| name | tdd |
| description | Use when implementing new features, fixing bugs, or when the user asks for test-driven development.
Triggers: "TDD", "test first", "red green refactor", "write tests", "test-driven".
NEVER write production code without a failing test first.
|
Test-Driven Development
MANDATORY: Follow RED-GREEN-REFACTOR. No exceptions.
THE RULE
No production code exists without a failing test that demanded it.
RED: Write a Failing Test
- Write the simplest test that describes the desired behavior
- Run it. It MUST fail. If it passes, the test is wrong or the feature already exists.
- The test name describes the behavior:
should [expected] when [condition]
- Test ONLY one behavior per test
GREEN: Make It Pass
- Write the MINIMUM production code to make the test pass
- Do not write code "for later" or "just in case"
- Hardcode return values if that's all it takes. Seriously.
- Run the test. It MUST pass now.
REFACTOR: Clean Up
- Remove duplication between test and production code
- Improve names, extract functions, simplify logic
- Run ALL tests after each refactor step — they must stay green
- Do not add behavior during refactoring
Cycle Speed
Each RED-GREEN-REFACTOR cycle should be 1-5 minutes. If a cycle takes longer:
- The step is too big. Break it down.
- Write a simpler test first.
RATIONALIZATION TABLE
| Excuse | Why It Fails |
|---|
| "I'll write tests after" | You won't. And the code won't be testable. |
| "This is too simple to test" | Simple untested code becomes complex untested code. |
| "I need to write the whole thing first" | No. One test, one behavior, one cycle. |
| "Testing slows me down" | Testing speeds you up. Debugging untested code is what's slow. |
| "The test would be trivial" | Good. Trivial tests catch non-trivial regressions. |
| "I know it works" | Prove it. With a test. |
ABSOLUTE PROHIBITION
- NEVER write production code before a failing test demands it.
- NEVER write more than one failing test at a time.
- NEVER skip the REFACTOR step — accumulated tech debt compounds.
- NEVER delete a failing test to make the suite green.
- NEVER write a test that can't fail.
