I've seen fairly poor results from people asking AI agents to fill in coverage holes. Too many tests that either don't make sense, or add coverage without meaningfully testing anything.
If you're already at a very high coverage, the remaining bits are presumably just inherently difficult.
I suppose it's mixed results but a coverage report should give you "these exact lines are uncovered" and it becomes pretty straightforward to see "ah yeah that error condition isn't tracked, the behavior should be X, go write that test".
If you're already at a very high coverage, the remaining bits are presumably just inherently difficult.