I write primarily Scala too. The frontier models seem reasonably good at it to me. One approach I sometimes use is to ask for it to write something complicated in Python, using only the standard library, and then once I have the Python refined I ask for a translation into Scala. This trick may not be applicable to your problems, but it works for me when I find an interesting algorithm in a paper and I'd like to use it but there is no existing Java/Scala implementation.

I use AI assistance to generate code and review code, but I haven't had success trying to use it to update a substantial existing code base in Scala. I have tried using both Claude Code and Cursor and the handful of times I tried there were so many oversights and mistakes that resolving the mess was more effort than doing it manually. I'll probably try again after the next big model releases.

Current frontier models have been the least useful to me when I'm asking them to review performance-critical code inside Scala. These are bits of my code base that I have written to use a lot of mutable variables, mutable data structures, and imperative logic to avoid allocations and other performance impediments. I only use this style for functions highlighted by profiling. I can ask for a code review and even include the reasoning for the unusual style in the docstring, but Claude and Gemini have still tried to nudge me back to much slower standard Scala idioms.