Player agency is always zero when GM decides.
Anyone bringing up ‘other rpgs’ should name them explicitly, so we can refer to the game texts.
Agency is the ability to change the fiction, usually through player-facing mechanics combined with established fiction which act as constraints.
Announcing an action isn’t agency - such a definition would be a nonsense. Of course, nonsense definitions suit the purposes of GMs wishing to conceal the lack of player agency in their games.
Going to use this post as a jumping off point to talk about this and I'm going to go about this in a roundabout way.
Agency, in anything, means the ability to put into effect one's will upon a thing.
Basketball players have agency during play of a game. Their opponent(s) also have agency. The referees also have agency.
Any member of the group above can arrest the agency of another member of the group. How does this happen? The following example hopefully does some work here:
* A wing player possesses the ball on the right elbow. He dribbles hard left to the middle, intent on (a) beating his man and either (b) scoring or (c) generating a scoring opportunity for a teammate.
All of (a - c) are component parts of that wing player's effort to effect his will upon that singular possession (and in-so-doing, effect the greater will of his team upon the game).
Now the defender manned up on that wing player opposes all of (a - c) above. If their will is done, none of those things will be realized (and what will be realized is a stifled possession leading to a bad shot or a Turnover).
Now, in a perfect world with a perfect referee, the ref has no agency. S/he is merely the rulebook given life. But we all know this isn't true, so the referee will invariably put into effect their will upon the game, even if its merely the unconscious will of "trying to call a fair and correct game." Unfortunately, they are human...and because they are human, something like "the Block vs Charge violation" paradigm (just to name one of many) is utterly beholden to their human inadequacies (cognitive biases, minor vision impairment, a flawed mental model of how two human bodies of differing velocities and angles of intercept interact). Or you could have more or less sinister or benign agency by a referee (willfully calling a play one way because of a bad relationship with a player or a coach or wanting the game to speed up so they can go get a drink after the game quicker).
All of agency (when it comes to games) is about authorship on the emerging work.
In a basketball game its about
this possession and then about the game in total.
In roleplaying games its also about authorship. You can map the above exactly to TTRPG scenarios.
A player wants thing x to happen and works to cement its place in the unfolding situation before them. They do this by orienting themselves to the game's parameters (the authority distribution, the resolution mechanics, their thematic/tactical/strategic interests, the GM's ethos, the present gamestate and the possible future gamestates, etc) and then declaring an action for their PC.
All other participants are similarly positioned except, like the referee in the above scenario, the GM has the unique ability to declare the game's default orientation toward authority distribution as null and void (even if in so doing they're hoisted with their own petard and lose their game).
This conversation MUST orbit around the expected authority distribution and attendant play procedures that put it into effect which are inherent to play (and distinct to a particular system). That is how a TTRPG's gamestate is changed from gamestate 1 to gamestate 2 and that is how the shared fiction emerges.