We're really handicapped by a lack of definition, here.
Let's try this: what would it take, no matter how outlandish or unlikely, for you to be completely convinced that a particular chat bot demonstrates full sentience?
I think for me, it would never be able to fully demonstrate sentience to my satisfaction, because even people cannot. I have to take it on faith, philosophically speaking, that you are sentient, for example.
So, for me, if a chat bot feels and behaves like it's sentient, it is sentient for all real, practical intents and purposes, irrespective of its internal processes. Whether it "really" feels, like I do, is as irrelevant as whether you yourself "really" feel like I do. Without a good reason to believe you are not sentient, I must behave as if you are. Likewise, if a chat bot claims sentience and seems to hold a conversation and react the way I expect a sentient creature would, it would be unethical for me to ignore that because I didn't feel its code was sufficiently complicated, particularly if I cannot say for certain why anything is sentient.