You're quick to say "to me" in your comparison.

My experience is very different than yours. Codex and CC yield very differenty result both because of the harness differencess and the model differences, but niether is noticeably better than the other.

Personally, I like Codex better just because I don't have to mess with any sort of planning mode. If I imply that it shouldn't change code yet, it doesn't. CC is too impatient to get started.

I guess yes, that's a harness difference, and you can also configure CC as a harness to behave very differently, but still with same harness and guidance, "to me" there's still a difference in terms of Opus 4.6 and e.g. GPT 5.4 or which GPT model do you use? I've been using Claude Code, Codex and OpenCode as harnesses presently, but for serious long running implementation I feel like I can only really rely on CC + Opus 4.6.