In Commander, does a player lose the game immediately when their life total reaches 0 even if they have replacement effects?

A player loses the game due to having 0 or less life as a state-based action, defined in CR 704.5a: 'If a player has 0 or less life, that player loses the game.' State-based actions are checked at specific moments — not continuously mid-spell or mid-ability — giving replacement effects time to apply first (CR 704.3).

Replacement effects (CR 614.1) modify an event before it occurs. If a replacement effect would change the outcome of reaching 0 life (for example, by setting your life total to a different value, or causing a different event to happen instead), that replacement applies before state-based actions ever see a life total of 0 or less.

A classic example: if you control Platinum Emperion, your life total cannot change. If damage would reduce you to 0, the replacement effect prevents that change entirely, so state-based actions never see you at 0. Similarly, effects like Lich's Mastery replace the loss-of-life event or the losing-the-game event itself.

Commander's 21 commander-combat-damage rule (a separate loss condition unique to the format) follows the same pattern: it is also checked as a state-based action (CR 903.10a), not mid-step, so replacement effects can interact with it as well.

In summary: losing at 0 life is never instantaneous mid-action. You survive until the game checks state-based actions, and only then if no replacement effect has altered the situation (CR 704.3, 704.5a, 614.1).