XPT (x.pushthrough) is used by some mining pools (e.g. ypool.net, primecoin). I have googled but could not find any reference to the details of this protocol. What is the protocol to be able to write a basic client?
It seems that XPT protocol was developed by ypool.net and there is no official specification of it.
But it is possible to get details from the source code of xptMiner or xptProxy.
You can start from https://github.com/jh000/xptProxy/blob/master/src/xptProxy/xptClient.cpp