Mutation testing tells you what coverage does not

Line coverage says a line executed during the suite. It does not say an assertion would have failed had that line been wrong, which is the question everyone thinks coverage answers.

# change > to >=, flip a boolean, remove a line — then run the tests
vendor/bin/infection --min-msi=60 --threads=4

# Mutation Score Indicator: 61%
# 412 mutants: 251 killed, 148 escaped, 13 not covered

An escaped mutant is a change to the code that no test noticed, and reading a handful of them is more informative about a suite than any coverage report. It is slow — the suite runs once per mutant — so it belongs on a nightly job over changed files rather than in the commit path. A file with 100% coverage and 30% MSI is the common and instructive result.