tdd

name	tdd
description	Drives development with a red-green-refactor loop, one test at a time. Use when the user wants to build a feature or fix a bug using TDD, mentions "red-green-refactor", or asks for test-first development.

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

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

Anti-Pattern: Horizontal Slices

Avoid writing all tests first, then all implementation. That 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 (data structures, function signatures) rather than user-facing behavior
• Tests become insensitive to real changes — they pass when behavior breaks, fail when behavior 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.

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 (small interface, deep implementation)
• Design interfaces for testability
• 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?"

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

Deep modules (from A Philosophy of Software Design): prefer a small interface hiding substantial implementation over a shallow module whose interface is nearly as complex as what it wraps.

┌─────────────────────┐
│   Small Interface   │  ← few methods, simple params
├─────────────────────┤
│  Deep Implementation│  ← complex logic hidden inside
└─────────────────────┘

When designing an interface, ask: can I reduce the number of methods, simplify the parameters, or hide more complexity inside?

2. Tracer Bullet

Write ONE test that confirms ONE thing about the system:

RED:    Write test for first behavior → test fails
VERIFY: Confirm it fails for the EXPECTED reason (missing feature, not a
        typo or syntax error)
GREEN:  Write minimal code to pass → test passes
VERIFY: Confirm it passes AND all other tests still pass, with clean output

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

3. Incremental Loop

For each remaining behavior:

RED:    Write next test → fails
VERIFY: Confirm it fails for the EXPECTED reason (missing feature, not a
        typo or syntax error)
GREEN:  Minimal code to pass → passes
VERIFY: Confirm it passes AND all other tests still pass, with clean output

4. Refactor

After all tests pass, look for refactor candidates:

• Duplication → extract a function or class
• Long methods → break into private helpers (keep tests on the public interface)
• Shallow modules → combine or deepen them
• Feature envy → move logic to where the data lives
• Primitive obsession → introduce value objects
• Existing code the new code reveals as problematic
• Apply SOLID principles where natural

Run tests after each refactor step. Never refactor while RED — get to GREEN first.

Constraints

• Never write all tests first. Vertical slices only — one test, one implementation, then repeat.
• Never refactor while RED. Get to GREEN first.
• One test at a time; write only enough code to pass the current test.
• Don't anticipate future tests or add speculative features.
• Test observable behavior through public interfaces — never private methods or internal collaborators.
• Get user approval on the behavior list before writing code.

Quality

• Once the feature or fix is done, run the shared self-review checklist on it. Surface issues in the chat only if found.