The example was the GM ruling no by fiat versus getting a failure on the dice. I had a chance with the dice, but it didn't pan out. I never had a chance with the GM. It wasn't about being able to figure the odds in a case where I know and can anticipate the GM. I'm not in the camp that thinks that the GM deciding automatically removes any possibility of agency. Otherwise I'd have to accept that my players in my 5e game do not have agency, because 5e's core mechanic is GM decides. But, I look at what happens in my game, and lo, I behold agency! So, yeah, you totes can have agency with the GM deciding, but it really rests on the GM following clear principles of play, choosing to use the game mechanics well, and limiting saying no to clear situations where it's obvious why.