Does the cleanup step allow players to cast instants or activate abilities?

Under CR 514.1 and 514.2, the cleanup step is designed to be entirely automatic: the active player discards down to their maximum hand size, then all damage is removed from permanents and 'until end of turn' effects end. No player receives priority during a normal cleanup step, so instants cannot be cast and abilities cannot be activated.

However, CR 514.3 creates an important exception. If a state-based action would be performed (CR 704) or a triggered ability would trigger during the cleanup step, the game first processes those, and then each player does receive priority in the usual order. This gives players a window to cast instants or activate abilities in response.

After all players pass priority with the stack empty following that extra priority window, another cleanup step begins (CR 514.3). This cycle repeats until a cleanup step ends with no state-based actions to perform and no triggered abilities on the stack, at which point the turn truly ends.

Concrete example: You have 8 cards in hand at the start of cleanup and discard one. That discarded card triggers your opponent's Waste Not (it triggers whenever an opponent discards). Because a triggered ability triggered during cleanup, CR 514.3 kicks in: both players receive priority and may cast instants or activate abilities before the triggered ability resolves. After the stack empties with no new triggers or state-based actions, a new cleanup step begins and the turn ends normally.