3.1 KiB
name, description
| name | description |
|---|---|
| tdd | Build or fix behavior test-first through red-green-refactor. Use when the user requests TDD or test-first development, not merely any integration-test task. |
Test-Driven Development
TDD is the red → green → refactor loop. Use behavior-level tests to guide small changes; choose checks proportional to the task.
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.
Reuse established test boundaries and any seams already agreed with the user. Choose a seam that exercises the actual behavior, including persistence or integration contracts where those are the subject of the test. State a new choice briefly; ask only if it introduces a consequential design trade-off that cannot be inferred from the request and code.
Prioritize critical paths, failure modes, and complex logic; do not block routine tests on repeated seam approval.
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.
Rules of the loop
- Red before green. Write the failing test first, then only enough code to pass it. Don't anticipate future tests or add speculative features.
- One slice at a time. One seam, one test, one minimal implementation per cycle.
- Refactor while green. Simplify the changed code and tests without changing behavior, then rerun the checks. Keep unrelated redesign outside the task.