If a creature is dealt damage equal to its toughness and then gets a toughness boost before state-based actions are checked, does it survive?

State-based actions are checked at specific points during the game — not continuously. When the game checks whether a creature has lethal damage, it compares the total damage marked on the creature against its current toughness at that exact moment (CR 704.5g). If a toughness boost arrives before that check occurs, the creature's new, higher toughness is what matters.

Damage is marked on a creature as a number, and it stays there until removed (usually at end of turn). The lethality check is simply: is the damage marked on this creature greater than or equal to its current toughness? (CR 704.5g). The game does not retroactively ask whether the damage was lethal when it was first dealt.

The timing window that makes this possible is within a single priority pass. If you respond to something that deals damage — or use a spell or ability with no intervening priority for the opponent — you can raise toughness before state-based actions get a chance to check. State-based actions are checked after each player receives priority (CR 704.3), so a response that increases toughness in the same phase can save the creature.

Concrete example: Your 2/2 creature is dealt 2 damage by Lightning Bolt (a 3/3 total would die, but let's say it's a 3/2 — it takes 2 damage, exactly equal to its toughness). Before you pass priority and before state-based actions check, you cast Giant Growth (+0/+3), making it a 3/5. Now only 2 damage is marked on a creature with 5 toughness. State-based actions see 2 < 5, so the creature does not die.

Note: This does not help against destruction effects or effects that say "destroy" — those don't use the damage/toughness comparison. Also, if the creature already has damage marked exceeding its original toughness due to deathtouch (CR 704.5h), the deathtouch rule still applies separately. But for normal lethal damage checks, boosting toughness in time absolutely saves the creature.