test-driven-development

name	test-driven-development
description	Use when implementing any feature or bugfix, before writing implementation code. Enforces RED-GREEN-REFACTOR.
type	skill

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.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

No exceptions without User permission.

Red-Green-Refactor

RED - Write Failing Test

Write one minimal test showing what should happen.

• One behavior
• Clear name
• Real code (no mocks unless unavoidable)

Verify RED - Watch It Fail

MANDATORY. Never skip. Confirm: Test fails (not errors) because feature is missing.

GREEN - Minimal Code

Write simplest code to pass the test. Do not over-engineer (YAGNI). Don't add features, refactor other code, or "improve" beyond the test.

Verify GREEN - Watch It Pass

MANDATORY. Confirm: Test passes and all other tests still pass.

REFACTOR - Clean Up

After green only: Remove duplication, improve names, extract helpers. Keep tests green. Don't add behavior.

Common Rationalizations

Excuse	Reality
"Too simple to test"	Simple code breaks. Test takes 30 seconds.
"I'll test after"	Tests passing immediately prove nothing.
"Tests after achieve same goals"	Tests-after = "what does this do?" Tests-first = "what should this do?"
"Already manually tested"	Ad-hoc ≠ systematic. No record, can't re-run.

Verification Checklist

Before marking work complete, ensure:

• Every new function/method has a test
• Watched each test fail before implementing
• Wrote minimal code to pass each test

If you are stuck, write a wished-for API or ask the User for help.

When to Use

• When this specific skillset is applicable to the current task.

Prerequisites

• Appropriate MCP servers and context.

Process

1. Review the problem statement.
2. Execute steps sequentially.
3. Validate the output.

Best Practices

• Always verify before completing the task.