No, the coffee cups are not leak proof. The lids, whilst preventing major splashes if knocked over, do have holes in them for drinking that cannot be closed. For this reason, we do not recommend carrying your coffee cup in a bag.
If your coffee cup is leaking from the seal between the lid and the cup itself, please check the o-ring to ensure it is fitted correctly. If it doesn't sit right on your lid, this may cause your coffee cup to leak.