Jason will be saving his costume for actual Halloween, but he did wear this. The cook does take requests, within reason. If anyone tries to get him to change his recipe for any reason other than, "I am allergic to X," or, "I know that I hate eating X," he will not be responsible for the knife that goes flying past their heads and into Dick's nice, clean walls.
Eventually, Jason did emerge from the kitchen and mingle, though he probably found his way to Dick's side or a quiet corner every now and then to decompress, since he's not quite used to the party scene these days, and big bro makes him feel safe (shh, don't tell anyone).
Ace (German Shepherd) stuck close to either Jason or Dick most of the night, though he worked the crowd for his share of treats, and got along peaceably with the other Bat pets. If everyone is cool with it, let's assume Ace was at least introduced to Alfred and Titus already, so meeting future!Ace went over more easily.