agent-skills-test-driven-development

Write a test that reproduces the bug first and confirm it fails, then implement the fix and confirm the test passes. This Prove-It Pattern guarantees the bug existed, the fix works, and a regression guard remains in the suite.

RED means writing a failing test for the desired behavior, GREEN means writing the minimal code to make it pass, and REFACTOR means cleaning up the implementation while keeping tests green. The cycle repeats for each new behavior.

Prefer real implementations first, then fakes, stubs, and finally interaction mocks as a last resort. Mock only when the real dependency is too slow, non-deterministic, or has uncontrollable side effects like external APIs or email sending.

Skip tests only for pure configuration changes, documentation updates, or static content with no behavioral impact. Any change to logic, behavior, or edge case handling should be covered by a test.

Tests that assert on internal method calls or implementation details break during refactoring even when behavior is unchanged. Assert on inputs and outputs (state-based testing) instead of interaction sequences.