What happens if a player has ten or more poison counters in the middle of resolving a spell?

A player loses the game if they have ten or more poison counters. However, this is a state-based action (CR 704.5c), and state-based actions are not checked during the resolution of a spell or ability. They are only checked once the spell or ability has finished resolving (CR 704.3).

So if a spell's resolution causes a player to reach ten or more poison counters partway through, the game does not stop mid-resolution. The spell continues to resolve fully, carrying out all remaining instructions. Only after the entire spell has finished resolving do state-based actions get checked, at which point the player with ten or more poison counters loses the game (CR 704.3, 704.5c).

This means any remaining effects of the spell — such as dealing damage, drawing cards, or creating tokens — still happen before the game checks whether anyone has lost. The loss is essentially 'queued' until the resolution window closes.

Example: Suppose a spell reads 'Target player gets 5 poison counters, then you draw 3 cards.' The target player already has 6 poison counters, so gaining 5 more brings them to 11. Even so, the active player still draws their 3 cards before state-based actions are checked and the poisoned player officially loses the game.

This interaction is important strategically: even if a spell will clearly kill a player via poison, all of that spell's effects resolve first before the losing condition is formally applied.