test-driven-development

name	test-driven-development
description	Use when implementing any feature or bugfix, before writing implementation code. Enforces RED-GREEN-REFACTOR cycle: write failing test first, watch it fail, write minimal code, watch it pass.
argument-hint	Describe the feature or bug to implement with TDD.

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Test-Driven Development (TDD)

Overview

Write the test first. Watch it fail. Write minimal code to pass.

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

Violating the letter of the rules is violating the spirit of the rules.

When to Use

Always:

• New features (new slice, new service method, new API endpoint)
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (ask your human partner):

• Throwaway prototypes
• Generated code
• Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over.

No exceptions:

• Don't keep it as "reference"
• Don't "adapt" it while writing tests
• Don't look at it
• Delete means delete

Implement fresh from tests. Period.

Red-Green-Refactor

RED - Write Failing Test

Write one minimal test showing what should happen.

Requirements:

• One behavior
• Clear name
• Real code (no mocks unless unavoidable)

For DevCD: tests live in tests/test_<slice>.py. Use pytest-asyncio for async routes.

Verify RED - Watch It Fail

MANDATORY. Never skip.

pytest tests/test_<slice>.py::test_name -v

Confirm:

• Test fails (not errors)
• Failure message is expected
• Fails because feature missing (not typos)

Test passes? You're testing existing behavior. Fix test. Test errors? Fix error, re-run until it fails correctly.

GREEN - Minimal Code

Write simplest code to pass the test.

Don't add features, refactor other code, or "improve" beyond the test.

For DevCD: add models to slices/<slice>/models.py, logic to service.py, routes to api.py.

Verify GREEN - Watch It Pass

MANDATORY.

pytest tests/test_<slice>.py::test_name -v

Confirm:

• Test passes
• Other tests still pass
• Output pristine (no errors, warnings)

Test fails? Fix code, not test. Other tests fail? Fix now.

REFACTOR - Clean Up

After green only:

• Remove duplication
• Improve names
• Extract helpers

Keep tests green. Don't add behavior.

Repeat

Next failing test for next feature.

Good Tests

Quality	Good	Bad
Minimal	One thing. "and" in name? Split it.	test('validates email and domain and whitespace')
Clear	Name describes behavior	test('test1')
Shows intent	Demonstrates desired API	Obscures what code should do

Red Flags - STOP and Start Over

• Code before test
• Test after implementation
• Test passes immediately
• Can't explain why test failed
• Tests added "later"
• Rationalizing "just this once"
• "I already manually tested it"
• "TDD is dogmatic, I'm being pragmatic"

All of these mean: Delete code. Start over with TDD.

Example: New Slice Endpoint (DevCD)

Feature: GET /state returns current host state

RED

def test_get_state_returns_200(client):
    response = client.get("/state")
    assert response.status_code == 200

Verify RED

pytest tests/test_state_engine.py::test_get_state_returns_200 -v
# FAIL: 404 Not Found

GREEN — add route to slices/host_state_engine/api.py

Verify GREEN

pytest tests/test_state_engine.py::test_get_state_returns_200 -v
# PASS

REFACTOR if needed.

Verification Checklist

Before marking work complete:

• Every new function/method has a test
• Watched each test fail before implementing
• Each test failed for expected reason (feature missing, not typo)
• Wrote minimal code to pass each test
• All tests pass (make check)
• Output pristine (no errors, warnings)
• Tests use real code (mocks only if unavoidable)
• Edge cases and errors covered

Can't check all boxes? You skipped TDD. Start over.

When Stuck

Problem	Solution
Don't know how to test	Write wished-for API. Write assertion first. Ask your human partner.
Test too complicated	Design too complicated. Simplify interface.
Must mock everything	Code too coupled. Use dependency injection.
Test setup huge	Extract helpers. Still complex? Simplify design.

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

Final Rule

Production code → test exists and failed first
Otherwise → not TDD

No exceptions without your human partner's permission.