Equivalent mutant
A mutant that changes the code but not what it does, so no test could ever catch it. It survives, and that says nothing bad about your tests.
So 100% is not always possible, and "kill every survivor" needs judgement. Push an agent too hard and it will write strange tests, or bend the code, to chase mutants that are not real bugs.
Say it to your agent
“For this survivor, compare the mutant with the original over a wide range of inputs. If you find an input where they differ, write the test for it. If you can't, say that it is equivalent and show me what you tried.”