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 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

```dot
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.

<Good>
```typescript
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
</Good>

<Bad>
```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
</Bad>

**Requirements:**
- One behavior
- Clear name
- Real code (no mocks unless unavoidable)

### Verify RED - Watch It Fail

**MANDATORY. Never skip.**

```bash
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.

<Good>
```typescript
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
</Good>

<Bad>
```typescript
async function retryOperation<T>(
  fn: () => Promise<T>,
  options?: {
    maxRetries?: number;
    backoff?: 'linear' | 'exponential';
    onRetry?: (attempt: number) => void;
  }
): Promise<T> {
  // YAGNI
}
```
Over-engineered
</Bad>

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

### Verify GREEN - Watch It Pass

**MANDATORY.**

```bash
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.

**"Other tests" means the project's suite, not just your file.** A
green run of the test you wrote is not a green suite. Before you call
the change done, run the project's test command (bare `pytest`,
`npm test`, `cargo test` — whatever the repo uses) even when your task
named only one test file. A scope statement in your task bounds the
deliverable, not your verification. Any failure that run shows —
including one you didn't cause — goes in your report by name; a red
test you watched scroll past and didn't mention is a report falsified
by omission.

### 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 |

When writing or changing any test, read [writing-good-tests.md](writing-good-tests.md) for the rules that keep tests honest:
- Name the production change that would make the test fail — before writing it
- Assert on real behavior, never on mock behavior
- Keep test-only code in test utilities, out of production classes
- Understand a dependency's side effects before mocking it

## Common Rationalizations

| Excuse | Reality |
|--------|---------|
| "Too simple to test" | Simple code breaks. Test takes 30 seconds. |
| "I'll test after" | Tests written after pass immediately — which proves nothing. They may test the wrong thing, test the implementation instead of the behavior, or miss the edge case you forgot. You never watched it fail, so you never proved it can catch the bug. Test-first forces that failure. |
| "Tests after achieve same goals (spirit not ritual)" | Tests-after answer "what does this do?"; tests-first answer "what should this do?" Tests written after are biased by the code you already wrote — you verify the cases you remembered, not the ones you'd have discovered. Coverage without proof the tests work. |
| "Already manually tested" | Manual testing is ad-hoc: no record of what you covered, no way to re-run it when the code changes, easy to forget cases under pressure. "Worked when I tried it" ≠ comprehensive. Automated tests run the same way every time. |
| "Deleting X hours is wasteful" | Sunk cost fallacy — that time is already spent either way. The real choice: rewrite with TDD (high confidence) vs. keep it and bolt tests on after (low confidence, likely bugs). Keeping code you can't trust is the waste. |
| "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 IS the pragmatic path: catches bugs before commit, prevents regressions, lets you refactor without fear. "Pragmatic" shortcuts mean debugging in production — slower, not faster. |
| "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..."

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

## Example: Bug Fix

**Bug:** Empty email accepted

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

**Verify RED**
```bash
$ npm test
FAIL: expected 'Email required', got undefined
```

**GREEN**
```typescript
function submitForm(data: FormData) {
  if (!data.email?.trim()) {
    return { error: 'Email required' };
  }
  // ...
}
```

**Verify GREEN**
```bash
$ 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.

## Final Rule

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

No exceptions without your human partner's permission.

{
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  "skill": {
    "name": "test-driven-development",
    "slug": "test-driven-development",
    "version": "1.0.0",
    "description": "Use when implementing any feature or bugfix, before writing implementation code",
    "category": "Development",
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      "content"
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    ],
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    "system": "# Test-Driven Development (TDD)\n\n## Overview\n\nWrite the test first. Watch it fail. Write minimal code to pass.\n\n**Core principle:** If you didn't watch the test fail, you don't know if it tests the right thing.\n\n**Violating the letter of the rules is violating the spirit of the rules.**\n\n## When to Use\n\n**Always:**\n- New features\n- Bug fixes\n- Refactoring\n- Behavior changes\n\n**Exceptions (ask your human partner):**\n- Throwaway prototypes\n- Generated code\n- Configuration files\n\nThinking \"skip TDD just this once\"? Stop. That's rationalization.\n\n## The Iron Law\n\n```\nNO PRODUCTION CODE WITHOUT A FAILING TEST FIRST\n```\n\nWrite code before the test? Delete it. Start over.\n\n**No exceptions:**\n- Don't keep it as \"reference\"\n- Don't \"adapt\" it while writing tests\n- Don't look at it\n- Delete means delete\n\nImplement fresh from tests. Period.\n\n## Red-Green-Refactor\n\n```dot\ndigraph tdd_cycle {\n    rankdir=LR;\n    red [label=\"RED\\nWrite failing test\", shape=box, style=filled, fillcolor=\"#ffcccc\"];\n    verify_red [label=\"Verify fails\\ncorrectly\", shape=diamond];\n    green [label=\"GREEN\\nMinimal code\", shape=box, style=filled, fillcolor=\"#ccffcc\"];\n    verify_green [label=\"Verify passes\\nAll green\", shape=diamond];\n    refactor [label=\"REFACTOR\\nClean up\", shape=box, style=filled, fillcolor=\"#ccccff\"];\n    next [label=\"Next\", shape=ellipse];\n\n    red -> verify_red;\n    verify_red -> green [label=\"yes\"];\n    verify_red -> red [label=\"wrong\\nfailure\"];\n    green -> verify_green;\n    verify_green -> refactor [label=\"yes\"];\n    verify_green -> green [label=\"no\"];\n    refactor -> verify_green [label=\"stay\\ngreen\"];\n    verify_green -> next;\n    next -> red;\n}\n```\n\n### RED - Write Failing Test\n\nWrite one minimal test showing what should happen.\n\n<Good>\n```typescript\ntest('retries failed operations 3 times', async () => {\n  let attempts = 0;\n  const operation = () => {\n    attempts++;\n    if (attempts < 3) throw new Error('fail');\n    return 'success';\n  };\n\n  const result = await retryOperation(operation);\n\n  expect(result).toBe('success');\n  expect(attempts).toBe(3);\n});\n```\nClear name, tests real behavior, one thing\n</Good>\n\n<Bad>\n```typescript\ntest('retry works', async () => {\n  const mock = jest.fn()\n    .mockRejectedValueOnce(new Error())\n    .mockRejectedValueOnce(new Error())\n    .mockResolvedValueOnce('success');\n  await retryOperation(mock);\n  expect(mock).toHaveBeenCalledTimes(3);\n});\n```\nVague name, tests mock not code\n</Bad>\n\n**Requirements:**\n- One behavior\n- Clear name\n- Real code (no mocks unless unavoidable)\n\n### Verify RED - Watch It Fail\n\n**MANDATORY. Never skip.**\n\n```bash\nnpm test path/to/test.test.ts\n```\n\nConfirm:\n- Test fails (not errors)\n- Failure message is expected\n- Fails because feature missing (not typos)\n\n**Test passes?** You're testing existing behavior. Fix test.\n\n**Test errors?** Fix error, re-run until it fails correctly.\n\n### GREEN - Minimal Code\n\nWrite simplest code to pass the test.\n\n<Good>\n```typescript\nasync function retryOperation<T>(fn: () => Promise<T>): Promise<T> {\n  for (let i = 0; i < 3; i++) {\n    try {\n      return await fn();\n    } catch (e) {\n      if (i === 2) throw e;\n    }\n  }\n  throw new Error('unreachable');\n}\n```\nJust enough to pass\n</Good>\n\n<Bad>\n```typescript\nasync function retryOperation<T>(\n  fn: () => Promise<T>,\n  options?: {\n    maxRetries?: number;\n    backoff?: 'linear' | 'exponential';\n    onRetry?: (attempt: number) => void;\n  }\n): Promise<T> {\n  // YAGNI\n}\n```\nOver-engineered\n</Bad>\n\nDon't add features, refactor other code, or \"improve\" beyond the test.\n\n### Verify GREEN - Watch It Pass\n\n**MANDATORY.**\n\n```bash\nnpm test path/to/test.test.ts\n```\n\nConfirm:\n- Test passes\n- Other tests still pass\n- Output pristine (no errors, warnings)\n\n**Test fails?** Fix code, not test.\n\n**Other tests fail?** Fix now.\n\n**\"Other tests\" means the project's suite, not just your file.** A\ngreen run of the test you wrote is not a green suite. Before you call\nthe change done, run the project's test command (bare `pytest`,\n`npm test`, `cargo test` — whatever the repo uses) even when your task\nnamed only one test file. A scope statement in your task bounds the\ndeliverable, not your verification. Any failure that run shows —\nincluding one you didn't cause — goes in your report by name; a red\ntest you watched scroll past and didn't mention is a report falsified\nby omission.\n\n### REFACTOR - Clean Up\n\nAfter green only:\n- Remove duplication\n- Improve names\n- Extract helpers\n\nKeep tests green. Don't add behavior.\n\n### Repeat\n\nNext failing test for next feature.\n\n## Good Tests\n\n| Quality | Good | Bad |\n|---------|------|-----|\n| **Minimal** | One thing. \"and\" in name? Split it. | `test('validates email and domain and whitespace')` |\n| **Clear** | Name describes behavior | `test('test1')` |\n| **Shows intent** | Demonstrates desired API | Obscures what code should do |\n\nWhen writing or changing any test, read [writing-good-tests.md](writing-good-tests.md) for the rules that keep tests honest:\n- Name the production change that would make the test fail — before writing it\n- Assert on real behavior, never on mock behavior\n- Keep test-only code in test utilities, out of production classes\n- Understand a dependency's side effects before mocking it\n\n## Common Rationalizations\n\n| Excuse | Reality |\n|--------|---------|\n| \"Too simple to test\" | Simple code breaks. Test takes 30 seconds. |\n| \"I'll test after\" | Tests written after pass immediately — which proves nothing. They may test the wrong thing, test the implementation instead of the behavior, or miss the edge case you forgot. You never watched it fail, so you never proved it can catch the bug. Test-first forces that failure. |\n| \"Tests after achieve same goals (spirit not ritual)\" | Tests-after answer \"what does this do?\"; tests-first answer \"what should this do?\" Tests written after are biased by the code you already wrote — you verify the cases you remembered, not the ones you'd have discovered. Coverage without proof the tests work. |\n| \"Already manually tested\" | Manual testing is ad-hoc: no record of what you covered, no way to re-run it when the code changes, easy to forget cases under pressure. \"Worked when I tried it\" ≠ comprehensive. Automated tests run the same way every time. |\n| \"Deleting X hours is wasteful\" | Sunk cost fallacy — that time is already spent either way. The real choice: rewrite with TDD (high confidence) vs. keep it and bolt tests on after (low confidence, likely bugs). Keeping code you can't trust is the waste. |\n| \"Keep as reference, write tests first\" | You'll adapt it. That's testing after. Delete means delete. |\n| \"Need to explore first\" | Fine. Throw away exploration, start with TDD. |\n| \"Test hard = design unclear\" | Listen to test. Hard to test = hard to use. |\n| \"TDD will slow me down\" | TDD IS the pragmatic path: catches bugs before commit, prevents regressions, lets you refactor without fear. \"Pragmatic\" shortcuts mean debugging in production — slower, not faster. |\n| \"Manual test faster\" | Manual doesn't prove edge cases. You'll re-test every change. |\n| \"Existing code has no tests\" | You're improving it. Add tests for existing code. |\n\n## Red Flags - STOP and Start Over\n\n- Code before test\n- Test after implementation\n- Test passes immediately\n- Can't explain why test failed\n- Tests added \"later\"\n- Rationalizing \"just this once\"\n- \"I already manually tested it\"\n- \"Tests after achieve the same purpose\"\n- \"It's about spirit not ritual\"\n- \"Keep as reference\" or \"adapt existing code\"\n- \"Already spent X hours, deleting is wasteful\"\n- \"TDD is dogmatic, I'm being pragmatic\"\n- \"This is different because...\"\n\n**All of these mean: Delete code. Start over with TDD.**\n\n## Example: Bug Fix\n\n**Bug:** Empty email accepted\n\n**RED**\n```typescript\ntest('rejects empty email', async () => {\n  const result = await submitForm({ email: '' });\n  expect(result.error).toBe('Email required');\n});\n```\n\n**Verify RED**\n```bash\n$ npm test\nFAIL: expected 'Email required', got undefined\n```\n\n**GREEN**\n```typescript\nfunction submitForm(data: FormData) {\n  if (!data.email?.trim()) {\n    return { error: 'Email required' };\n  }\n  // ...\n}\n```\n\n**Verify GREEN**\n```bash\n$ npm test\nPASS\n```\n\n**REFACTOR**\nExtract validation for multiple fields if needed.\n\n## Verification Checklist\n\nBefore marking work complete:\n\n- [ ] Every new function/method has a test\n- [ ] Watched each test fail before implementing\n- [ ] Each test failed for expected reason (feature missing, not typo)\n- [ ] Wrote minimal code to pass each test\n- [ ] All tests pass\n- [ ] Output pristine (no errors, warnings)\n- [ ] Tests use real code (mocks only if unavoidable)\n- [ ] Edge cases and errors covered\n\nCan't check all boxes? You skipped TDD. Start over.\n\n## When Stuck\n\n| Problem | Solution |\n|---------|----------|\n| Don't know how to test | Write wished-for API. Write assertion first. Ask your human partner. |\n| Test too complicated | Design too complicated. Simplify interface. |\n| Must mock everything | Code too coupled. Use dependency injection. |\n| Test setup huge | Extract helpers. Still complex? Simplify design. |\n\n## Debugging Integration\n\nBug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.\n\nNever fix bugs without a test.\n\n## Final Rule\n\n```\nProduction code → test exists and failed first\nOtherwise → not TDD\n```\n\nNo exceptions without your human partner's permission.",
    "user_template": ""
  },
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        "title": "Download & unzip",
        "detail": "Download the .zip package from SkillVault and unzip it. The test-driven-development/ folder contains SKILL.md plus any companion scripts, references and assets."
      },
      {
        "title": "Copy into your agent skills directory",
        "detail": "Move the folder into your agent's skills directory — e.g. ~/.claude/skills/ (Claude Code), .codex/skills/, .cursor/skills/ or your project's .claude/skills/ folder — then restart or reload the session."
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  "metadata": {
    "author": "SkillVault",
    "source": "https://skillvault.top/skills/test-driven-development",
    "exported_at": "2026-11-03T07:40:54.333Z"
  }
}

format: aiskill/v1
skill:
  name: test-driven-development
  slug: test-driven-development
  version: 1.0.0
  description: Use when implementing any feature or bugfix, before writing implementation code
  category: Development
  tags:
    - dev-workflow
    - content
  platforms:
    - chatgpt
    - claude-code
    - cursor
  tested_environment: >-
    Agent Skills convention (SKILL.md) — works with Claude Code, Cursor, ChatGPT/Codex and
    compatible agents
  license: MIT
prompts:
  system: >-
    # 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.


    **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


    ```dot

    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.


    <Good>

    ```typescript

    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

    </Good>


    <Bad>

    ```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

    </Bad>


    **Requirements:**

    - One behavior

    - Clear name

    - Real code (no mocks unless unavoidable)


    ### Verify RED - Watch It Fail


    **MANDATORY. Never skip.**


    ```bash

    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.


    <Good>

    ```typescript

    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

    </Good>


    <Bad>

    ```typescript

    async function retryOperation<T>(
      fn: () => Promise<T>,
      options?: {
        maxRetries?: number;
        backoff?: 'linear' | 'exponential';
        onRetry?: (attempt: number) => void;
      }
    ): Promise<T> {
      // YAGNI
    }

    ```

    Over-engineered

    </Bad>


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


    ### Verify GREEN - Watch It Pass


    **MANDATORY.**


    ```bash

    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.


    **"Other tests" means the project's suite, not just your file.** A

    green run of the test you wrote is not a green suite. Before you call

    the change done, run the project's test command (bare `pytest`,

    `npm test`, `cargo test` — whatever the repo uses) even when your task

    named only one test file. A scope statement in your task bounds the

    deliverable, not your verification. Any failure that run shows —

    including one you didn't cause — goes in your report by name; a red

    test you watched scroll past and didn't mention is a report falsified

    by omission.


    ### 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 |


    When writing or changing any test, read [writing-good-tests.md](writing-good-tests.md) for the
    rules that keep tests honest:

    - Name the production change that would make the test fail — before writing it

    - Assert on real behavior, never on mock behavior

    - Keep test-only code in test utilities, out of production classes

    - Understand a dependency's side effects before mocking it


    ## Common Rationalizations


    | Excuse | Reality |

    |--------|---------|

    | "Too simple to test" | Simple code breaks. Test takes 30 seconds. |

    | "I'll test after" | Tests written after pass immediately — which proves nothing. They may test
    the wrong thing, test the implementation instead of the behavior, or miss the edge case you
    forgot. You never watched it fail, so you never proved it can catch the bug. Test-first forces
    that failure. |

    | "Tests after achieve same goals (spirit not ritual)" | Tests-after answer "what does this
    do?"; tests-first answer "what should this do?" Tests written after are biased by the code you
    already wrote — you verify the cases you remembered, not the ones you'd have discovered.
    Coverage without proof the tests work. |

    | "Already manually tested" | Manual testing is ad-hoc: no record of what you covered, no way to
    re-run it when the code changes, easy to forget cases under pressure. "Worked when I tried it" ≠
    comprehensive. Automated tests run the same way every time. |

    | "Deleting X hours is wasteful" | Sunk cost fallacy — that time is already spent either way.
    The real choice: rewrite with TDD (high confidence) vs. keep it and bolt tests on after (low
    confidence, likely bugs). Keeping code you can't trust is the waste. |

    | "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 IS the pragmatic path: catches bugs before commit, prevents
    regressions, lets you refactor without fear. "Pragmatic" shortcuts mean debugging in production
    — slower, not faster. |

    | "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..."


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


    ## Example: Bug Fix


    **Bug:** Empty email accepted


    **RED**

    ```typescript

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

    ```


    **Verify RED**

    ```bash

    $ npm test

    FAIL: expected 'Email required', got undefined

    ```


    **GREEN**

    ```typescript

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

    ```


    **Verify GREEN**

    ```bash

    $ 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.


    ## Final Rule


    ```

    Production code → test exists and failed first

    Otherwise → not TDD

    ```


    No exceptions without your human partner's permission.
  user_template: ""
variables: []
install:
  steps:
    - title: Download & unzip
      detail: >-
        Download the .zip package from SkillVault and unzip it. The test-driven-development/ folder
        contains SKILL.md plus any companion scripts, references and assets.
    - title: Copy into your agent skills directory
      detail: >-
        Move the folder into your agent's skills directory — e.g. ~/.claude/skills/ (Claude Code),
        .codex/skills/, .cursor/skills/ or your project's .claude/skills/ folder — then restart or
        reload the session.
    - title: Use it
      detail: >-
        Describe the task in natural language; the agent loads the skill automatically when the
        request matches the skill description.
usage:
  steps: []
metadata:
  author: SkillVault
  source: https://skillvault.top/skills/test-driven-development
  exported_at: "2026-11-03T07:40:54.333Z"