tdd

---
name: tdd
description: Test-driven development. Use when the user wants to build features or fix bugs test-first, mentions "red-green-refactor", or wants integration tests.
---

# 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.

**ALWAYS check `.ab-method/structure/index.yaml` FIRST** for where the domain docs live — paths are user-configurable. Look for it under the project root first; only if the project has none (AB Method installed as a plugin), read the bundled default at `../../../.ab-method/structure/index.yaml` relative to this `SKILL.md`. Every path the index names is relative to the project root. From there, read `CONTEXT.md` and `UBIQUITOUS_LANGUAGE.md` (if they exist) so test names and interface vocabulary match the project's domain language, and respect the 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 it survives refactors because it doesn't care about internal structure.

See [tests.md](tests.md) for examples and [mocking.md](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, so 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 no user is there to confirm** (an autonomous run such as `ab-start-task` or `ab-start-roadmap`, or a mission subagent), the plan is the agreement: test at the public interfaces the mission's objective and the grill already settled. If the mission needs a seam the plan never named, take the narrowest public interface that exposes the behavior and name it in the mission summary, so the review stage can challenge it. Never drop to internals to avoid the question.

When the shape of that interface is itself in question (how deep the module is, where the seam belongs, what the interface should expose, how to make it testable), call the Skill tool with "codebase-design" 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.

## 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.
- **Refactoring is not part of the loop.** It belongs to the review stage: the `review-implementation` skill runs over the task's whole diff once its missions are green, and applies cleanups with the tests as the safety net. Not the red → green implementation cycle.

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