Test-Driven Development (TDD)

Overview

Write the test first. Watch it fail. Write minimal code to pass.

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

This skill enforces strict test-first development following the RED/GREEN/REFACTOR cycle. Violating the letter of the rules is violating the spirit of the rules.

When to Use This Skill

Always:

• New features
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (ask human partner):

• Throwaway prototypes
• Generated code
• Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over.

No exceptions:

• Don't keep it as "reference"
• Don't "adapt" it while writing tests
• Delete means delete

Core Principles

1. RED: Write a failing test first
2. GREEN: Write minimal code to make test pass
3. REFACTOR: Improve code while keeping tests green
4. NEVER: Write implementation before tests

Quick Start

The RED/GREEN/REFACTOR Cycle

RED → Verify RED → GREEN → Verify GREEN → REFACTOR → Repeat

1. RED: Write one minimal test showing desired behavior
2. Verify RED: Run test, confirm it fails for right reason
3. GREEN: Write simplest code to pass test
4. Verify GREEN: Run test, confirm it passes
5. REFACTOR: Clean up while keeping tests green
6. Repeat: Next test for next feature

Cycle Details

RED: Write one minimal test (one behavior, clear name, real code) Verify RED: MANDATORY - watch it fail for right reason GREEN: Write simplest code to pass (no extras) Verify GREEN: MANDATORY - watch it pass, all tests pass REFACTOR: Clean up while keeping tests green (optional)

Navigation

For detailed information:

• Workflow: Complete RED/GREEN/REFACTOR workflow with detailed examples
• Examples: Real-world TDD scenarios with step-by-step walkthroughs
• Philosophy: Why order matters and why tests-after don't work
• Anti-patterns: Common mistakes, rationalizations, and red flags
• Integration: Using TDD with debugging and other skills

Key Reminders

• ALWAYS write the test BEFORE implementation
• Make each test fail FIRST to verify it's testing something
• Keep implementation minimal - just enough to pass tests
• Refactor only when tests are green
• One cycle at a time - small steps
• If test passes immediately, it's not testing new behavior

Red Flags - STOP and Start Over

If you catch yourself:

• Writing code before test
• Test passes immediately
• Can't explain why test failed
• "I'll test after"
• "Keep as reference"
• "Already spent X hours, deleting is wasteful"
• "Tests after achieve the same purpose"

ALL of these mean: Delete code. Start over with TDD.

Why Order Matters

Tests-after pass immediately (proves nothing), test-first fail then pass (proves it works). See Philosophy for detailed explanation.

Integration with Other Skills

• systematic-debugging: Create failing test in Phase 4 (bug reproduction)
• verification-before-completion: Verify tests exist and watched them fail
• defense-in-depth: Add validation tests after implementing feature

Real-World Impact

From TDD practice:

• Test-first: 95%+ first-time correctness
• Test-after: 40% first-time correctness
• TDD time: 25-45 minutes per feature (including tests)
• Non-TDD time: 15 minutes coding + 60-120 minutes debugging

TDD is pragmatic - finds bugs before commit, prevents regressions, documents behavior, enables refactoring.