Test Driven Development

Test-Driven Development

Overview

Write a failing test first, then implement the minimal code to pass it, then refactor.

Core principle: NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST.

This is mandatory. No exceptions for production code.

The Red-Green-Refactor Cycle

RED: Write Failing Test

1. Write a minimal test demonstrating desired behavior
2. Run test - it MUST fail
3. Verify it fails for the right reason

GREEN: Implement Minimal Code

1. Write the simplest code to pass the test
2. No extra features, no optimization
3. Run test - it MUST pass

REFACTOR: Clean Up

1. Improve code while maintaining test pass
2. Remove duplication
3. Improve naming
4. Run tests to ensure nothing breaks

Why This Matters

• Tests written after code pass immediately - proving nothing
• Manual testing is ad-hoc and non-repeatable
• TDD catches bugs before deployment
• Tests become safety net for future changes

Critical Requirements

Tests must be:

• Minimal - one behavior each
• Clear - descriptive naming
• Real - avoiding unnecessary mocks

Verification Checklist

• Test fails before implementation (RED)
• Test passes after implementation (GREEN)
• Refactoring maintains pass
• Test actually exercises the code

Red Flags - STOP

• Code written before test
• Tests that pass immediately
• Manual verification instead of tests
• "I'll write tests after"
• Skipping the cycle

If you wrote code before test: Delete it. Start over with test first.

Common Mistakes

Integration

• Phase 4.1 of debugging - Create failing test before fixing
• Plan execution - Each task includes test-first step
• Subagent-driven development - Implementation includes test creation

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