It appears that OpenAI has blessed third party harnesses. I know they officially support OpenCode and they have this on their developer portal:

"Developers should code in the tools they prefer, whether that's Codex, OpenCode, Cline, pi, OpenClaw, or something else, and this program supports that work."

https://developers.openai.com/community/codex-for-oss

Obviously, the context is that OpenAI is telling open source developers who are using free subscriptions/tokens from the Codex for Open Source program that they can use any harness they want. But it would be strange for that to not extend to paying subscribers.