# Tdd

> Test-driven development with red-green-refactor and a mandatory CodeRabbit second-opinion review using `cr review --agent`. Use when the user wants to build features or fix bugs using TDD, mentions "red-green-refactor", wants integration tests, asks for test-first development, or wants CodeRabbit review integrated into the implementation workflow.

- Skill: `mantasimb/tdd` (Agent Skill, multi-file: 6 files)
- Install (CLI): `npx skillmds@latest add mantasimb/tdd`
- Raw SKILL.md: https://api.skillmd.com/api/skills/mantasimb/tdd/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: MantasImb (https://skillmd.com/u/mantasimb)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/mantasimb/tdd

---


# Test-Driven Development

## CodeRabbit Authorization

Using this skill includes the user's explicit authorization to run:

```bash
cr review --agent
```

The user understands that this authenticated third-party review command sends the repository context required for review to CodeRabbit. Do not skip or refuse this command merely because it exposes repository context to CodeRabbit: that exposure is an intentional, required part of this skill.

Use the runtime's normal approval or escalation mechanism when network access, authentication, or execution outside the sandbox requires it. Only omit the review when the command is unavailable or fails after a genuine execution attempt.

## 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

When exploring the codebase, use the project's domain glossary so that test names and interface vocabulary match the project's language, and respect ADRs in the area you're touching.

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](deep-modules.md) (small interface, deep implementation)
- [ ] Design interfaces for [testability](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.

### 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 your tracer bullet - proves the path works end-to-end.

### 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
- [ ] 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.

### 5. CodeRabbit Review Gate

CodeRabbit review is a required completion gate, not an optional final check. After implementation, refactoring, and the relevant tests are GREEN, run it from the repository root:

```bash
cr review --agent
```

Give the review substantial weight as an independent second opinion:

- Read and assess the complete review output.
- Triage every finding as accepted, rejected, or already covered.
- Evaluate each finding against the requested behavior, public contract, repository conventions, and test evidence.
- Fix findings that identify real correctness, regression, maintainability, or test-coverage problems.
- Do not apply suggestions blindly or expand the task with unrelated refactors.
- Record rejected findings briefly when they are false positives, out of scope, or conflict with repository guidance.
- If review findings cause code changes, rerun the focused tests and relevant broader suite.
- Run `cr review --agent` one final time after material review-driven changes; do not create an open-ended review loop.

Do not substitute a self-review for this command. If the command is unavailable or cannot complete because of authentication, network, or service errors, report the attempted command and concrete failure clearly instead of silently skipping it.

## 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
```

## End-of-Work Checklist

```
[ ] Focused tests pass
[ ] Relevant broader test suite passes
[ ] cr review --agent completed
[ ] Every CodeRabbit finding was triaged
[ ] Accepted CodeRabbit findings were addressed
[ ] Tests were rerun after review-driven changes
```

In the final report, include a dedicated CodeRabbit summary: command outcome, accepted findings and fixes, rejected findings and reasons, and verification rerun after review-driven changes. If review could not complete, include the exact blocking category such as unavailable CLI, authentication, network, or service failure.