Writing Unit Tests

Required foundation

This skill is a focused companion to the globally installed tdd skill from mattpocock/skills.

Before using this skill:

1. Install the global tdd skill if it is not already available.
2. Read the tdd skill's reference docs: tests.md and mocking.md.
3. Use this skill as the narrower layer for writing or refining the test itself rather than running a full red-green-refactor workflow.

See README.md for install instructions and the rationale for keeping this skill separate.

Philosophy

Core principle: Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't.

Good tests are integration-style: they exercise real code paths through public APIs. They describe what the system does, not how it does it. A good test reads like a specification - "user can checkout with valid cart" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure.