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
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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
| Error | Cause | Solution |
|---|---|---|
| Mutation run takes hours | Too many files in scope or slow test suite | Narrow mutate scope to critical modules; use --incremental mode; parallelize with --concurrency |
| All mutants survive | Tests only check for truthiness, not specific values | Strengthen assertions -- use toBe(42) instead of toBeTruthy(); add boundary checks |
| Equivalent mutant false positive | Mutation produces functionally identical code (e.g., x >= 0 vs x > -1) | Mark as equivalent in config; ignore in score calculation; document rationale |
| Out of memory during run | Too many concurrent mutation workers | Reduce --concurrency setting; increase Node.js --max-old-space-size; reduce shard size |
| Stryker "initial test run failed" | Test suite does not pass cleanly before mutations begin | Fix all failing tests first; ensure npm test exits 0; check test runner configuration |
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 42Resources
- 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