# Tdd

> 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.

- Skill: `somtougeh/tdd` (Agent Skill, multi-file: 6 files)
- Install (CLI): `npx skillmds@latest add somtougeh/tdd`
- Raw SKILL.md: https://api.skillmd.com/api/skills/somtougeh/tdd/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Integrations & APIs
- Author: SomtoUgeh (https://skillmd.com/u/somtougeh)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/somtougeh/tdd

---


# 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** assert incidental implementation rather than the contract. Prefer public behavior; direct database or filesystem checks are appropriate when persistence, constraints, migrations, or external effects are the contract. The warning sign is a test changing solely because a private function was renamed while observable behavior stayed the same.

See [tests.md](tests.md) for examples and [mocking.md](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 (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. Because you just wrote the code, you know exactly what behavior 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:

- [ ] Infer the requested interface and priority behaviors from the user's specification and existing code
- [ ] Identify opportunities for [deep modules](deep-modules.md) (small interface, deep implementation)
- [ ] Design interfaces for [testability](interface-design.md)
- [ ] List the behaviors to test (not implementation steps)
- [ ] Ask only when a missing choice materially changes the public interface or behavior

If the request already specifies the interface and behavior, start the first red-green cycle without re-asking. Focus testing effort on critical paths and complex logic rather than mirroring every implementation detail.

### 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 the first verified path through the selected test boundary. Call it end-to-end only if the test actually exercises the complete runtime flow. Confirm RED fails for the intended missing behavior, not a syntax, import, environment, or setup error.

### 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](refactoring.md):

- [ ] Extract duplication where it represents the same responsibility
- [ ] 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 behavior, not implementation
[ ] Test uses public interface only
[ ] Test would survive internal refactor
[ ] Code is minimal for this test
[ ] No speculative features added
```