Essential Principles

Every behavior starts with a failing test. No exceptions — because a test that has never been red cannot confirm it detects the right failure.

RED first. Write a failing test before any implementation. The test must fail for the right reason (missing behavior, not syntax error). Run it. See it fail.
GREEN minimally. Write the minimum code to pass the failing test. Nothing more.
REFACTOR safely. Clean up code while tests stay green. No new behavior during REFACTOR.
Small cycles. One behavior per RED-GREEN-REFACTOR cycle. If the cycle takes more than a few minutes, the step is too big.
Run tests constantly. After every RED, GREEN, and REFACTOR step — silent phases hide regressions, so running tests at each boundary proves the phase achieved its goal.

Quick Start

Situation	First Action
No test runner configured	Set one up first — install framework, create test directory, verify runner with a trivial test
New feature	Identify the simplest behavior → write a failing test → implement
Bugfix	Write a test that reproduces the defect → confirm it fails → fix it