What new or existing metric would better assess test coverage than line coverage and branch coverage?

I wrote a tool called tert - I guess it's called an agent harness now - to run various test runners and log test output and coverage output to disk. FWIU stripping spaces from JSON does save tokens. It seems like feeding coverage lines-missing maps into the prompt results in better output, better LLM-authored tests.

"Refactor these tests for maintainability and coverage. Use fixtures, mocks, and parametrization"