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Mutation Testing

proffesor-for-testing/agentic-qe

  • 96 installs
  • 433 repo stars
  • Updated August 4, 2026
  • proffesor-for-testing/agentic-qe

mutation-testing is a Claude Code skill for testing & qa.

About

mutation-testing is a Claude Code skill for testing & qa. It helps solo builders move faster with AI-assisted development.

  • mutation-testing
  • Testing & QA
  • AI-coding skill

Mutation Testing by the numbers

  • 96 all-time installs (skills.sh)
  • +3 installs in the week ending Aug 4, 2026 (Skillselion tracking)
  • Ranked #1,015 of 2,153 Testing & QA skills by installs in the Skillselion catalog
  • Data as of Aug 5, 2026 (Skillselion catalog sync)
npx skills add https://github.com/proffesor-for-testing/agentic-qe --skill mutation-testing

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Listed on Skillselion
Installs96
repo stars★ 433
Last updatedAugust 4, 2026
Repositoryproffesor-for-testing/agentic-qe ↗

How do I helps with testing & qa tasks.?

Helps with testing & qa tasks.

Who is it for?

Best when you're working on testing & qa and need structured help with mutation testing.

Skip if: Teams with no testing & qa needs, or anyone wanting a generic chat assistant without this specific workflow.

When should I use this skill?

When you need to helps with testing & qa tasks., or when mutation-testing is a claude code skill for testing & qa.

What you get

Structured output aligned to mutation-testing: mutation-testing, Testing & QA.

Files

SKILL.mdMarkdownGitHub ↗

Mutation Testing

<default_to_action> When validating test quality or improving test effectiveness: 1. MUTATE code (change + to -, >= to >, remove statements) 2. RUN tests against each mutant 3. VERIFY tests catch mutations (kill mutants) 4. IDENTIFY surviving mutants (tests need improvement) 5. 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

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

Common Mutation Operators

CategoryOriginalMutant
Arithmetica + ba - b
Relationalx >= 18x > 18
Logicala && b`a \
Conditionalif (x)if (true)
Statementreturn 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.

Run History

After each mutation test run, append results to run-history.json in this skill directory:

node -e "
const fs = require('fs');
const h = JSON.parse(fs.readFileSync('.claude/skills/mutation-testing/run-history.json'));
h.runs.push({date: new Date().toISOString().split('T')[0], mutation_score_pct: SCORE, killed: KILLED, survived: SURVIVED});
fs.writeFileSync('.claude/skills/mutation-testing/run-history.json', JSON.stringify(h, null, 2));
"

Read run-history.json before each run to track score improvements over time.

Skill Composition

  • Before mutation testing → Run /qe-test-generation to ensure tests exist
  • After mutation results → Use /qe-coverage-analysis to prioritize improvement areas
  • Quality gate → Feed results into /qe-quality-assessment for ship/no-ship decision

Gotchas

  • Stryker requires --testRunner jest explicitly if both jest and vitest are installed
  • Mutating >= to > in date comparisons rarely gets killed — add boundary tests
  • Running on files >500 LOC will timeout; use --mutate to target specific functions
  • --concurrency defaults to CPU count which OOMs in containers — set to 2

Related skills

mattpocockTddFollow test-driven development with a strict red-green-refactor loop when creating reliable features or fixing bugs.544k196kobraTest Driven DevelopmentEnforce writing failing tests before any production implementation code.181k264kmattpocockQaRun conversational QA sessions that turn user-reported bugs into well-written, domain-aware GitHub issues without manual ticket writing.179k196kmattpocockMigrate To ShoehornAutomatically update TypeScript test files that rely on unsafe `as` type assertions by replacing them with type-safe partial objects from @total-typescript/shoehorn.164k196kanthropicsWebapp TestingVerify frontend behavior, debug UI issues, capture screenshots, and inspect logs of a running local web application using Playwright.123k165kmicrosoftPlaywright CliRun browser automation, generate element snapshots, inspect DOM attributes, and execute Playwright tests from the terminal.102k12.3k

FAQ

What does mutation-testing do?

mutation-testing is a Claude Code skill for testing & qa.

When should I use mutation-testing?

When you need to helps with testing & qa tasks., or when mutation-testing is a claude code skill for testing & qa.

What are the main capabilities?

mutation-testing; Testing & QA; AI-coding skill.

Testing & QAtesting
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