Summoner's Pact and Priority

[Bongo]

Hi!

Is it possible to play Flash, wait for counters (opponent plays no counters and passes priority) and then play Summoner's Pact?
Or is my priority window over after my opponent passes priority, in other words: do I have to play the Pact before the Flash?

[ELD]

The short answer is yes, you do have to play Summoner's Pact before Flash. 

If you play Flash, and then pass priority, your opponent may respond.  If they do not respond, then Flash resolves.  In order to keep priority, you would need to announce the Summoner's Pact in the same sentence, making very clear that you intend to respond to your own spell.  To prevent this kind of issue, you can just play Summoner's Pact first, then Flash.