Lupine Prototype’s restriction applies only at the moment that attackers and blockers are chosen. If a player puts a card into their hand after Lupine Prototype has attacked or blocked, it won’t be removed from combat.

If a creature can’t attack or block unless you return a permanent to your hand, and your hand is empty, you can attack or block with that creature and Lupine Prototype. This is because the legality of chosen attackers and blockers is checked before costs are paid.