Here's another example (which
@AbdulAlhazred already posted 20 or 30 pages upthread):
The GM narrates
you come to a dead end.
Now I can't describe the following action for my character (assuming that I'm not ethereal or similar):
I keep walking straight ahead.
Thus the GM's description is a limit on "baseline" agency.
And this sort of thing happens in D&D all the time.
The bigger point is this: "baseline" agency is constrained by the fictional position of the PC; and that fictional position contains both internal and external elements.