/tdd

---
name: tdd
description: Drive a change through a red-green-refactor loop - failing test first, minimal code to pass, then clean up. Use when implementing a feature or fixing a bug where correctness matters and a test can pin the behavior. Says "TDD", "test first", "red green refactor", "write the test first".
user-invocable: true
---

# tdd

The agent codes better with a tight feedback loop than with a long
specification. A failing test is the tightest loop there is: it states the
target, and the target either goes green or it does not.

## The loop

1. **Red.** Write one failing test that states the next slice of behavior.
   Run it. Confirm it fails for the right reason - a test that passes before
   you write the code is testing nothing. If it errors instead of failing on
   the assertion, fix the test setup first.
2. **Green.** Write the least code that makes the test pass. Not the general
   solution, not the abstraction - the minimal move. Run the test. Green.
3. **Refactor.** Now clean up with the test as a net: remove duplication,
   name things, simplify. Re-run after every edit. Behavior is frozen; only
   the shape changes.
4. Repeat for the next slice. Small slices. The rate of feedback is the
   speed limit - never take on a step too big to hold a single test.

## What makes a test worth writing