State-Based Actions in Magic: The Gathering, Explained

State-based actions are automatic checks the game performs to keep the rules consistent, without anyone needing to do anything. They don't use the stack, can't be responded to, and aren't controlled by any player. Instead, the game constantly looks for certain conditions and corrects them immediately. Common examples include a player at 0 or less life losing the game, a creature with 0 toughness being put into its owner's graveyard, and a creature with lethal damage marked being destroyed.

Other state-based actions include the legend rule (if a player controls two or more legendary permanents with the same name, they choose one to keep and put the rest into the graveyard), an Aura attached to something illegal being put into the graveyard, a token that has left the battlefield ceasing to exist, and a player who attempted to draw from an empty library losing the game. Each is a fixed rule the game enforces automatically.

State-based actions are checked whenever a player would receive priority. This means the game looks for applicable conditions before any player can cast a spell or activate an ability, both during the resolution boundaries of spells and abilities and at the start of each step or phase where priority is given. They are not checked continuously between those moments, which is why a creature can briefly have 0 toughness mid-resolution and only die when the next check occurs.

When the game checks, it applies all applicable state-based actions simultaneously as a single event, then checks again. If new conditions appeared because of those actions, another round happens. This loop repeats until no state-based actions are pending, and only then does a player actually receive priority. Because they happen before priority, you cannot respond to a creature dying to lethal damage; it is already gone by the time you could act.

Lethal damage and 0 toughness are the most frequently encountered. A creature is destroyed by state-based action if it has damage marked on it equal to or greater than its toughness, and a creature with 0 or less toughness is put into the graveyard without being destroyed (so regeneration and indestructible do not save it). These checks use the creature's current toughness after all continuous effects are applied.

The legend rule and 'plus/minus counters cancel out' are also state-based. If a permanent has both +1/+1 and -1/-1 counters, that many of each are removed as a state-based action until only one kind remains. Knowing that these are state-based, not triggered, explains why they happen the instant the game checks, with no opportunity for an opponent to respond in between.