Tdd

Test-Driven Development

TDD is the red to 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 any project context docs (a CONTEXT.md, architecture notes, or ADRs in the area you are touching) if they exist, so test names and interface vocabulary match the project's domain language and conventions.

For the deep-module vocabulary that governs where seams go, run /codebase-design.

What a good test is

Tests verify behaviour through public interfaces, not implementation details. Code can change entirely; tests should not. A good test reads like a specification — "user can checkout with valid cart" tells you exactly what capability exists — and survives refactors because it does not 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 behaviour 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 cannot 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 behaviour has not changed.
• Tautological — the assertion recomputes the expected value the way the code does (expect(add(a, b)).toBe(a + b), a snapshot derived by hand the same way, a constant asserted equal to itself), so it passes by construction and can never disagree with the code. Expected values must come from an independent source of truth — a known-good literal, a worked example, the spec.
• Horizontal slicing — writing all tests first, then all implementation. Bulk tests verify imagined behaviour: you test the shape of things rather than user-facing behaviour, the tests go insensitive to real changes, and you commit to test structure before understanding the implementation. Work in vertical slices instead — one test then one implementation then repeat, each test a tracer bullet that responds to what the last cycle taught you.

Rules of the loop

• Red before green. Write the failing test first, then only enough code to pass it. Do not anticipate future tests or add speculative features.
• One slice at a time. One seam, one test, one minimal implementation per cycle.
• Run tests through the project's toolchain. In this repo that means the pinned environment — for example devenv shell -- bun test. Confirm the runner before you start looping so red and green are reliable signals.
• Refactoring is not part of the loop. It belongs to the review stage (run /code-review), not the red to green implementation cycle.

Adapted from mattpocock/skills (MIT).