mutation-testing

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"]
implementation_status	optimized
optimization_version	1
last_optimized	2025-12-02T00:00:00.000Z
dependencies	[]
quick_reference_card	true
tags	["mutation","stryker","test-quality","kill-rate","assertions","effectiveness"]

Mutation Testing

<default_to_action> When validating test quality or improving test effectiveness:

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

Quick Mutation Metrics:

• Mutation Score = Killed / (Killed + Survived)
• Target: > 80% mutation score
• Surviving mutants = weak tests

Critical Success Factors:

• High coverage ≠ good tests (100% coverage, 0% assertions)
• Mutation testing proves tests actually catch bugs
• Focus on critical code paths first </default_to_action>

Quick Reference Card

When to Use

• Evaluating test suite quality
• Finding gaps in test assertions
• Proving tests catch bugs
• Before critical releases

Mutation Score Interpretation

Score	Interpretation
90%+	Excellent test quality
80-90%	Good, minor improvements
60-80%	Needs attention
< 60%	Significant gaps

Common Mutation Operators

Category	Original	Mutant
Arithmetic	a + b	a - b
Relational	x >= 18	x > 18
Logical	a && b	a || b
Conditional	if (x)	if (true)
Statement	return x	(removed)