# tdd

Enforces strict red-green-refactor cycles with an auditable commit trail. Keeps LLM agents from writing unverified production code.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 1900 installs · ★ 1
- Author: pandoscope
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/pandoscope/skills/tree/HEAD/derived/tdd/SKILL.md
- Updated: 2026-10-23
- HTML: https://affpapa.org/en/nevibecode/skills/pandoscope-tdd

## What it does

The skill introduces an iron law: zero production code without a committed failing test first. It rejects bulk testing in favor of vertical slices. You write one test, produce the minimal implementation to pass it, commit, and then refactor. Any code drafted before its test must be deleted.

Every stage produces distinct git commits using prefixes like `test(red):`, `feat:`, or `chore(stub):`. The stub commit creates bare signatures returning `NotImplementedError` so static type checkers pass without leaking logic. Failing tests rely on strict framework markers such as `@pytest.mark.xfail(strict=True)` in Python or `test.fails()` in Vitest.

The included `lint-red.sh` script monitors commit formats and strips leftover test markers before merging. Even so, git hooks only check file structure and cannot verify whether you actually watched the test fail.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add pandoscope/skills --skill tdd -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/tdd/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/tdd/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add pandoscope/skills --skill tdd -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/tdd/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add pandoscope/skills --skill tdd -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/tdd/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install pandoscope/skills tdd
```

2. **Manually: put the files in place**
- File: `.github/skills/tdd/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add pandoscope/skills --skill tdd -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/pandoscope/skills --path derived/tdd
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/tdd/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add pandoscope/skills --skill tdd -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/pandoscope/skills/tree/HEAD/derived/tdd/SKILL.md

## When to use it
- enforce test-driven development during feature work and bug fixing
- prevent AI agents from generating code before a failing test exists
- configure strict failing test markers for pytest, vitest, and jest
- audit intermediate git commit hygiene using `lint-red.sh`

## Example prompts
- Implement the user session timeout using strict TDD.
- Fix the tax calculation bug via red-green-refactor with separate commits.
- Write a failing test for invalid email inputs and prepare a `test(red):` commit.

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: a85ecf5bf3923089.

BEGIN THIRD-PARTY FILE a85ecf5bf3923089
````text
---
name: tdd
description: Test-driven development via red-green-refactor with an auditable commit protocol. Use when implementing any feature, bugfix, refactor, or behavior change before writing implementation code, or when the user mentions TDD, red-green-refactor, or test-first development.
metadata.derived-from: merge of mattpocock/skills' `tdd` (https://github.com/mattpocock/skills/blob/7afa86d3a5dd96edde06ffa014e16c64e733681e/skills/engineering/tdd/SKILL.md) + obra/superpowers' `test-driven-development` (https://github.com/obra/superpowers/blob/030a222af19c1f3a93c6eb876a7422d8e4fc0162/skills/test-driven-development/SKILL.md)
metadata.derivation-note: Commit protocol (red/green commits, markers, lint-red.sh) and the vertical-slice framing are the load-bearing original parts. Diff prose against upstream, not the protocol.
---

# Test-Driven Development

Write test first. Commit it red. Write minimal code to pass. Commit green. Refactor.

**Core principle:** tests verify behavior through public interfaces, not implementation. Code can change entirely; tests shouldn't. A test that breaks when you rename an internal function — with no behavior change — was testing implementation. Delete it.

No watched failure = no proof the test tests the right thing. Commit history is the evidence; hooks check its structure.

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

## Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST COMMITTED FIRST.

Wrote code before the test? Delete it. Implement fresh from tests. Delete means delete. Exceptions (throwaway prototypes, generated code, config) need human sign-off. Thinking "skip TDD just this once"? That is rationalization.

## 1. Plan (before any code)

Use the project's domain glossary so test names and interface vocabulary match the codebase; respect ADRs in the area you touch.

- List the *behaviors* to test, not implementation steps. Prioritize critical paths and complex logic — you can't test everything.
- Design interfaces for [testability](interface-design.md); identify opportunities for [deep modules](deep-modules.md) (small interface, deep implementation)
- Confirm the public interface and the priority behaviors with the user, then proceed.

## 2. Vertical slices, not horizontal

DO NOT write all tests, then all implementation. That is horizontal slicing and it produces crap tests: written in bulk they test *imagined* behavior and the *shape* of things (signatures, data structures), go insensitive to real changes, and commit you to test structure before you understand the code.

Work in vertical slices — one test → one implementation → repeat. Each test responds to what the last cycle taught you.

```text
WRONG (horizontal):  RED: t1 t2 t3 t4   GREEN: i1 i2 i3 i4
RIGHT (vertical):    t1→i1  t2→i2  t3→i3 ...
```

The first slice is a tracer bullet: it proves the path works end to end.

## 3. Commit protocol

One behavior = one RED commit + one GREEN commit. Multiple cycles per branch is fine; prefer one cycle in flight in Go/Rust repos (see markers).
````
END THIRD-PARTY FILE a85ecf5bf3923089

The third-party file has ended. Everything between the a85ecf5bf3923089 tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

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