Running Mutation Tests

Mutation Test Runner

Overview

Execute mutation testing to evaluate the effectiveness of a test suite by systematically introducing small code changes (mutants) and checking whether existing tests detect them. A killed mutant means the tests caught the change; a surviving mutant reveals a testing gap.

Prerequisites

• Mutation testing framework installed (Stryker, mutmut, PITest, or go-mutesting)
• Existing test suite with reasonable pass rate (all tests must pass before mutation testing)
• Source code with functions and logic suitable for mutation (conditionals, arithmetic, return values)
• Sufficient CI resources (mutation testing runs the test suite once per mutant -- CPU-intensive)
• Configuration file for the mutation tool specifying target files and test commands

Instructions

1. Verify the existing test suite passes completely:
   • Run the full test suite and confirm 100% pass rate.
   • Fix any failing or skipped tests before proceeding.
   • Mutation testing is meaningless if the baseline tests are broken.
2. Configure the mutation testing tool:
   • Stryker: Create stryker.config.mjs with mutate patterns, test runner, and thresholds.
   • mutmut: Configure setup.cfg or pyproject.toml with [mutmut] section.
   • PITest: Add Maven/Gradle plugin with target classes and test configurations.
3. Select target files for mutation:
   • Focus on business logic modules (not configuration, constants, or type definitions).
   • Exclude auto-generated code, third-party wrappers, and test utilities.
   • Start with a small scope (one module) to validate setup before expanding.
4. Run the mutation testing suite:
   • Execute npx stryker run, mutmut run, or mvn pitest:mutationCoverage.
   • Monitor progress -- expect long execution times (10-100x normal test runtime).
   • Use incremental mode if available to skip already-tested mutants.
5. Analyze the mutation report:
   • Killed mutants: Tests detected the change -- indicates strong test coverage.
   • Survived mutants: Tests did not catch the change -- indicates a testing gap.
   • Timed out mutants: Mutation caused an infinite loop -- generally acceptable.
   • No coverage mutants: The mutated code is not exercised by any test.
6. For each surviving mutant, determine the appropriate action:
   • Write a new test that specifically catches the mutation.
   • Or determine the mutation is equivalent (functionally identical to original) and mark as ignored.
7. Set mutation score thresholds (recommended: 80% kill rate) and integrate into CI as a quality gate.

Output

• Mutation testing report (HTML or JSON) with killed/survived/timed-out counts
• Mutation score percentage (killed / total non-equivalent mutants)
• Surviving mutant inventory with file, line, mutation type, and suggested test
• New test cases written to kill surviving mutants
• CI configuration with mutation score threshold enforcement

Error Handling

Examples

Stryker configuration for TypeScript project:

// 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
};

Example surviving mutant and fix:

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);
});

mutmut for Python:

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

# View surviving mutants
mutmut results

# Inspect a specific mutant
mutmut show 42

Resources

• Stryker Mutator: https://stryker-mutator.io/
• mutmut (Python): https://github.com/boxed/mutmut
• PITest (Java): https://pitest.org/
• go-mutesting: https://github.com/zimmski/go-mutesting
• Mutation testing theory: https://en.wikipedia.org/wiki/Mutation_testing

Source: jeremylongshore/claude-code-plugins-plus-skills → skills/.curated/running-mutation-tests/SKILL.md

Also appears in: jeremylongshore/claude-code-plugins-plus-skills/plugins/testing/mutation-test-runner/skills/running-mutation-tests/SKILL.md