Mutation Testing
Expert knowledge for mutation testing - validating that your tests actually catch bugs by introducing deliberate code mutations.
When to Use This Skill
| Use this skill when... | Use another skill instead when... |
|---|---|
| Validating test effectiveness | Writing unit tests (use vitest-testing) |
| Finding weak/insufficient tests | Analyzing test smells (use test-quality-analysis) |
| Setting up Stryker or mutmut | Writing E2E tests (use playwright-testing) |
| Improving mutation score | Generating test data (use property-based-testing) |
| Checking if tests catch real bugs | Setting up code coverage only |
Core Expertise
Mutation Testing Concept
- Mutants: Small code changes (mutations) introduced automatically
- Killed: Test fails with mutation (good - test caught the bug)
- Survived: Test passes with mutation (bad - weak test)
- Coverage: Tests execute mutated code but don't catch it
- Score: Percentage of mutants killed (aim for 80%+)
What Mutation Testing Reveals
- Tests that don't actually verify behavior
- Missing assertions or edge cases
- Overly permissive assertions
- Dead code or unnecessary logic
- Areas needing stronger tests
TypeScript/JavaScript (Stryker)
Installation
# Using Bun
bun add -d @stryker-mutator/core
# For Vitest
bun add -d @stryker-mutator/vitest-runner
# For Jest
bun add -d @stryker-mutator/jest-runner
Running Stryker
npx stryker run # Run mutation testing
npx stryker run --incremental # Only changed files
npx stryker run --mutate "src/utils/**/*.ts" # Specific files
npx stryker run --reporters html,clear-text # HTML report
open reports/mutation/html/index.html # View report
Understanding Results
Mutation score: 82.5%
- Killed: 66 (tests caught the mutation)
- Survived: 14 (tests passed despite mutation - weak tests!)
- No Coverage: 0 (mutated code not executed)
- Timeout: 0 (tests took too long)
Example: Weak vs Strong Test
// Source code
function calculateDiscount(price: number, percentage: number): number {
return price - (price * percentage / 100)
}
// WEAK: Test passes even if we mutate the calculation
test('applies discount', () => {
const result = calculateDiscount(100, 10)
expect(result).toBeDefined() // Too weak!
})
// STRONG: Test catches mutation
test('applies discount correctly', () => {
expect(calculateDiscount(100, 10)).toBe(90)
expect(calculateDiscount(100, 20)).toBe(80)
expect(calculateDiscount(50, 10)).toBe(45)
})
Python (mutmut)
Installation
uv add --dev mutmut # Using uv
pip install mutmut # Using pip
Running mutmut
uv run mutmut run # Run mutation testing
uv run mutmut run --paths-to-mutate=src/calculator.py # Specific files
uv run mutmut results # Show results
uv run mutmut summary # Summary
uv run mutmut show 1 # Show specific mutant
uv run mutmut apply 1 # Apply mutant manually
uv run mutmut html # HTML report
Understanding Results
Status: 45/50 mutants killed (90%)
- Killed: 45 (tests caught the mutation)
- Survived: 5 (tests passed despite mutation)
Mutation Score Targets
| Score | Quality | Action |
|---|---|---|
| 90%+ | Excellent | Maintain quality |
| 80-89% | Good | Small improvements |
| 70-79% | Acceptable | Focus on weak areas |
| 60-69% | Needs work | Add missing tests |
| < 60% | Poor | Major test improvements needed |
Agentic Optimizations
| Context | Command |
|---|---|
| Quick TS mutation | npx stryker run --incremental --reporters clear-text |
| Targeted TS mutation | npx stryker run --mutate "src/core/**/*.ts" |
| Quick Python mutation | uv run mutmut run --paths-to-mutate=src/core/ |
| View survived | uv run mutmut results | grep Survived |
| CI mode | npx stryker run --reporters json |
For detailed examples, advanced patterns, and best practices, see REFERENCE.md.
See Also
vitest-testing- Unit testing frameworkpython-testing- Python pytest testingtest-quality-analysis- Detecting test smellsapi-testing- HTTP API testing
References
- Stryker: https://stryker-mutator.io/
- mutmut: https://github.com/boxed/mutmut
- Mutation Testing Intro: https://en.wikipedia.org/wiki/Mutation_testing
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