Xanathar, Guild Kingpin

The target opponent may still cast spells in response to Xanathar, Guild Kingpin's ability in your upkeep. If that player has somehow become an illegal target for the ability as it tries to resolve, the ability does nothing. They may cast spells and you cannot look at or play the top card of their library.

You'll still pay all costs for spells cast this way, including additional costs. You may also pay alternative costs if any are available.

During your turn, you can look at the top card of the chosen opponent's library whenever you want (with one restriction; see below), even if you don't have priority. This action doesn't use the stack. Knowing what that card is becomes part of the information you have access to, just like you can look at the cards in your hand.