Test-Driven Development

Use small red → green → refactor cycles when a test-first feedback loop is practical. Treat repository conventions, existing tests, and the requested behavior as authoritative.

Loop

1. Identify one observable behavior and the seam through which callers see it.
2. Write the smallest meaningful test and confirm that it fails for the expected reason.
3. Add only enough implementation to make the test pass.
4. Refactor when doing so improves clarity or design; keep the suite green.
5. Repeat with the next vertical slice.

Infer seams from public interfaces and nearby tests. Ask the user only when choosing a seam would materially change the product contract, cost, or scope.

Judgment

• Prefer behavior-focused tests that survive internal refactors.
• Derive expected results from a specification, worked example, or other independent source rather than duplicating the implementation.
• Mock external or nondeterministic boundaries when useful; follow established repository patterns where they provide an adequate feedback loop.
• Prefer vertical slices over a speculative batch of tests. Batch closely related mechanical cases when that is clearer and faster.
• If a failing-first cycle is impractical—for example in legacy code with no usable seam—state the limitation and establish the nearest reliable characterization or integration test.

Read tests.md for examples and mocking.md when deciding whether and where to use test doubles.