TDD — Test-First Execution

The Iron Law

No production code without a failing test first. This is enforcement, not advice:

• Code written before its test gets deleted and redone test-first. Not "test added after" — deleted, then rebuilt from the test.
• A test that passes on first run is wrong. You never saw it fail, so you don't know it can fail. See the red before you make the green.

Step 0 — whose code is this?

Touching untested legacy code? STOP. Run safe-incremental-coding to build the characterization net first, then return here. That skill deliberately opposes the Iron Law (no fine-grained TDD tests on legacy structure — they bake in the bad shape), which is exactly why it is a separate skill. New code, or code with tests you can stand on, proceeds directly.

The loop

Run one loop per increment of behavior. Keep each pass small enough that you could throw it away without grief.