By "agency", are you referring to physical control of the body? As far as I can tell, the process of "switching" (allowing the tulpa to control the host's body temporarily) is a very rare process which is a good deal more difficult than just creating a tulpa, and which many people who have tulpas cannot do at all even if they try.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.