Mutation Testing

What it does

When your agent uses it

• Evaluating test quality
• Identifying weak tests
• Proving tests actually catch bugs

Example prompts

• “/mutation-testing”

Requirements

• Node.js

Workflow steps

5 steps, taken from the first numbered list in SKILL.md.

1. 1MUTATE code (change + to -, >= to >, remove statements)
2. 2RUN tests against each mutant
3. 3VERIFY tests catch mutations (kill mutants)
4. 4IDENTIFY surviving mutants (tests need improvement)
5. 5STRENGTHEN tests to kill surviving mutants

What it can do on your machine

Read from SKILL.md and the folder at commit 829d030. It shows what the files ask for, not the result of running them.

• Tool permissions

  Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

  From allowed-tools in the SKILL.md frontmatter.

• Runs code

  Ships 1 file in scripts/, which the agent can run.

  Shell commands in SKILL.md call:

  • npx
  • npm
  • node

  From the folder's file list and the shell code blocks in SKILL.md.

• Network

  No URLs in SKILL.md. Its commands use npx and npm, which can reach the network depending on how they are called.

  From URLs in SKILL.md, links to its own repository left out.

• Credentials

  Names no API keys, tokens, secrets or passwords.

  From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Mutation Testing loads about 1.7k tokens when it runs, and up to ~2.1k if it reads all its reference files. Until then it costs about 63 tokens; SKILL.md has 355 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it

~63

When it runs · the whole SKILL.md, loaded when a task matches

~1.7k

With references · SKILL.md plus every file in references/, read only if the agent opens them

~2.1k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); the scripts in this folder are not scanned.

SKILL.md

The full file from proffesor-for-testing/agentic-qe at commit 829d030, republished under its MIT licence (© proffesor-for-testing). 355 words, ~1,742 tokens.

name

mutation-testing

description

Test quality validation through mutation testing, assessing test suite effectiveness by introducing code mutations and measuring kill rate. Use when evaluating test quality, identifying weak tests, or proving tests actually catch bugs.

category

specialized-testing

priority

high

tokenEstimate

900

agents

qe-test-generator, qe-coverage-analyzer, qe-quality-analyzer, qe-mutation-tester

implementation_status

optimized

optimization_version

1

last_optimized

2025-12-02

quick_reference_card

true

tags

mutation, stryker, test-quality, kill-rate, assertions, effectiveness

trust_tier

3

// 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

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

# Initialize
npx stryker init

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

npx stryker run

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

// 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
});

// 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");

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

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'
});

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));
"

© proffesor-for-testing, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 7 other files (scripts, references) in assets/skills/mutation-testing of proffesor-for-testing/agentic-qe.

• SKILL.md
• config.json
• evals/mutation-testing.yaml
• references/mutation-operators.md
• run-history.json
• schemas/output.json
• scripts/validate-config.json
• test-data/sample-output.json

Open the folder on GitHubat commit 829d030