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

// 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

Install

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

Initialize

npx stryker init

Configuration:

{ "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:

npx stryker run

Output:

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

Fixing Surviving Mutants

// 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

// 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

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.