Does a player lose the game immediately when they reach 0 life or at the next state-based action check?

A player does not lose the game the instant their life total becomes 0 or less. Instead, losing due to 0 or less life is a state-based action (SBA), meaning it is only checked and applied at specific moments defined by the rules — not mid-spell or mid-ability resolution.

According to CR 704.3, state-based actions are checked whenever a player would receive priority. CR 704.5a specifies the relevant SBA: "If a player has 0 or fewer life points, that player loses the game." The game engine checks this condition, sees it is true, and then the player loses — but only at that checkpoint, not the moment the life total changed.

This timing matters practically. If two effects happen simultaneously (e.g., a spell causes both players to lose life), all results of that spell are applied first, SBAs are then checked all at once, and if both players are at 0 or less life simultaneously, the game is a draw rather than a sequential pair of losses.

Concrete example: Your opponent is at 2 life and you cast Lightning Bolt dealing 3 damage. While Lightning Bolt is resolving, your opponent is not yet eliminated — their life total drops to -1, the spell finishes resolving, and then the game checks state-based actions. At that checkpoint, CR 704.5a applies and your opponent loses the game.

This also means replacement effects or triggered abilities (like Platinum Angel's effect) can still interact before SBAs are checked, potentially preventing the loss entirely if the condition is removed before priority is passed.