I promise: This is not a trap.
If a GM pre-establishes agreement (dunno how better to phrase that) does that change where you think agency is?
Not worried about traps because I'm pretty comfortable with my understanding. If you discover something new or incongruent, that gives me an opportunity to re-evaluate. So, fire away!
In this case, I'm not sure it really changes much. If this is an agreement about the themes and tropes expected in the game, I think this is just fine provided the explanations are of reasonable completeness. This is exactly what happens when I run Blades; we all agree to the tropes and themes of the game and carry on. It's what happens when someone suggests play 5e -- there are a lot of these baked into the system, despite any claims of the malleability of the setting material, the game itself hard codes a lot of this. So, no, I don't really find an agreement to play a game to be much of a problem because we have to agree to play a game to even look at agency within the game.
What I'm speaking of here is the GM decides core resolution mechanic present in the D&D-alike game set. Any game that features the ability for the GM to determine, based on their own take of the fiction only, whether or not an action fails (ie, action negation) you have an issue with player agency. And, again, this might not be a problem! I think D&D is very clearly an autocracy, with the GM seated on the high chair, but you can have benevolent dictators that are pleasant as well as tin-pot dictators that are not. If I'm more concerned about being able to put what I want in the game, though, no amount of benevolence from the dictator is going to satisfy that -- it's going to rub wrong, at best. This is, as best as I can tell, where
@pemerton,
@Campbell,
@Aldarc sit -- they don't care to find out how nice the autocrat is going to be, they want to have a say that's impossible in such systems without GM approval. I'm less adamant, probably because I'm usually the autocrat in this situation. I can say that the last time I played 5e, I was rubbed wrong by a good number of things, usually involving GM negation of actions or PC interests. I even ran across an old text message conversation with one of the other players where this was being discussed and I was very unsure how I felt that my character was still alive because it was a clear use of GM Force. I mean, I liked my character, but....
So, I hope this addresses your question? If not, feel free to ask again.