I think it's important not to explain everything away by saying the players agreed to X, yeah. Mostly because while players do agree to the pitch, and build characters, they aren't necessarily privy to how the GM does business, nor are they necessarily conversant enough in the mechanics in question to know exactly what the stakes are. That's completely ok in most cases of course, that level of understanding isn't even remotely necessary to play and enjoy an RPG. It is, however, the root of comments at the table like what do you mean I can't do that? which is exactly the sort of thing we're talking about.
I think there's a difference between the active agency that you get in authoring a character, and the ability to make decisions in range X about that character, versus the output in terms of agency when you power that avatar up and start having it do things. Upstream I was talking about the second kind, and then @Lanefan mentioned the former. I think it's possible to keep those two ideas separate and discrete in terms of explanatory power.
I think there's a difference between the active agency that you get in authoring a character, and the ability to make decisions in range X about that character, versus the output in terms of agency when you power that avatar up and start having it do things. Upstream I was talking about the second kind, and then @Lanefan mentioned the former. I think it's possible to keep those two ideas separate and discrete in terms of explanatory power.