Test Driven Development

Test-Driven Development

A disciplined cycle for writing code one behavior at a time: red (failing test) → green (minimal code) → refactor.

The Iron Law

No production code without a failing test first. Not even one line. Not even "I'll just sketch this out." Not even "the test is obvious — I'll add it after."

The Cycle

1. Write the failing test

Write a single test for one behavior. The test should:

• Describe what you want, not how it's done.
• Fail in a meaningful way (assertion error — not syntax error, not missing import).
• Be small — favor one method's behavior over a whole class.

2. Confirm red

Run the test. Confirm it fails for the expected reason.

• Unexpected failure reason: diagnose first; don't paper over with implementation.
• Passes without any implementation: the behavior already exists, or the test isn't testing what you think. Investigate.

3. Write the minimal implementation

Write the simplest code that makes the test pass. Resist the urge to add more.

• No "while I'm here" additions.
• No defensive code for cases not yet tested.
• No abstractions for hypothetical future tests.

4. Confirm green

Run the test. Confirm it passes. Run the full suite to check for regressions.

5. Refactor

Look at the code you just wrote and the code around it:

• Is there duplication to extract?
• Are names clear and intention-revealing?
• Is the code as simple as it can be while remaining clear?
• Any code smells?

If you see an improvement, make it. Re-run tests. If nothing needs refactoring, say so out loud and move on.

Common Rationalizations

| Excuse | Reality | |---|---| | "Test is trivial, I'll skip it" | Trivial tests catch real regressions later. Write it. | | "I already manually tested it" | Manual tests don't run in CI. Write the automated test. | | "I'll write the test after" | Tests-after asks "what does this do?" Tests-first asks "what should this do?" Different exercises. | | "Refactor isn't needed here" | Say so explicitly. Don't silently skip Step 5. | | "I'll batch the refactor at the end" | Refactor with context fresh, not after you've moved on. |

Red Flags — STOP

• Code before test.
• "I'll just sketch this out and add the test later."
• Test added after the code already passed manually.
• Implementing more than one behavior per cycle.
• Skipping Step 5 without saying so.

All of these mean: stop, delete the unprotected code, start the cycle properly.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: infews
• Source: infews/pxp_skill
• License: MIT

Install and usage instructions live in the source repository linked above.