How do you mean OpenAI lets you use your own harness? I'm under the impression that a custom harness requires the OpenAI SDK, which requires api tokens rather than plus/pro accounts.
"A little secret. About 5% of our production traffic is on the Pi harness, about another 5% is on OpenCode. Reminder you can use your ChatGPT account in a flourishing set of other tools.
We’ll continue to make Codex awesome, but you have options."
Claude web definitely feels like it's the same models behind as in API, with much less extra behavior/layers that make it behave differently.
I think they even partnered with opencode or something like that (don't remember).
OpenAI officially allows that with subscriptions.
OpenAI's harness is fully open source[1], and (AFAIK) doesn't come with any kind of signed-build request integrity verification like Claude does. And by that logic, if you're allowed to use their API with a fork of Codex that you yourself compiled, there's nothing stopping you from making some other harness act like such a fork.
Warning: I use this daily myself for work, but beyond me arguing with it to get the CDP architecture, this was completely vibe-coded. I mostly use vim mode (jump to line, jump to prev/next user message) and the account switcher.