Do state-based actions check continuously during the resolution of a spell or only after it finishes resolving?

State-based actions (SBAs) are checked only at specific moments — never in the middle of a spell or ability resolving. According to CR 704.3, the game checks SBAs whenever a player would receive priority. Spells and abilities must fully resolve before any player receives priority again.

This means that even if a creature's toughness drops to 0 or a player's life total reaches 0 partway through a spell's resolution, nothing immediately happens. The spell finishes resolving completely first, and only then are SBAs checked (CR 704.3, 608.2).

This has important practical consequences: a spell can, for example, reduce a creature to 0 toughness and then give it +3/+3 all within the same effect, and the creature will survive because SBAs never saw it at 0 toughness during the resolution.

Concrete example: Your opponent casts Searing Wind dealing 10 damage to your 2/2 creature, but the same spell text also says 'then put a +10/+10 counter on that creature.' Even with lethal damage marked mid-resolution, SBAs aren't checked until the spell fully resolves. If the +10/+10 counter is granted as part of the same resolution, the creature survives when SBAs finally check (assuming the counter is added before the spell ends).

In short: think of spell resolution as an uninterruptible sequence. SBAs, triggers going on the stack, and priority are all paused until the spell is completely done (CR 608.2b, 704.3).