practicing-test-driven-development

Test-Driven Development (TDD)

Contents

• Overview
• The Iron Law
• Red-Green-Refactor
• Good Tests
• Red Flags
• Verification Checklist
• Multi-Model Test Generation

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.

Protocol Threshold (Required)

Follow the [CP Protocol Threshold] injected by hooks:

• Before the first Task call: output a standalone [CP1 Assessment] block (the block itself must not include tool calls; tool calls may follow in the same reply)
• Before claiming completion / requesting review / claiming verification passed: output a standalone [CP3 Assessment] block (the block itself must not include tool calls; the conclusion may follow in the same reply)

If unmet -> immediately perform the CP assessment, then continue the flow right away; do not stop or interrupt.

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.

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

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.

Hard reminder: before your first Task tool call, you must output a standalone 【CP1 Assessment】 block (fixed format with fields).

► Checkpoint 1 (Task Analysis): Before writing test, apply checkpoint logic from coordinating-multi-model-work/checkpoints.md:

• Check critical task conditions → Match: invoke domain expert for test design
• Complex testing scenario → invoke cross-validation for comprehensive test coverage

✅ Good example:

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

❌ Bad example:

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)

Verify RED - Watch It Fail

MANDATORY. Never skip.

npm test path/to/test.test.ts

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.

► Checkpoint 3 (Quality Gate): After implementation passes, apply checkpoint logic from coordinating-multi-model-work/checkpoints.md:

• Implementation complete → invoke domain expert for code review
• Critical business logic → invoke cross-validation for security/performance review

✅ Good example:

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

❌ Bad example:

async function retryOperation<T>(
  fn: () => Promise<T>,
  options?: {
    maxRetries?: number;
    backoff?: "linear" | "exponential";
    onRetry?: (attempt: number) => void;
  },
): Promise<T> {
  // YAGNI
}

Over-engineered

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

Verify GREEN - Watch It Pass

MANDATORY.

Hard reminder: before claiming completion or verification passed, you must output a standalone 【CP3 Assessment】 block (fixed format with fields).

npm test path/to/test.test.ts

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

Keep tests green. Don't add behavior.

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

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..."

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

Example: Bug Fix

Bug: Empty email accepted

RED

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

Verify RED

$ npm test
FAIL: expected 'Email required', got undefined

GREEN

function submitForm(data: FormData) {
  if (!data.email?.trim()) {
    return { error: "Email required" };
  }
  // ...
}

Verify GREEN

$ npm test
PASS

REFACTOR Extract validation for multiple fields if needed.

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 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.

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

Multi-Model Test Generation

Related skill: superpowers:coordinating-multi-model-work

At checkpoints, apply semantic routing using coordinating-multi-model-work/routing-decision.md:

• Frontend component tests → GEMINI (Gemini MCP mcp__gemini__gemini)
• Backend logic tests → CODEX (Codex MCP mcp__codex__codex)
• Full-stack tests → CROSS_VALIDATION (call both MCP tools)

Full checkpoint logic: See coordinating-multi-model-work/checkpoints.md

See coordinating-multi-model-work/INTEGRATION.md for invocation templates.

CRITICAL: Generated tests must still follow TDD cycle - run them to confirm they fail (RED) before implementing.

Fallback (Fail-Closed): If external models are unavailable or time out, STOP and follow coordinating-multi-model-work/GATE.md (do not proceed with generated tests).

Final Rule

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

No exceptions without your human partner's permission.