In Commander, do you lose if your life total reaches 0 even before state-based actions are checked?

State-based actions (SBAs) are the mechanism that actually causes a player to lose the game for having 0 or less life. They are checked at specific moments — after each spell or ability finishes resolving, before a player would receive priority, and at other defined game checkpoints — not continuously at every instant (CR 704.3).

This means the moment your life total becomes 0 or less is not itself the losing event. The loss only happens when the game next checks SBAs and finds your life at 0 or less (CR 704.5a). Until that check occurs, you are still in the game.

A practical consequence: if something causes your life total to drop to 0 and then immediately raises it back above 0 within the same period before SBAs are checked, you do not lose. For example, if a spell deals 5 damage to you (dropping you to 0) and you have a triggered ability that says 'whenever you are dealt damage, you gain that much life' — but that trigger hasn't resolved yet — SBAs are checked after the damage spell finishes resolving, at which point you are still at 0. You would lose before your trigger gets a chance to resolve.

Commander adds the additional Commander damage loss condition (21 combat damage from a single commander) and the 'poison counters' condition, but these are also checked as SBAs under the same timing rules (CR 704.6a, 704.6b).

Example: Your life total is 3. An opponent casts Lightning Bolt targeting you. When Bolt resolves, your life total becomes 0. SBAs are then checked immediately — finding your life at 0, you lose the game right there, before anyone can cast another spell or activate an ability to save you.