Mutation Testing Lab: How Good Is Your Test Suite?

Code coverage tells you which lines ran — it says nothing about whether your tests would actually catch a bug there. Mutation testing answers that harder question by seeding synthetic defects into the code and rerunning the suite against each one: if nothing fails, the suite has a gap. This lab renders a 3D grid of mutants across four real mutation-operator families (arithmetic, relational, logical, boundary), lets you tune test coverage, assertion strength and the unavoidable equivalent-mutant rate, and animates each mutant being killed, surviving, or turning out semantically equivalent — with a live mutation score computed exactly as PIT, Stryker or mutmut would report it.