tdd

# Test-Driven Development TDD is the red → green loop. This skill is the reference that makes that loop produce tests worth keeping: what a good test is, where tests go, the anti-patterns, and the rules of the loop. Every section applies on every cycle — consult them before and during the loop, not after. When exploring the codebase, read `CONTEXT.md` (if it exists) so test names and interface vocabulary match the project's domain language, and respect ADRs in the area you're touching. ## What a good test is Tests verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't. A good test reads like a specification — "user can checkout with valid cart" tells you exactly what capability exists — and survives refactors because it doesn't care about internal structure. See [tests.md](tests.md) for examples and [mocking.md](mocking.md) for mocking guidelines. ## Seams — where tests go A **seam** is the public boundary you test at: the interface where you observe behavior without reaching inside. Tests live at seams, never against internals. **Test only at pre-agreed seams.** Before writing any test, write down the seams under test and confirm them with the user. No test is written at an unconfirmed seam. You can't test everything — agreeing the seams up front is how testing effort lands on the critical paths and complex logic instead of every edge case. Ask: "What's the public interface, and which seams should we test?" ...