Test Driven Development

This puts tests first in your development workflow, red-green-refactor style. Write a failing test, make it pass with minimal code, then clean up. The real win is the Prove-It Pattern for bugs: reproduce the issue with a test before you fix anything, so you know it's actually fixed and won't regress. The guide includes practical stuff like the test pyramid (80% unit, 15% integration, 5% E2E), testing state over interactions, and when to use real implementations versus mocks. If you're working with an AI agent on a codebase without tests, you're flying blind. This gives you the proof that changes actually work.

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Install

$ npx -y skills add addyosmani/agent-skills --skill test-driven-development --agent claude-code

Installs into .claude/skills of the current project.

Files

Test-Driven Development

Overview

Write a failing test before writing the code that makes it pass. For bug fixes, reproduce the bug with a test before attempting a fix. Tests are proof — "seems right" is not done. A codebase with good tests is an AI agent's superpower; a codebase without tests is a liability.

When to Use

Implementing any new logic or behavior
Fixing any bug (the Prove-It Pattern)
Modifying existing functionality
Adding edge case handling
Any change that could break existing behavior

When NOT to use: Pure configuration changes, documentation updates, or static content changes that have no behavioral impact.

Related: For browser-based changes, combine TDD with runtime verification using Chrome DevTools MCP — see the Browser Testing section below.

Discover the Stack First

The TDD cycle is universal; the commands are not. Before writing the first test, discover how this repository tests, and use its commands for every RED, GREEN, and verification step:

SKILL.md on GitHub