test1, test2, test3, test4, test5
  GREEN: impl1, impl2, impl3, impl4, impl5