# TDD

AI DevKit · Test-driven development, write a failing test before writing production code.

- Repository: https://github.com/codeaholicguy/ai-devkit (1656 stars)
- Category: Testing
- License: Apache-2.0
- Last updated: 2026-11-01
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add codeaholicguy/ai-devkit --skill tdd -g -a claude-code -y`
Codex: `npx skills add codeaholicguy/ai-devkit --skill tdd -g -a codex -y`
Cursor: `npx skills add codeaholicguy/ai-devkit --skill tdd -g -a cursor -y`
Gemini CLI: `npx skills add codeaholicguy/ai-devkit --skill tdd -g -a gemini-cli -y`

## Description

AI DevKit · Test-driven development, write a failing test before writing production code. Use when implementing new functionality, adding behavior, or fixing bugs during active development.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

# TDD

Red. Green. Refactor. In that order, every time.

## Hard Rules

- No production code without a failing test first.
- If production code was written before its test, delete it and start over with a failing test.
- Never skip the red step. A test that has never failed proves nothing.

## Cycle

For each unit of behavior:

1. **Red**, Write a test for the next behavior. Run it. It must fail. Read the failure message, it should describe the missing behavior.
2. **Green**, Write the minimum production code to make the test pass. Nothing more. Run the test. Apply the `verify` skill.
3. **Refactor**, Clean up both test and production code. Run the test again. Still green? Done. Apply the `verify` skill.

Then pick the next behavior and repeat.

## Rules for Each Step

**Red:**
- Test one behavior, not one function. Name the test after what the system should do, not what the function is called.
- The test must fail for the right reason, a missing method, wrong return value, unmet condition. Not a syntax error or import failure.
- If the test passes immediately, it's not testing new behavior. Delete it or pick a different behavior.

**Green:**
- Write the simplest code that passes. Hardcode if needed, the next test will force generalization.
- Do not add code "while you're in there." If it's not required by a failing test, it doesn't exist yet.
- Do not refactor during green. Pass first, clean second.

**Refactor:**
- Remove duplication between test and production code.
- Extract only when you see real duplication, not predicted duplication.
- Tests must still pass after every refactor move. Run them after each change.

## Anti-Patterns

The rest: https://github.com/codeaholicguy/ai-devkit/blob/HEAD/skills/tdd/SKILL.md