# Test Driven Development

> Use for any feature, bug fix, refactor, or behavior change — enforces RED-GREEN-REFACTOR. Write the failing test first, watch it fail, write minimal code to pass, refactor. Carve out only throwaway prototypes, generated code, configuration files. Adapted from Hermes Agent / obra/superpowers.

- Skill: `fabioc-aloha/test-driven-development` (Agent Skill)
- Install (CLI): `npx skillmds@latest add fabioc-aloha/test-driven-development`
- Raw SKILL.md: https://api.skillmd.com/api/skills/fabioc-aloha/test-driven-development/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: fabioc-aloha (https://skillmd.com/u/fabioc-aloha)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/fabioc-aloha/test-driven-development

---


# Test-Driven Development (TDD)

## Overview

Write the test first. Watch it fail. Write minimal code to pass.

**Core principle:** If you didn't watch the test fail, you don't know if it tests the right thing.

**Violating the letter of the rules is violating the spirit of the rules.**

## When to Use

**Always:**

- New features
- Bug fixes
- Refactoring
- Behavior changes

**Exceptions (ask the user first):**

- Throwaway prototypes (use [spike](../spike/SKILL.md) instead)
- Generated code
- Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

## The Iron Law

```text
NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST
```

Write code before the test? Delete it. Start over.

**No exceptions:**

- Don't keep it as "reference"
- Don't "adapt" it while writing tests
- Don't look at it
- Delete means delete

Implement fresh from tests. Period.

## Red-Green-Refactor Cycle

### RED — Write Failing Test

Write one minimal test showing what should happen.

**Good test:**

```python
def test_retries_failed_operations_3_times():
    attempts = 0
    def operation():
        nonlocal attempts
        attempts += 1
        if attempts < 3:
            raise Exception('fail')
        return 'success'

    result = retry_operation(operation)

    assert result == 'success'
    assert attempts == 3
```

Clear name, tests real behavior, one thing.

**Bad test:**

```python
def test_retry_works():
    mock = MagicMock()
    mock.side_effect = [Exception(), Exception(), 'success']
    result = retry_operation(mock)
    assert result == 'success'  # What about retry count? Timing?
```

Vague name, tests mock not real code.

**Requirements:**

- One behavior per test
- Clear descriptive name ("and" in name? Split it)
- Real code, not mocks (unless truly unavoidable)
- Name describes behavior, not implementation

### Verify RED — Watch It Fail

**MANDATORY. Never skip.**

Run the specific test through the workspace terminal:

```bash
pytest tests/test_feature.py::test_specific_behavior -v
```

Confirm:

- Test fails (not errors from typos)
- Failure message is expected
- Fails because the feature is missing

**Test passes immediately?** You're testing existing behavior. Fix the test.

**Test errors?** Fix the error, re-run until it fails correctly.

### GREEN — Minimal Code

Write the simplest code to pass the test. Nothing more.

**Good:**

```python
def add(a, b):
    return a + b  # Nothing extra
```

**Bad:**

```python
def add(a, b):
    result = a + b
    logging.info(f"Adding {a} + {b} = {result}")  # Extra!
    return result
```

Don't add features, refactor other code, or "improve" beyond the test.

**Cheating is OK in GREEN:**

- Hardcode return values
- Copy-paste
- Duplicate code
- Skip edge cases

We'll fix it in REFACTOR.

### Verify GREEN — Watch It Pass

**MANDATORY.**

```bash
# Run the specific test
pytest tests/test_feature.py::test_specific_behavior -v

# Then run ALL tests to check for regressions
pytest tests/ -q
```

Confirm:

- Test passes
- Other tests still pass
- Output pristine (no errors, warnings)

**Test fails?** Fix the code, not the test.

**Other tests fail?** Fix regressions now.

### REFACTOR — Clean Up

After green only:

- Remove duplication
- Improve names
- Extract helpers
- Simplify expressions

Keep tests green throughout. Don't add behavior.

**If tests fail during refactor:** Undo immediately. Take smaller steps.

### Repeat

Next failing test for next behavior. One cycle at a time.

## Why Order Matters

**"I'll write tests after to verify it works"**

Tests written after code pass immediately. Passing immediately proves nothing:

- Might test the wrong thing
- Might test implementation, not behavior
- Might miss edge cases you forgot
- You never saw it catch the bug

Test-first forces you to see the test fail, proving it actually tests something.

**"I already manually tested all the edge cases"**

Manual testing is ad-hoc. You think you tested everything but:

- No record of what you tested
- Can't re-run when code changes
- Easy to forget cases under pressure
- "It worked when I tried it" ≠ comprehensive

Automated tests are systematic. They run the same way every time.

**"Deleting X hours of work is wasteful"**

Sunk cost fallacy. The time is already gone. Your choice now:

- Delete and rewrite with TDD (high confidence)
- Keep it and add tests after (low confidence, likely bugs)

The "waste" is keeping code you can't trust.

**"TDD is dogmatic, being pragmatic means adapting"**

TDD IS pragmatic:

- Finds bugs before commit (faster than debugging after)
- Prevents regressions (tests catch breaks immediately)
- Documents behavior (tests show how to use code)
- Enables refactoring (change freely, tests catch breaks)

"Pragmatic" shortcuts = debugging in production = slower.

**"Tests after achieve the same goals — it's spirit not ritual"**

No. Tests-after answer "What does this do?" Tests-first answer "What should this do?"

Tests-after are biased by your implementation. You test what you built, not what's required. Tests-first force edge case discovery before implementing.

## Common Rationalizations

| Excuse | Reality |
|---|---|
| "Too simple to test" | Simple code breaks. Test takes 30 seconds. |
| "I'll test after" | Tests passing immediately prove nothing. |
| "Tests after achieve same goals" | Tests-after = "what does this do?" Tests-first = "what should this do?" |
| "Already manually tested" | Ad-hoc ≠ systematic. No record, can't re-run. |
| "Deleting X hours is wasteful" | Sunk cost fallacy. Keeping unverified code is technical debt. |
| "Keep as reference, write tests first" | You'll adapt it. That's testing after. Delete means delete. |
| "Need to explore first" | Fine. Throw away exploration, start with TDD. |
| "Test hard = design unclear" | Listen to the test. Hard to test = hard to use. |
| "TDD will slow me down" | TDD faster than debugging. Pragmatic = test-first. |
| "Manual test faster" | Manual doesn't prove edge cases. You'll re-test every change. |
| "Existing code has no tests" | You're improving it. Add tests for the code you touch. |

## Red Flags — STOP and Start Over

If you catch yourself doing any of these, delete the code and restart with TDD:

- Code before test
- Test after implementation
- Test passes immediately on first run
- Can't explain why test failed
- Tests added "later"
- Rationalizing "just this once"
- "I already manually tested it"
- "Tests after achieve the same purpose"
- "Keep as reference" or "adapt existing code"
- "Already spent X hours, deleting is wasteful"
- "TDD is dogmatic, I'm being pragmatic"
- "This is different because..."

**All of these mean: Delete code. Start over with TDD.**

## Verification Checklist

Before marking work complete:

- [ ] Every new function/method has a test
- [ ] Watched each test fail before implementing
- [ ] Each test failed for expected reason (feature missing, not typo)
- [ ] Wrote minimal code to pass each test
- [ ] All tests pass
- [ ] Output pristine (no errors, warnings)
- [ ] Tests use real code (mocks only if unavoidable)
- [ ] Edge cases and errors covered

Can't check all boxes? You skipped TDD. Start over.

## When Stuck

| Problem | Solution |
|---|---|
| Don't know how to test | Write the wished-for API. Write the assertion first. Ask the user. |
| Test too complicated | Design too complicated. Simplify the interface. |
| Must mock everything | Code too coupled. Use dependency injection. |
| Test setup huge | Extract helpers. Still complex? Simplify the design. |

## Integration With Other Skills

### With delegated work

When dispatching a worker subagent, enforce TDD in the goal:

> "Implement [feature] using strict TDD. Follow test-driven-development skill: write failing test FIRST, run to verify failure, write minimal code to pass, run to verify pass, refactor if needed, commit. Project test command: `pytest tests/ -q`."

### With systematic-debugging

Bug found? Write failing test reproducing it. Follow TDD cycle. The test proves the fix and prevents regression. See [systematic-debugging](../systematic-debugging/SKILL.md) — never fix bugs without a test.

### With plan

[plan](../plan/SKILL.md) calls for TDD per task. Every implementation task in a plan should start with RED.

## Testing Anti-Patterns

- **Testing mock behavior instead of real behavior** — mocks should verify interactions, not replace the system under test
- **Testing implementation details** — test behavior/results, not internal method calls
- **Happy path only** — always test edge cases, errors, and boundaries
- **Brittle tests** — tests should verify behavior, not structure; refactoring shouldn't break them

## Final Rule

```text
Production code → test exists and failed first
Otherwise → not TDD
```

No exceptions without the user's explicit permission.

## Related

- [systematic-debugging](../systematic-debugging/SKILL.md) — bug-found path that produces the failing test
- [plan](../plan/SKILL.md) — every plan task should embed RED-GREEN-REFACTOR
- [spike](../spike/SKILL.md) — TDD exception lane for throwaway feasibility experiments
- [code-review](../code-review/SKILL.md) — post-write companion; TDD is pre-write, code-review is the review gate

## Would Revise If

- **Event-based**: ≥3 heir feedback files flagging the Iron Law as obstruction within 90 days; OR zero RED-GREEN-REFACTOR cycle citations in heir commits by 2026-09-07 (skill is decorative — heirs ignore it). First trigger sinks to Mall (still installable on demand) rather than removing entirely.
- **Date-based**: 2026-09-07 (90 days from adoption). If the "When to Use" carve-outs (throwaway prototypes, generated code, configuration files) consistently mis-classify heir work and produce friction, broaden the carve-outs explicitly.
- **Counter-evidence**: if a heir reports that TDD enforcement caused a real regression (test-first led to a worse design that test-after would have caught), document the case and revise the Iron Law's absoluteness.

## Attribution

Adapted from [Hermes Agent](https://github.com/NousResearch/hermes-agent) (Nous Research, MIT), which itself adapted the discipline from [obra/superpowers](https://github.com/obra/superpowers) (MIT). Both upstream sources MIT-licensed.