# Test Driven Development

> Test-driven development (TDD) with red-green-refactor loop. Use when user wants to build features, fix bugs, mentions "tdd", wants integration tests, or implementing all new code.

- Skill: `christopher-buss/test-driven-development` (Agent Skill, multi-file: 6 files)
- Install (CLI): `npx skillmds@latest add christopher-buss/test-driven-development`
- Raw SKILL.md: https://api.skillmd.com/api/skills/christopher-buss/test-driven-development/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Integrations & APIs
- Author: christopher-buss (https://skillmd.com/u/christopher-buss)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/christopher-buss/test-driven-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](./references/tests.md) for examples and
[mocking.md](./references/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.

```text
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](./references/deep-modules.md)
  (small interface, deep implementation)
- [ ] Design interfaces for [testability](./references/interface-design.md)
- [ ] 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.

Stub out the test file with the test descriptions using test.todo (without
implementation) to get user feedback on coverage before writing any code.

### 2. Tracer Bullet

Write ONE test that confirms ONE thing about the system:

```text
RED:   Write test for first behavior → test fails
GREEN: Write minimal code to pass → test passes
COMMIT: Commit this working test and code
```

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

### 3. Incremental Loop

For each remaining behavior:

```text
RED:   Write next test → fails
GREEN: Minimal code to pass → passes
COMMIT: Commit this working test and code
```

Rules:

- One test at a time
- Only enough code to pass current test
- Don't anticipate future test behavior
- Keep tests focused on observable behavior
- Commit history shows a clear progression of behavior being added

### 4. Refactor

After all tests pass, look for [refactor candidates](./references/refactoring.md):

- [ ] Extract duplication
- [ ] Deepen modules (move complexity behind simple interfaces)
- [ ] 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

```text
[ ] 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
[ ] Commit has been made with working test and code
```