It is the current practice to treat an S Corporation as a resident of the U.S. for purposes of the Canada-U.S. Convention provided that it is a qualifying person or otherwise eligible for benefits under paragraph 3 or 6 of Article XXIX - A of that Convention.
However, treaty benefits under Article X(6) claimed by an LLC with respect to an amount of profit attributable to a Canadian permanent establishment will be recognized by CRA only if the amount is considered to be derived, pursuant to Article IV(6), by a corporation that is a resident of the U.S. and a qualifying person (or entitled to benefits under paragraph 3 or 6 of Article XXIX-A).