at most it will carry poop bags and small amount of treats. the pockets are pretty small.
he does wear a harness at the beach every weekend (full harness with handle) so I know it doesn't bother him, but a harness, martingale, and normal collar all at the same time gets a bit much. He only wears the martingale while walking/hiking, his regular collar with ID and such is a flat breakaway one that he can't get into trouble with.