I'm using Claude on APIs on which it was not trained, and it's doing well. The techniques it uses to debug and find solutions aren't that different from older APIs. "Let me dump all of this response and guess which field might be what" sometimes, "let me get a stack trace just before it goes wrong", many familiar development techniques.

I need to fill in timesheets, which uses a proprietary API. Claude guessed and iterated until it found out how to use that API, and filling in those sheets is now much less bothersome. I only need to enter a password every three days instead of four times per day.