superpowers-test-driven-development

name	superpowers-test-driven-development
description	Use when implementing any feature or bugfix, before writing implementation code

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 your human partner):

• Throwaway prototypes
• Generated code
• Configuration files

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

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

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

Tool Usage Rule:

• Always use write_file to create test files BEFORE any implementation code
• Never use patch to create new implementation code (that's writing before testing)
• Use terminal to run tests and watch them fail before writing implementation

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.

Hermes Tools for TDD

As an AI assistant, you have specific tools to execute TDD properly:

• write_file: Create test files and implementation code
• read_file: Read test results and source files
• terminal: Run tests (pytest, npm test, etc.)
• patch: Make targeted edits to existing code

Tool Examples

# RED - Create a test file using write_file
write_file(
    path="tests/test_calculator.py",
    content="""
import pytest
from calculator import add

def test_adds_two_numbers():
    assert add(2, 3) == 5
"""
)

# Verify RED - Run the test using terminal
terminal(command="pytest tests/test_calculator.py -v")

# GREEN - Create implementation using write_file
write_file(
    path="calculator.py",
    content="""
def add(a, b):
    return a + b
"""
)

# Verify GREEN - Run tests again
terminal(command="pytest tests/test_calculator.py -v")

Critical: Always use write_file to create test files BEFORE creating any implementation code. Never use patch to add implementation code - that violates TDD.

Red-Green-Refactor

digraph tdd_cycle {
    rankdir=LR;
    red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"];
    verify_red [label="Verify fails\ncorrectly", shape=diamond];
    green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"];
    verify_green [label="Verify passes\nAll green", shape=diamond];
    refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"];
    next [label="Next", shape=ellipse];

    red -> verify_red;
    verify_red -> green [label="yes"];
    verify_red -> red [label="wrong\nfailure"];
    green -> verify_green;
    verify_green -> refactor [label="yes"];
    verify_green -> green [label="no"];
    refactor -> verify_green [label="stay\ngreen"];
    verify_green -> next;
    next -> red;
}

RED - Write Failing Test

Write one minimal test showing what should happen.

Create the test file using write_file:

write_file(
    path="tests/retry.test.ts",
    content="""
import { retryOperation } from '../retry';

test('retries failed operations 3 times', async () => {
  let attempts = 0;
  const operation = () => {
    attempts++;
    if (attempts < 3) throw new Error('fail');
    return 'success';
  };

  const result = await retryOperation(operation);

  expect(result).toBe('success');
  expect(attempts).toBe(3);
});
"""
)

Clear name, tests real behavior, one thing

```typescript test('retry works', async () => { const mock = jest.fn() .mockRejectedValueOnce(new Error()) .mockRejectedValueOnce(new Error()) .mockResolvedValueOnce('success'); await retryOperation(mock); expect(mock).toHaveBeenCalledTimes(3); }); ``` Vague name, tests mock not code

Requirements:

• One behavior
• Clear name
• Real code (no mocks unless unavoidable)
• Use write_file to create the test file before any implementation code exists

Verify RED - Watch It Fail

MANDATORY. Never skip.

Use the terminal tool to run tests:

terminal(command="npm test path/to/test.test.ts")

For Python pytest:

terminal(command="pytest tests/test_feature.py -v")

Confirm:

• Test fails (not errors)
• Failure message is expected
• Fails because feature missing (not typos)

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

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

GREEN - Minimal Code

Write simplest code to pass the test.

Create implementation using write_file:

write_file(
    path="retry.ts",
    content="""
export async function retryOperation<T>(fn: () => Promise<T>): Promise<T> {
  for (let i = 0; i < 3; i++) {
    try {
      return await fn();
    } catch (e) {
      if (i === 2) throw e;
    }
  }
  throw new Error('unreachable');
}
"""
)

Just enough to pass

```typescript async function retryOperation( fn: () => Promise, options?: { maxRetries?: number; backoff?: 'linear' | 'exponential'; onRetry?: (attempt: number) => void; } ): Promise { // YAGNI } ``` Over-engineered

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

Important: Use write_file for implementation code, never use patch to add new implementation code (that violates TDD - you're writing code before tests).

Verify GREEN - Watch It Pass

MANDATORY.

Use the terminal tool to run tests:

terminal(command="npm test path/to/test.test.ts")

For Python pytest:

terminal(command="pytest tests/test_feature.py -v")

Confirm:

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

Test fails? Fix code, not test.

Other tests fail? Fix now.

REFACTOR - Clean Up

After green only:

• Remove duplication
• Improve names
• Extract helpers

Use patch for refactoring existing green code:

patch(
    mode="replace",
    path="retry.ts",
    old_string="    if (i === 2) throw e;",
    new_string="    if (i === maxRetries - 1) throw e;"
)

Keep tests green. Don't add behavior.

Never use patch to add new implementation code - that violates TDD.

Repeat

Next failing test for next feature.

Good Tests

Quality	Good	Bad
Minimal	One thing. "and" in name? Split it.	test('validates email and domain and whitespace')
Clear	Name describes behavior	test('test1')
Shows intent	Demonstrates desired API	Obscures what code should do

Why Order Matters

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

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

• Might test 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 (X more hours, high confidence)
• Keep it and add tests after (30 min, low confidence, likely bugs)

The "waste" is keeping code you can't trust. Working code without real tests is technical debt.

"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. You verify remembered edge cases, not discovered ones.

Tests-first force edge case discovery before implementing. Tests-after verify you remembered everything (you didn't).

30 minutes of tests after ≠ TDD. You get coverage, lose proof tests work.

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 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 existing code.

Red Flags - STOP and Start Over

• Code before test
• Test after implementation
• Test passes immediately
• Can't explain why test failed
• Tests added "later"
• Rationalizing "just this once"
• "I already manually tested it"
• "Tests after achieve the same purpose"
• "It's about spirit not ritual"
• "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..."
• Using patch to add new implementation code (instead of write_file)
• Creating implementation files without creating test files first
• Running tests in terminal after writing implementation (wrong order)

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

Example: Bug Fix

Bug: Empty email accepted

RED Create test file with write_file:

write_file(
    path="tests/email_validation.test.ts",
    content="""
import { submitForm } from '../form';

test('rejects empty email', async () => {
  const result = await submitForm({ email: '' });
  expect(result.error).toBe('Email required');
});
"""
)

Verify RED

terminal(command="npm test tests/email_validation.test.ts")

Expected output:

FAIL: expected 'Email required', got undefined

GREEN Create implementation with write_file:

write_file(
    path="form.ts",
    content="""
interface FormData {
  email?: string;
}

export async function submitForm(data: FormData) {
  if (!data.email?.trim()) {
    return { error: 'Email required' };
  }
  // rest of implementation
}
"""
)

Verify GREEN

terminal(command="npm test tests/email_validation.test.ts")

Expected output:

PASS

REFACTOR Use patch to extract validation for multiple fields if needed:

patch(
    mode="replace",
    path="form.ts",
    old_string="  if (!data.email?.trim()) {\n    return { error: 'Email required' };\n  }",
    new_string="  const validationError = validateEmail(data.email);\n  if (validationError) return validationError;"
)

write_file(
    path="validation.ts",
    content="""
export function validateEmail(email?: string) {
  if (!email?.trim()) {
    return { error: 'Email required' };
  }
  return null;
}
"""
)

Verification Checklist

Before marking work complete:

• Created test files using write_file BEFORE any implementation
• Ran tests using terminal and watched them fail (test execution command shown)
• 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 using write_file (not patch for new features)
• Ran tests using terminal and verified all pass
• Output pristine (no errors, warnings)
• Tests use real code (mocks only if unavoidable)
• Edge cases and errors covered
• Used patch only for refactoring existing green code

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

When Stuck

Problem	Solution
Don't know how to test	Write wished-for API. Write assertion first. Ask your human partner.
Test too complicated	Design too complicated. Simplify interface.
Must mock everything	Code too coupled. Use dependency injection.
Test setup huge	Extract helpers. Still complex? Simplify design.
Need to examine existing code	Use read_file to inspect the source files before writing tests
Need to check test output	Use terminal to run tests and read the output carefully

Using read_file for investigation:

# Read existing implementation to understand structure
read_file(path="src/calculator.py")

# Read test files to see existing test patterns
read_file(path="tests/test_calculator.py")

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

Testing Anti-Patterns

When adding mocks or test utilities, read @testing-anti-patterns.md to avoid common pitfalls:

• Testing mock behavior instead of real behavior
• Adding test-only methods to production classes
• Mocking without understanding dependencies

Final Rule

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

No exceptions without your human partner's permission.