# Tdd

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

- Skill: `zenobi-us/tdd` (Agent Skill, multi-file: 4 files)
- Install (CLI): `npx skillmds@latest add zenobi-us/tdd`
- Raw SKILL.md: https://api.skillmd.com/api/skills/zenobi-us/tdd/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: zenobi-us (https://skillmd.com/u/zenobi-us)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/zenobi-us/tdd

---


## Language

All agent-authored prose MUST follow **ASD-STE100 Simplified Technical English**. Apply it to questions, explanations, recommendations, issue text, reports, handoffs, and procedures.

- Use short, complete sentences and active voice.
- Use one term for one meaning.
- Put conditions before commands.
- Use imperative sentences for procedures.
- Keep code, identifiers, commands, quoted text, product names, and exact domain terms unchanged.
- Use the `simple-english` skill for the full rule set.

# Test-Driven Development

TDD is the red → green → refactor 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. The implementation loop stops at green. Refactoring happens during review unless the user asks for it during the cycle.

Before resolving domain artifacts, follow [ALIGNMENT-ROOT.md](../../ALIGNMENT-ROOT.md).

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](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 — 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?"

## 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 (see the `code-review` skill), not the red → green implementation cycle.

## Local repository rules

Before reading or writing workflow or domain artifacts:

1. Follow [ALIGNMENT-ROOT.md](../../ALIGNMENT-ROOT.md).
2. Resolve alignment paths against `ALIGNMENT_ROOT`.
3. Keep source code and ordinary project files relative to `repository-root`.
4. Do not silently mix alignment roots.
5. All agent-authored prose MUST follow ASD-STE100 Simplified Technical English.