Does a player lose the game immediately when their life total reaches zero 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 is handled by state-based actions, which are checked whenever a player would receive priority — that is, after a spell or ability finishes resolving, before any player may act (CR 704.3).

CR 704.3a states: 'If a player's life total is 0 or less, that player loses the game.' This is a state-based action, meaning the game engine checks for it at specific moments, not continuously mid-resolution.

This distinction matters in practice: if multiple things happen during a single spell's resolution — say, you lose 5 life and simultaneously gain 5 life — state-based actions aren't checked until after the whole spell finishes. If your life ends up above 0 by then, you don't lose.

Concrete example: You are at 2 life. An opponent's spell deals 5 damage to you (dropping you to -3), but the same spell also causes you to gain 10 life. Since both happen during resolution, state-based actions are only checked after the spell fully resolves. Your life total is now 7, so you do not lose the game.

State-based actions are also checked after each step and phase ends, so there are regular 'checkpoints' throughout a turn. The key takeaway: 0 life triggers a loss only at the next SBA check, giving a small but important window where effects can save you.