You cannot use a ChatGPT subscription with a CLI tool, if you want to build your own harness you have to go through the API. I'm unsure about Gemini. Claude Code seems to be a special case because it is itself a CLI tool and so it becomes much easier to build a custom harness around, but its not surprising or unusual that it would have restrictions.

Subscription products normally have terms of use that limit how you use it that are shaped by the infrastructure they rely on. A harness is often tuned to usage that fits with the constraints of the service, the backend that supports the tool is engineered for that usage. A custom harness could easily bypass that tuning and become unsustainable.

On top of that, the API tends to be a much more flexible product to use directly. I can understand why you'd have more expectations paying for the max product, but this doesn't sound unusual or unreasonable to me.