tdd

name	tdd
description	Test-driven development with red-green-refactor loop. Use when user wants to build features or fix bugs using TDD, mentions "red-green-refactor", wants integration tests, or asks for test-first development.

Test-Driven Development

NB: referenced sibling files (tests.md, mocking.md, deep-modules.md, interface-design.md, refactoring.md) are cohort-only content at the source. DR-specific versions will be written under DR-724-B; until then, use the skill's inline guidance and fall back to your own judgement. (Matt's upstream copies of these files are also bundled here as a courtesy — they are terse but usable starter content and will be replaced by DR versions.)

Philosophy

Core principle: Tests should verify behaviour 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 (like querying a database directly instead of using the interface). The warning sign: your test breaks when you refactor, but behaviour hasn't changed. If you rename an internal function and tests fail, those tests were testing implementation, not behaviour.

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 behaviour, not actual behaviour
• You end up testing the shape of things (data structures, function signatures) rather than user-facing behaviour
• Tests become insensitive to real changes - they pass when behaviour breaks, fail when behaviour is fine
• You outrun your headlights, committing to test structure before understanding the implementation

Correct approach: Vertical slices via tracer bullets. One test -> one implementation -> repeat. Each test responds to what you learned from the previous cycle. Because you just wrote the code, you know exactly what behaviour matters and how to verify it.

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 behaviours to test (prioritise)
• Identify opportunities for deep modules (small interface, deep implementation)
• Design interfaces for testability
• List the behaviours to test (not implementation steps)
• Get user approval on the plan

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

You can't test everything. Confirm with the user exactly which behaviours matter most. Focus testing effort on critical paths and complex logic, not every possible edge case.

2. Tracer Bullet

Write ONE test that confirms ONE thing about the system:

RED:   Write test for first behaviour -> 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 behaviour:

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 behaviour

4. Refactor

After all tests pass, look for refactor candidates:

• Extract duplication
• Deepen modules (move complexity behind simple interfaces)
• Apply SOLID principles where natural
• Consider what new code reveals about existing code
• Run tests after each refactor step

Never refactor while RED. Get to GREEN first.

Checklist Per Cycle

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