I use Kimi K3 and GLM 5.3 to find old bugs and improve old code, and would try Fable 5.1 if I could afford $50 for 1M output tokens (I am retired and all my research is self funded now)

I like to start by prompting with “examine this code base for problems and improvements and write to IMPROVEMENTS.MD” and then carefully look over the suggestions, and either fix myself or let the model+coding harness try.