I'm not sure what can or cannot affect an agenda

I ran into someone playing Stannis and I am running Bloodthirst - I wasn't sure whether Stannis prevents cards being drawn via an agenda

Meanwhile, If I defend With Brienne against my opponent runng Siege, can they claim 2 power if they win?

I can see a difference between Brienne preventing the response being triggered and the passive effect of Bloodthirst, which once it happens cannot be cancelled/prevented - i.e. if the agenda action can be applied, it is then immune

Stannis

"Players cannot draw cards through card effects

Bloodthirst

At the end of the challenge phase, the player who controls the fewest characters draws 2 cards .

Brienne

While Brienne of Tarth is participating in a challenge, opponents cannot trigger effects

Siege

Response: After you win a Military challenge, claim 2 power for your House

"Cannot" is absolute. Stannis and Brienne will prevent agenda effects from going off if their abilities are turned on. Agenda effects only cannot be canceled. They can still be prevented.

Thanks - I expectged that the Cannot would be the crux issue