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."
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.
"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.