# Test Driven Development

Drives development with tests.

- Repository: https://github.com/cloudflare/computer (9770 stars)
- Category: Testing
- License: MIT
- Last updated: 2026-10-29
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add cloudflare/computer --skill test-driven-development -g -a claude-code -y`
Codex: `npx skills add cloudflare/computer --skill test-driven-development -g -a codex -y`
Cursor: `npx skills add cloudflare/computer --skill test-driven-development -g -a cursor -y`
Gemini CLI: `npx skills add cloudflare/computer --skill test-driven-development -g -a gemini-cli -y`

## Description

Drives development with tests. Use when implementing any logic, fixing any bug, or changing any behavior. Use when you need to prove that code works, when a bug report arrives, or when you're about to modify existing functionality.

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

# 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.

## The TDD Cycle

```
    RED                GREEN              REFACTOR
 Write a test    Write minimal code    Clean up the
 that fails  ──→  to make it pass  ──→  implementation  ──→  (repeat)
      │                  │                    │
      ▼                  ▼                    ▼
   Test FAILS        Test PASSES         Tests still PASS
```

### Step 1: RED, Write a Failing Test

Write the test first. It must fail. A test that passes immediately proves nothing.

```typescript
// RED: This test fails because createTask doesn't exist yet
describe('TaskService', () => {
  it('creates a task with title and default status', async () => {
    const task = await taskService.createTask({ title: 'Buy groceries' });

    expect(task.id).toBeDefined();
    expect(task.title).toBe('Buy groceries');
    expect(task.status).toBe('pending');
    expect(task.createdAt).toBeInstanceOf(Date);
  });
});
```

### Step 2: GREEN, Make It Pass

Write the minimum code to make the test pass. Don't over-engineer:

```typescript
// GREEN: Minimal implementation
export async function createTask(input: { title: string }): Promise<Task> {
  const task = {
    id: generateId(),
    title: input.title,
    status: 'pending' as const,
    createdAt: new Date(),
  };
  await db.tasks.insert(task);
  return task;
}
```

The rest: https://github.com/cloudflare/computer/blob/HEAD/.agents/skills/test-driven-development/SKILL.md