---
name: tdd
description: Test-driven development. Use when a slice is implemented test-first, test quality or seams are in question, or the red-green loop is being discussed.
---

# /tdd - Test-Driven Development

Every message to the operator follows the `operator-communication`
skill: load it before your first message.

If a `tw_*` tool or the workflow misbehaves, refuses you, or otherwise surprises you, invoke the `workflow-failure` skill before improvising.

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.

Where it fits in this pipeline:

- The **tdd-worker** consults this skill at authoring time via the `skill: tdd`
  parameter.
- The **slice-verifier** does **not** consult this skill; it stays a strict
  pass/fail gate on the slice's acceptance criteria.
- The **code-review** skill may use this vocabulary for quality judgment.

See [tests.md](tests.md) for concrete examples and [mocking.md](mocking.md) for
mocking guidelines.

## What a good test is

Tests verify behavior 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 it survives refactors because it does not care about
internal structure.

- Use public APIs only.
- Name tests after the behavior they protect.
- Assert on outcomes callers or users can observe.

## 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 cannot 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 is 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), delegate to the
`codebase-design` skill for the vocabulary. It is the shared source of the
module, interface, depth, seam, adapter, leverage and locality terms.

- **Features:** the seam is agreed in the effort's architecture document,
  which `to-tickets` produces. If a seam is not in the architecture, confirm
  it before writing a test at that boundary.
- **Bugs:** the reproducing exception or failure is the implicit seam. Write a
  regression test that reproduces the bug through the same public interface the
  user hit.

## 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 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, or 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. Do not anticipate future tests or add speculative behavior.
- **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 (see
  the `code-review` skill), not the red → green implementation cycle.