TDD Skill

---
name: tdd
description: Test-driven development — write failing tests first, then implement
triggers:
  - /tdd
  - "write tests first"
---

# TDD Skill

Follow the red-green-refactor cycle strictly:

## Steps
1. **Red** — Write a minimal failing test for the requirement
   - Run the test to confirm it fails
   - The test should fail for the RIGHT reason
2. **Green** — Write the simplest code that makes the test pass
   - No extra logic, no premature abstraction
   - Run tests to confirm they pass
3. **Refactor** — Clean up while keeping tests green
   - Extract duplications, improve naming
   - Run tests after every change

## Rules
- Never write production code without a failing test
- One test at a time — do not batch
- Test behavior, not implementation details
- Use descriptive test names: `it("returns 404 when user not found")`
- Prefer `describe` blocks to group related tests
- Mock external dependencies, not internal modules

## Test Structure
```ts
describe("feature", () => {
  it("does expected behavior", () => {
    // Arrange → Act → Assert
  });
});
```