# testing-anti-patterns

A skill for spotting tests that verify mock behavior, add test-only production methods, or use incomplete mocks.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 368 installs · ★ 78
- Author: bobmatnyc
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/bobmatnyc/claude-mpm-skills/tree/HEAD/universal/testing/testing-anti-patterns/SKILL.md
- Updated: 2026-07-18
- HTML: https://affpapa.org/en/nevibecode/skills/testing-anti-patterns

## What it does

testing-anti-patterns guides test writing and review around a simple rule: verify real behavior, not the behavior of a mock. It flags assertions aimed at mock calls, methods added to production classes only for tests, and mocking done before the dependencies and side effects are understood.

The material covers five problem groups: testing mock behavior, test-only production methods, uninformed mocking, incomplete mocks, and tests treated as an afterthought. It includes TypeScript and Jest examples, Python and pytest examples, plus a checklist for reviewing a test before commit.

When mock setup takes more than half of a test, the skill points toward considering an integration test with real components. It is disabled for direct user invocation and does not run automatically.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add bobmatnyc/claude-mpm-skills --skill testing-anti-patterns -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/testing-anti-patterns/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/testing-anti-patterns/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add bobmatnyc/claude-mpm-skills --skill testing-anti-patterns -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/testing-anti-patterns/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add bobmatnyc/claude-mpm-skills --skill testing-anti-patterns -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/testing-anti-patterns/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install bobmatnyc/claude-mpm-skills testing-anti-patterns
```

2. **Manually: put the files in place**
- File: `.github/skills/testing-anti-patterns/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add bobmatnyc/claude-mpm-skills --skill testing-anti-patterns -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/bobmatnyc/claude-mpm-skills --path universal/testing/testing-anti-patterns
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/testing-anti-patterns/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add bobmatnyc/claude-mpm-skills --skill testing-anti-patterns -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/bobmatnyc/claude-mpm-skills/tree/HEAD/universal/testing/testing-anti-patterns/SKILL.md

## When to use it
- check whether a test verifies real behavior
- find production methods that exist only for tests
- spot mocks missing fields used downstream
- trace dependencies before adding a mock
- move testing earlier into a TDD workflow

## Example prompts
- Review this test and tell me whether it checks real behavior or only mock behavior.
- Find test-only methods in the production classes changed in this branch and suggest test utilities instead.
- Inspect the mocks in `src/service.test.ts`; compare them with the dependency API and list missing fields.

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 86e3c0aec4ede73e.

BEGIN THIRD-PARTY FILE 86e3c0aec4ede73e
````text
---
name: testing-anti-patterns
description: Never test mock behavior. Never add test-only methods to production classes. Understand dependencies before mocking. Language-agnostic principles with TypeScript/Jest and Python/pytest examples.
user-invocable: false
disable-model-invocation: true
when_to_use: when writing or changing tests, adding mocks, or tempted to add test-only methods to production code
version: 3.0.0
tags:
  - testing
  - typescript
  - javascript
  - python
  - pytest
  - jest
  - mocking
  - tdd
  - anti-patterns
progressive_disclosure:
  entry_point:
    summary: "Avoid testing mocks, test-only production methods, and incomplete mocking. Test real behavior, not mock behavior. Covers TypeScript/Jest and Python/pytest."
    when_to_use: "When writing tests, adding mocks, reviewing test failures, or tempted to add test-only methods to production code."
    quick_start: "1. Ask: 'Am I testing real behavior?' 2. Check: 'Is this method only for tests?' 3. Verify: 'Do I understand what I'm mocking?' 4. Confirm: 'Is my mock complete?' 5. Apply: TDD prevents these patterns"
  references:
    - core-anti-patterns.md
    - completeness-anti-patterns.md
    - detection-guide.md
    - tdd-connection.md
    - python-examples.md
related_skills:
  - toolchains-typescript-testing-jest
  - toolchains-typescript-testing-vitest
  - toolchains-python-testing-pytest
  - universal-debugging-systematic-debugging
  - universal-debugging-verification-before-completion
---

# Testing Anti-Patterns

## Overview

Tests must verify real behavior, not mock behavior. Mocks are a means to isolate, not the thing being tested.

**Core principle:** Test what the code does, not what the mocks do.

**Following strict TDD prevents these anti-patterns.** See the Test-Driven Development skill (available in the skill library) for the complete TDD workflow.

## When to Use This Skill

Activate this skill when:
- **Writing or changing tests** - Verify tests cover real behavior
- **Adding mocks** - Ensure mocking is necessary and correct
- **Reviewing test failures** - Check if mock behavior is the issue
- **Tempted to add test-only methods** - STOP and reconsider
- **Tests feel overly complex** - Sign of over-mocking

## The Iron Laws

```
1. NEVER test mock behavior
2. NEVER add test-only methods to production classes
3. NEVER mock without understanding dependencies
4. NEVER create incomplete mocks
5. NEVER treat tests as afterthought
```

## Core Anti-Pattern Categories

### 1. Testing Mock Behavior
Asserting on mock elements instead of real behavior. **Fix:** Test real component or don't mock it.
**→** [core-anti-patterns.md](references/core-anti-patterns.md#anti-pattern-1-testing-mock-behavior)
````
END THIRD-PARTY FILE 86e3c0aec4ede73e

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