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 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 first writing tests at a seam, name that public boundary and obtain user confirmation. Reuse an approved test plan or explicit confirmation of the same seam in the current task; adding test cases within it does not require another confirmation. A new or materially changed seam requires confirmation. No test is written at an unconfirmed seam. This keeps testing effort on agreed critical paths and complex logic.

For a seam not yet approved, ask: "What's the public interface, and which seams should we test?"