# 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: `tiennguyen1203/tdd` (Agent Skill, multi-file: 6 files)
- Install (CLI): `npx skillmds@latest add tiennguyen1203/tdd`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tiennguyen1203/tdd/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Integrations & APIs
- Author: tiennguyen1203 (https://skillmd.com/u/tiennguyen1203)
- Updated: 2026-09-22
- Page: https://skillmd.com/skills/tiennguyen1203/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** 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](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 (MANDATORY — never skip)

**STOP. You MUST use `AskUserQuestion` before writing any test code.** This applies to both new test files and adding tests to existing files.

Steps:

1. Read the relevant source code and any existing test file
2. Draft a list of test suites/cases you plan to write (use `describe`/`it` naming)
3. **Use `AskUserQuestion`** to present the proposed test suites to the user for confirmation
4. Wait for user approval before writing any code

When adding incremental tests to an existing file, still present the new test cases for confirmation first.

Example `AskUserQuestion` usage:

```
question: "Here are the test cases I plan to write for `checkout`. Do these look right?"
options:
  - "Approve and proceed"
  - "I want to adjust the test cases"
```

Include the proposed test structure in the question description so the user can review it.

**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.

Also consider during planning:

- Interface changes needed
- Opportunities for [deep modules](deep-modules.md)
- [Testability](interface-design.md) of the design

### 2. Tracer Bullet

After user approves the test plan, write ONE test that confirms ONE thing:

```
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 approved 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
- [ ] 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
```