What is Mutation Testing? A Practical Guide for QA Engineers

Line coverage is a liar. Your tests can cover 100% of your code and still miss critical bugs. Coverage tells you which lines ran -- not which bugs your tests actually catch . Mutation testing fixes this gap. It answers a harder question: "If I introduce a bug into this code, will my tests detect it?" How Mutation Testing Works Start with correct code -- the "golden" implementation Generate mutants -- AI or tools create variants with subtle bugs (off-by-one errors, wrong operators, missing null checks) Run your tests against each mutant Score -- if your test fails on a mutant, that mutant is "killed." Your kill ratio = killed / total mutants A Simple Example Given a function that calculates shipping cost: def calculate_shipping ( weight , distance ): base = 5.0 if weight > 10 : base += weight * 0.5 if distance > 100 : base += distance * 0.1 return round ( base , 2 ) A mutant might change weight > 10 to weight >= 10 or weight > 11 . If your tests don't cover the boundary at exactly weigh