Can a player play a reaction when the opponent proceeds to the end step (i.e., when changing phases)?

Ruling: No, you cannot play a reaction when changing phases. You can only play a reaction during an opponent's turn if the chain is not empty and you get priority, or if there is a showdown and you get focus.

Nuances:

• This differs from Magic: The Gathering, where players commonly perform actions during the opponent's end step.