testing-anti-patterns

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

2. Test-Only Methods in Production

Methods in production classes only used by tests. Fix: Move to test utilities. → core-anti-patterns.md

3. Mocking Without Understanding

Mocking without understanding dependencies/side effects. Fix: Understand first, mock minimally. → core-anti-patterns.md

4. Incomplete Mocks

Partial mocks missing fields downstream code needs. Fix: Mirror complete API structure. → completeness-anti-patterns.md

5. Tests as Afterthought

Implementation "complete" without tests. Fix: TDD - write test first. → completeness-anti-patterns.md

Quick Detection Checklist