# Test Driven Development

> Test-driven development workflow (red → green → refactor) for features, bug fixes, and refactors. Use when implementing behavior changes and you want to drive design via tests, prevent regressions, and keep code modular and well-factored.

- Skill: `vadimcomanescu/test-driven-development` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add vadimcomanescu/test-driven-development`
- Raw SKILL.md: https://api.skillmd.com/api/skills/vadimcomanescu/test-driven-development/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Productivity
- Author: vadimcomanescu (https://skillmd.com/u/vadimcomanescu)
- Updated: 2026-09-22
- Page: https://skillmd.com/skills/vadimcomanescu/test-driven-development

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# Test-Driven Development

Write the test first, then write the smallest change that makes it pass, then refactor safely.

## Core loop
1) **Red**: write a failing test that expresses the desired behavior.
2) **Green**: implement the minimum change to pass.
3) **Refactor**: improve structure while keeping tests green.

## Quick Start
- Start from the public interface (API, function, UI behavior), not private helpers.
- Prefer “behavioral” test names (what), not “implementation” names (how).
- When fixing bugs: reproduce with a test first.

## Guardrails
- Don’t over-mock: prefer integration at boundaries; mock only slow/flaky externals.
- Keep tests deterministic: control time, randomness, and network.
- If a test is hard to write, your design likely needs an interface seam.

## Red/Green/Refactor checklist
- Red: test fails for the right reason (not setup errors).
- Green: smallest change to pass, no extra behavior.
- Refactor: improve structure without changing behavior or tests.

## References
- Common anti-patterns: `references/testing-anti-patterns.md`