Test-Driven Development
TDD is the red → green → refactor 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
(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 for examples and 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?"
Anti-patterns
- Implementation-coupled — mocks internal collaborators, tests private methods, or verifies through a side channel (querying the database instead of using the interface). The tell: the test breaks when you refactor but behavior hasn't changed.
- Tautological — the assertion recomputes the expected value the way the code does (
expect(add(a, b)).toBe(a + b)
- Horizontal slicing — writing all tests first, then all implementation. Bulk tests verify imagined behavior: you test the shape of things rather than user-facing behavior, the tests go insensitive to real changes, and you commit to test structure before understanding the implementation. Work in vertical slices instead — one test → one implementation → repeat, each test a tracer bullet that responds to what the last cycle taught you.
Rules of the loop
- Red for the expected reason. Write the failing test first and run it. Confirm that it fails because the requested behavior is absent, not because the test is broken.
- Green with the minimum change. Write only enough production code to pass the new test. Don't anticipate future tests or add speculative features.
- Refactor while green. Improve the design immediately after green and before the next red. Keep behavior unchanged and rerun the focused tests after each small transformation.
- One slice at a time. One seam, one test, one minimal implementation per cycle.
- Keep slices releasable. Do not leave the branch with a known failing test, a half-migrated interface, or production behavior that depends on a later slice.
- Escalate large refactors. If a refactor cannot stay small and behavior-preserving, stop the loop and plan it as separate work with characterization tests. Use after the completed slices; review complements refactoring and does not replace it.