Mutation Test Skill

Purpose

Run mutation testing to measure test suite effectiveness. Mutation testing introduces small changes (mutants) to code and checks if tests catch them. High coverage with low mutation score indicates weak tests.

Research Foundation

When This Skill Applies

• User asks to "validate test quality" or "check test effectiveness"
• User mentions "mutation testing" or "mutation score"
• User wants to know if tests are "actually testing anything"
• High coverage but bugs still escaping
• Assessing test suite health
• Pre-release quality validation

Trigger Phrases

Mutation Testing Concepts

What is a Mutant?

A mutant is a small code change that should cause tests to fail:

// Original
if (age >= 18) { return "adult"; }

// Mutant 1: Changed >= to >
if (age > 18) { return "adult"; }

// Mutant 2: Changed >= to ==
if (age == 18) { return "adult"; }

// Mutant 3: Changed "adult" to ""
if (age >= 18) { return ""; }

Mutation Operators

Mutation Score

Mutation Score = (Killed Mutants / Total Mutants) × 100

Implementation Process

1. Detect Project and Install Tool

def setup_mutation_tool(project_type):
    if project_type == "javascript":
        # Install Stryker
        return "npx stryker init"
    elif project_type == "python":
        # Install mutmut
        return "pip install mutmut"
    elif project_type == "java":
        # PITest via Maven/Gradle
        return "Add pitest plugin to pom.xml"

2. Configure Mutation Testing

Stryker (JavaScript):

// stryker.config.json
{
  "mutate": ["src/**/*.ts", "!src/**/*.test.ts"],
  "testRunner": "vitest",
  "reporters": ["html", "progress"],
  "coverageAnalysis": "perTest",
  "thresholds": {
    "high": 80,
    "low": 60,
    "break": 50
  }
}

mutmut (Python):

# setup.cfg
[mutmut]
paths_to_mutate=src/
tests_dir=tests/
runner=pytest

PITest (Java):

<!-- pom.xml -->
<plugin>
    <groupId>org.pitest</groupId>
    <artifactId>pitest-maven</artifactId>
    <version>1.15.0</version>
    <configuration>
        <targetClasses>
            <param>com.example.*</param>
        </targetClasses>
        <mutationThreshold>80</mutationThreshold>
    </configuration>
</plugin>

3. Run Mutation Analysis

# JavaScript
npx stryker run

# Python
mutmut run

# Java
mvn org.pitest:pitest-maven:mutationCoverage

4. Parse and Report Results

def parse_mutation_results(report_path):
    """Parse mutation testing report"""
    return {
        "total_mutants": 150,
        "killed": 120,
        "survived": 25,
        "timeout": 5,
        "mutation_score": 80.0,
        "survivors": [
            {
                "file": "src/auth/validate.ts",
                "line": 45,
                "mutator": "RelationalOperator",
                "original": "age >= 18",
                "mutant": "age > 18",
                "status": "survived"
            }
            # ... more survivors
        ]
    }

Output Format

## Mutation Testing Report

**Module**: src/auth/
**Test Suite**: test/auth/

### Summary

| Metric | Value |
|--------|-------|
| Total Mutants | 150 |
| Killed | 120 (80%) |
| Survived | 25 (17%) |
| Timeout | 5 (3%) |
| **Mutation Score** | **80%** |

### Status: PASSED (threshold: 80%)

### Survived Mutants (Highest Priority)

#### 1. `src/auth/validate.ts:45`
```diff
- if (age >= 18) { return "adult"; }
+ if (age > 18) { return "adult"; }

Problem: Boundary condition not tested Fix: Add test case for age = 18

2. src/auth/login.ts:23

- if (attempts < maxAttempts) { allow(); }
+ if (attempts <= maxAttempts) { allow(); }

Problem: Off-by-one boundary not tested Fix: Add test for attempts = maxAttempts

Recommended Test Improvements

1. Add boundary tests for validate.ts (3 survivors)
2. Add error path tests for login.ts (2 survivors)
3. Test null/undefined cases in session.ts (1 survivor)

Coverage vs Mutation Score

High coverage with low mutation score indicates weak assertions

## Integration with CI

### GitHub Actions Integration

```yaml
- name: Run mutation testing
  run: npx stryker run --reporters json

- name: Check mutation threshold
  run: |
    SCORE=$(jq '.metrics.mutationScore' reports/mutation/stryker-incremental.json)
    if (( $(echo "$SCORE < 80" | bc -l) )); then
      echo "::error::Mutation score $SCORE% below 80% threshold"
      exit 1
    fi

Optimization Tips

Incremental Mutation Testing

Only test changed code:

# Stryker incremental
npx stryker run --incremental

# PITest history
mvn pitest:mutationCoverage -DwithHistory

Target Critical Modules First

{
  "mutate": [
    "src/auth/**/*.ts",
    "src/payment/**/*.ts",
    "src/validation/**/*.ts"
  ]
}

Related Skills

• tdd-enforce - Enforce test-first development
• flaky-detect - Identify unreliable tests
• test-sync - Maintain test-code alignment

Script Reference

mutation_runner.py

Run mutation testing for project:

python scripts/mutation_runner.py --module src/auth

mutation_analyzer.py

Analyze and prioritize survivors:

python scripts/mutation_analyzer.py --report stryker-report.json