Mash. Agent, that is a tired old vacuous argument– “which of the 39 Melachos is being broken?”
Surely someone who considers himself an agent of one so great, is aware that there are entire categories of actions that are prohibited on Shabbos that do not fall under any of the 39 Melachos.
(For starters, even a Bais Yakov graduate who cannot back herself up with Rishonim and Achronim, can quote the relevant passuk in Yeshaya. You might even say it yourself when you make Kiddush.)