It now returns "COPY TO is not supported": https://github.com/olt/libpq/commit/0bdf7d34c11ed1e129057b14fe0c03e02dff77d5
Before that the error was "unknown response for copy query: 'H'"

I can check if I can integrate it into prepareTo and then use the server response to decide if it's a COPY FROM. Not sure if this will work.