Running Mutation Tests

What it does

When your agent uses it

• Performing specialized testing
• With phrases like run mutation tests
• Validate test effectiveness

Example prompts

• “run mutation tests”
• “test the tests”
• “validate test effectiveness”
• “/running-mutation-tests”

Requirements

• Python 3
• Node.js
• Compatibility (from SKILL.md): Designed for Claude Code
• Pre-approved tools (allowed-tools): Read, Write, Edit, Grep, Glob, Bash(test:mutation-*)

Workflow steps

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

1. 1Verify the existing test suite passes completely
2. 2Configure the mutation testing tool
3. 3Select target files for mutation
4. 4Run the mutation testing suite
5. 5Analyze the mutation report
6. 6For each surviving mutant, determine the appropriate action
7. 7Set mutation score thresholds (recommended: 80% kill rate) and integrate into CI as a quality gate.

What it can do on your machine

Read from SKILL.md and the folder at commit cfae287. It shows what the files ask for, not the result of running them.

• Tool permissions

  Pre-approves these tools, so the agent can use them without asking each time:

  • Read
  • Write
  • Edit
  • Grep
  • Glob
  • Bash(test:mutation-*)

  From allowed-tools in the SKILL.md frontmatter.

• Runs code

  Ships 2 files in scripts/ (Python), which the agent can run.

  Shell commands in SKILL.md call:

  • npx
  • mvn
  • npm

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

• Network

  Links to these hosts (documentation or services it may open):

  • github.com
  • stryker-mutator.io
  • pitest.org
  • en.wikipedia.org

  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.

• Compatibility

  Designed for Claude Code

  From compatibility in the SKILL.md frontmatter.

Context cost

Running Mutation Tests loads about 1.4k tokens when it runs, and up to ~1.4k if it reads all its reference files. Until then it costs about 57 tokens; SKILL.md has 544 words of instructions outside code blocks.

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

~57

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

~1.4k

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

~1.4k

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 jeremylongshore/tons-of-skills-marketplace at commit cfae287, republished under its MIT licence (© jeremylongshore). 544 words, ~1,425 tokens.

name

running-mutation-tests

description

Execute mutation testing to evaluate test suite effectiveness. Use when performing specialized testing. Trigger with phrases like "run mutation tests", "test the tests", or "validate test effectiveness".

allowed-tools

Read, Write, Edit, Grep, Glob, Bash(test:mutation-*)

compatibility

Designed for Claude Code

version

1.27.0

author

Jeremy Longshore <jeremy@intentsolutions.io>

license

MIT

tags

testing, mutation-tests

// stryker.config.mjs
export default {
  mutate: ['src/**/*.ts', '!src/**/*.d.ts', '!src/**/index.ts'],
  testRunner: 'jest',
  jest: { configFile: 'jest.config.ts' },
  reporters: ['html', 'clear-text', 'progress'],
  thresholds: { high: 80, low: 60, break: 50 },
  concurrency: 4,
  timeoutMS: 10000,  # 10000: 10 seconds in ms
};

Mutant: src/utils/discount.ts:15 -- ConditionalExpression
  Original:  if (total > 100)
  Mutant:    if (total >= 100)
  Status:    SURVIVED

Fix -- add boundary test:
it('does not apply discount at exactly 100', () => {
  expect(calculateDiscount(100)).toBe(0);
});
it('applies discount above 100', () => {
  expect(calculateDiscount(101)).toBe(10.1);
});

# Run mutation testing
mutmut run --paths-to-mutate=src/ --tests-dir=tests/

# View surviving mutants
mutmut results

# Inspect a specific mutant
mutmut show 42

© jeremylongshore, 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, assets) in skills/.curated/running-mutation-tests of jeremylongshore/tons-of-skills-marketplace.

• SKILL.md
• assets/README.md
• assets/config_template.yaml
• assets/example_mutation_results.json
• assets/mutation_report_template.md
• references/README.md
• scripts/README.md
• scripts/mutation_analyzer.py

Open the folder on GitHubat commit cfae287