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.

TDD applies to any language and any framework. The commands change; the cycle does not.

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.

Related: For browser-based changes, combine TDD with runtime verification using Chrome DevTools — see the browser-testing-with-devtools skill.