Mutation Testing

<default_to_action> When validating test quality or improving test effectiveness:

MUTATE code (change + to -, >= to >, remove statements)
RUN tests against each mutant
VERIFY tests catch mutations (kill mutants)
IDENTIFY surviving mutants (tests need improvement)
STRENGTHEN tests to kill surviving mutants

Quick Mutation Metrics:

Mutation Score = Killed / (Killed + Survived)
Target: > 80% mutation score
Surviving mutants = weak tests

Critical Success Factors:

High coverage ≠ good tests (100% coverage, 0% assertions)
Mutation testing proves tests actually catch bugs
Focus on critical code paths first </default_to_action>

Quick Reference Card

When to Use

Evaluating test suite quality
Finding gaps in test assertions
Proving tests catch bugs
Before critical releases

Mutation Score Interpretation

Score	Interpretation
90%+	Excellent test quality
80-90%	Good, minor improvements
60-80%	Needs attention
< 60%	Significant gaps

Common Mutation Operators

Category	Original	Mutant
Arithmetic	`a + b`	`a - b`
Relational	`x >= 18`	`x > 18`
Logical	`a && b`	`a \|\| b`
Conditional	`if (x)`	`if (true)`
Statement	`return x`	(removed)

How Mutation Testing Works

javascript

// Original code
function isAdult(age) {
  return age >= 18; // ← Mutant: change >= to >
}

// Strong test (catches mutation)
test('18 is adult', () => {
  expect(isAdult(18)).toBe(true); // Kills mutant!
});

// Weak test (mutation survives)
test('19 is adult', () => {
  expect(isAdult(19)).toBe(true); // Doesn't catch >= vs >
});
// Surviving mutant → Test needs boundary value

Using Stryker

bash

# Install
npm install --save-dev @stryker-mutator/core @stryker-mutator/jest-runner

# Initialize
npx stryker init

Configuration:

json

{
  "packageManager": "npm",
  "reporters": ["html", "clear-text", "progress"],
  "testRunner": "jest",
  "coverageAnalysis": "perTest",
  "mutate": [
    "src/**/*.ts",
    "!src/**/*.spec.ts"
  ],
  "thresholds": {
    "high": 90,
    "low": 70,
    "break": 60
  }
}

Run:

bash

npx stryker run

Output:

Mutation Score: 87.3%
Killed: 124
Survived: 18
No Coverage: 3
Timeout: 1

Fixing Surviving Mutants

javascript

// Surviving mutant: >= changed to >
function calculateDiscount(quantity) {
  if (quantity >= 10) { // Mutant survives!
    return 0.1;
  }
  return 0;
}

// Original weak test
test('large order gets discount', () => {
  expect(calculateDiscount(15)).toBe(0.1); // Doesn't test boundary
});

// Fixed: Add boundary test
test('exactly 10 gets discount', () => {
  expect(calculateDiscount(10)).toBe(0.1); // Kills mutant!
});

test('9 does not get discount', () => {
  expect(calculateDiscount(9)).toBe(0); // Tests below boundary
});

Agent-Driven Mutation Testing

typescript

// Analyze mutation score and generate fixes
await Task("Mutation Analysis", {
  targetFile: 'src/payment.ts',
  generateMissingTests: true,
  minScore: 80
}, "qe-test-generator");

// Returns:
// {
//   mutationScore: 0.65,
//   survivedMutations: [
//     { line: 45, operator: '>=', mutant: '>', killedBy: null }
//   ],
//   generatedTests: [
//     'test for boundary at line 45'
//   ]
// }

// Coverage + mutation correlation
await Task("Coverage Quality Analysis", {
  coverageData: coverageReport,
  mutationData: mutationReport,
  identifyWeakCoverage: true
}, "qe-coverage-analyzer");

Agent Coordination Hints

Memory Namespace

aqe/mutation-testing/
├── mutation-results/*   - Stryker reports
├── surviving/*          - Surviving mutants
├── generated-tests/*    - Tests to kill mutants
└── trends/*             - Mutation score over time

Fleet Coordination

typescript

const mutationFleet = await FleetManager.coordinate({
  strategy: 'mutation-testing',
  agents: [
    'qe-test-generator',     // Generate tests for survivors
    'qe-coverage-analyzer',  // Coverage correlation
    'qe-quality-analyzer'    // Quality assessment
  ],
  topology: 'sequential'
});

Related Skills

tdd-london-chicago - Write effective tests first
test-design-techniques - Boundary value analysis
quality-metrics - Measure test effectiveness

Remember

High code coverage ≠ good tests. 100% coverage but weak assertions = useless. Mutation testing proves tests actually catch bugs.

Focus on critical paths first. Don't mutation test everything - prioritize payment, authentication, data integrity code.

With Agents: Agents run mutation analysis, identify surviving mutants, and generate missing test cases to kill them. Automated improvement of test quality.

Search AI Tools

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SKILL.md