Agency is what a player has his PC believes and what actions he takes.
In all social situations, since agency is belief, I have to have complete choice or my agency is gone.
With the dex check, I can decide my PC think he will 100% succeed, possibly succeed or fail, or 100% has no chance. My agency is preserved with regard to deciding what the PC believes. Now to agency in deciding what the PC does. What he does is try to walk a tightrope. Does he try to walk a tightrope? Yes that is the action he takes. Success or failure isn't an action. That's action resolution, which is not a part of player agency.
Telling the player that his PC can't try to walk the tightrope is a no no, since it removes his agency. Telling him that the result of his attempt to walk the tightrope is that he falls, is not a no no, since action resolution does not take away player agency.