Assuming you logged in with OAuth, I am guessing you are trying to use gpt-5.5?

In my tests, it worked using gpt-5.4 for me and I assumed gpt-5.5 is not available to me because I am on the free plan

Do you have the subscription that allows 5.5? If so, I can look into what changed in API. Sorry I rarely use openAI so it is a bit of an untrodden path

Yes I'm on ChatGPT Pro (OAuth) and I'm trying to use gpt-5.5-xhigh.

That was the issue, 5.4 works just fine.

Support for service: priority (GPT /fast mode) would also be cool!