Funny I had the opposite experience. The Claude models seemed equivalent to GPT-5.4/5 in a generic harness like Copilot CLI or Opencode or Pi, but Claude Code the app/harness is so much better than all the others that I switched at work, even though I'd much prefer to use a non-proprietary harness (and eventually I do want to get Pi set up to be comparable).
Well, maybe I shouldn't have assumed the harness wasn't a big deal. When Microsoft changes its pricing on 6/1 I'll try some of the others.