Test-Driven Development (TDD)

Follow the red-green-refactor cycle: write one minimal failing test, run it to confirm it fails for the expected reason, write the simplest code that passes, verify all tests pass, then refactor while keeping tests green. Repeat for each new behavior.

Red means writing a failing test that defines desired behavior. Green means writing the minimal code to make it pass. Refactor means cleaning up duplication and names while keeping all tests passing, without adding new behavior.

Write tests before code. Tests written after implementation pass immediately, which proves nothing about whether they test the right thing. Watching a test fail first confirms it actually exercises the missing behavior.

Exceptions are limited to throwaway prototypes, generated code, and configuration files, and only with explicit agreement from your human partner. Exploration is fine, but the exploratory code should be discarded and rewritten test-first.

An immediately passing test means you are testing existing behavior or the test is wrong. Fix the test so it fails for the expected reason, such as a missing feature, before writing any implementation code.

Write a failing test that reproduces the bug first, then implement the minimal fix until the test passes. This proves the fix works and creates a regression test that prevents the bug from returning.