Can you cast a spell during the cleanup step?

Under normal circumstances, no player receives priority during the cleanup step, which means no one can cast spells, activate abilities, or take any game actions during it. This is explicitly stated in CR 514.3: the cleanup step proceeds without any player receiving priority.

The cleanup step has two main functions: the active player discards down to their maximum hand size (CR 514.1), and all damage is removed from permanents and 'until end of turn' effects end (CR 514.2). These happen automatically without player input.

There is one important exception: if a triggered ability triggers during the cleanup step — for example, because a 'until end of turn' effect wore off and caused a state-based action, or a triggered ability fired — then a new cleanup step begins after players receive priority to handle those triggers (CR 514.3). During that priority window, players could cast instants or activate abilities. But this is a rare, secondary cleanup step, not the normal case.

Example: Your opponent controls a creature enchanted with a temporary Control Magic effect that expires at end of turn. When the enchantment leaves during cleanup, a triggered ability fires. Players now receive priority in a new cleanup step — your opponent could cast a Lightning Bolt with that priority window before another cleanup step resolves.

So the practical answer for everyday play is: you cannot cast spells during the cleanup step unless an unusual trigger forces an additional cleanup step and creates a priority window.