version: "1.0.0" name: tdd description: Test-driven development. Use when the user wants to build features or fix bugs test-first, mentions "red-green-refactor", or wants integration tests.

Test-Driven Development

Philosophy

Core principle: Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't.

Good tests are integration-style: they exercise real code paths through public APIs. They describe _what_ the system does, not _how_ it does it. A good test reads like a specification — "user can checkout with valid cart" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure.

Bad tests are coupled to implementation. They mock internal collaborators, test private methods, or verify through external means. The warning sign: your test breaks when you refactor, but behavior hasn't changed.

See tests.md for examples and mocking.md for mocking guidelines.

Anti-Pattern: Horizontal Slices

DO NOT write all tests first, then all implementation. This is "horizontal slicing" — treating RED as "write all tests" and GREEN as "write all code."

This produces crap tests:

• Tests written in bulk test _imagined_ behavior, not _actual_ behavior
• You end up testing the _shape_ of things rather than user-facing behavior
• Tests become insensitive to real changes

Correct approach: Vertical slices via tracer bullets. One test → one implementation → repeat.

WRONG (horizontal):
  RED:   test1, test2, test3, test4, test5
  GREEN: impl1, impl2, impl3, impl4, impl5
 
RIGHT (vertical):
  RED→GREEN: test1→impl1
  RED→GREEN: test2→impl2
  RED→GREEN: test3→impl3
  ...

Workflow

1. Planning

Before writing any code:

• [ ] Confirm with user what interface changes are needed
• [ ] Confirm with user which behaviors to test (prioritize)
• [ ] Identify opportunities for deep modules — run /codebase-design for the vocabulary
• [ ] List the behaviors to test (not implementation steps)
• [ ] Get user approval on the plan

Ask: "What should the public interface look like? Which behaviors are most important to test?"

2. Tracer Bullet

Write ONE test that confirms ONE thing about the system:

RED:   Write test for first behavior → test fails
GREEN: Write minimal code to pass → test passes

This is your tracer bullet — proves the path works end-to-end.

3. Incremental Loop

For each remaining behavior:

RED:   Write next test → fails
GREEN: Minimal code to pass → passes

Rules:

• One test at a time
• Only enough code to pass current test
• Don't anticipate future tests
• Keep tests focused on observable behavior

4. Refactor

After all tests pass, look for refactor candidates:

• [ ] Extract duplication
• [ ] Deepen modules (move complexity behind simple interfaces)
• [ ] Apply SOLID principles where natural
• [ ] Run tests after each refactor step

Never refactor while RED. Get to GREEN first.

Checklist Per Cycle

[ ] Test describes behavior, not implementation
[ ] Test uses public interface only
[ ] Test would survive internal refactor
[ ] Code is minimal for this test
[ ] No speculative features added