Can you not? I built a custom harness that uses my Claude subscription. You can pass -p I think is the flag, to feed it a prompt. And it’s been a little while so I don’t fully remember how but you can also have it print the output to stdout. I even pass it custom tools as args and it works with the cache, and you can even append messages to conversations
You can work around it like that for sure; you can call it from the CLI, and you get free credits also to spend.
But I meant more like, you know: hey just use the whole sub in another harness. That you can't do.