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 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?"
When the shape of that interface is itself in question — how deep the module is, where the seam belongs, what the interface should expose — use the /codebase-design skill for the vocabulary. It is the shared source of the module, interface, depth, seam, adapter, leverage and locality terms, and it is a reference to consult, not a session to run.
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), 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 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.
Workflow
1. Planning
When exploring the codebase, use the project's domain glossary so that test names and interface vocabulary match the project's language, and respect ADRs in the area you're touching. Follow the project's commit style when committing through the loop, and the project's breadcrumb convention when capturing in-impl decisions for later review — both are documented in the project's agent docs if the project uses them.
Before writing any code:
- Shape the module and its interface — depth, seam placement, and designing for testability all come from the
/codebase-designskill - List the behaviors to test (not implementation steps)
- If the slice renders UI, route through frontend-verification.md for the visual-cycle rules and the agent-browser-vs-design-source gate
2. Tracer Bullet
Write ONE test that confirms ONE thing about the system:
RED: Write test for first behavior → test fails
GREEN: Write minimal code to pass → test passes
This is your tracer bullet - proves the path works end-to-end.
For visual slices, the RED artifact may be a story or a dev-route render rather than a test, and GREEN is closed by the visual gate. Same vertical-slicing rule. See frontend-verification.md.
3. Incremental Loop
For each remaining behavior:
RED: Write next test → fails
GREEN: Minimal code to pass → passes
Rules:
- One test at a time
- Only enough code to pass current test
- Don't anticipate future tests
- Keep tests focused on observable behavior
4. Refactor
After all tests pass, look for refactor candidates:
- Extract duplication
- Deepen modules (move complexity behind simple interfaces)
- Apply SOLID principles where natural
- Consider what new code reveals about existing code
- Run tests after each refactor step
Never refactor while RED. Get to GREEN first.
Checklist Per Cycle
[ ] Test describes behavior, not implementation
[ ] Test uses public interface only
[ ] Test would survive internal refactor
[ ] Code is minimal for this test
[ ] No speculative features added
[ ] Commit follows the project's commit style (one logical change per commit)
[ ] Any non-obvious in-impl decision is captured as a breadcrumb
Breadcrumbs
On a non-obvious in-impl choice, append an entry to .scratch/<feature>/decisions.md per the project's breadcrumb convention. Skip mechanical or obvious choices — noise makes reviewers stop reading. Big architectural calls belong in an ADR, not a breadcrumb.
When the loop ends, the next step is /prepare-for-review.