---
name: jor-tdd
description: Test-driven development. Build a feature or fix test-first, one red-green slice at a time, at existing public or user-agreed seams.
disable-model-invocation: true
metadata:
  based-on: "mattpocock/skills skills/engineering/tdd @ 3f599139104dfceabc7754eeb91319802f85e042 (SKILL.md, tests.md, mocking.md)"
---

In Codex, use `$jor-...` for these commands, or `$jor-stack:jor-...` when the skill picker shows the plugin namespace.

# Test-Driven Development

TDD is the red → green loop. This skill is what makes that loop produce tests worth keeping. Every section applies on every cycle: consult them before and during the loop, not after.

When exploring the codebase, read `GLOSSARY.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 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.

**Reuse existing public seams.** For a clear, authorized change, use the relevant public interface and existing test pattern without another approval. Honor seams already agreed with the user. For a new or unclear seam, state what it catches and misses, then confirm it with the user before writing tests.

When the shape of that interface is itself in question (how deep the module is, where the seam belongs, what the interface should expose), read `../jor-design/SKILL.md` and follow it for the vocabulary.

## 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.** Report refactoring candidates for `/jor-review`; implement accepted changes in a separate authorized pass.
- **Stop at green.** When the selected seams are covered and green, return to `jor-implement` for its automatic peer review when it is the parent workflow. When used standalone, recommend `/jor-peer-review` next. Browser review and full `/jor-review` stay user-invoked; full review is optional for large changes after any requested browser review and fixes. Start, stop, or restart dev servers only on explicit request.