Does a creature die immediately if a continuous effect reduces its toughness to zero mid-combat?

Whenever a game event occurs, the game checks state-based actions (SBAs) before any player receives priority. One of those SBAs, defined in CR 704.5f, states: "If a creature has toughness 0 or less, it's put into its owner's graveyard." This check happens continuously, not just at end of turn or after combat.

Continuous effects that reduce toughness — such as a Diminish effect, a static ability like Gloom Surgeon's layer interactions, or an enchantment like Oathsworn Giant being removed — are applied through the layer system (CR 613). Once the effect resolves or changes, the game immediately re-evaluates SBAs at the next opportunity (CR 704.3: SBAs are checked whenever a player would receive priority).

This means that even in the middle of the combat phase — say, between the Declare Blockers step and the Combat Damage step — if an instant or triggered ability resolves that reduces a creature's toughness to 0, SBAs are checked right after that ability finishes resolving, and the creature is put into the graveyard before combat damage is even assigned.

Example: Your opponent attacks with a 2/2. You cast -1/-1 Shrink effect (assume it gives -2/-2 until end of turn) during the combat damage step before damage resolves. The creature becomes a 0/0. The game checks SBAs immediately after the spell resolves, sees the 0 toughness, and the creature dies — it never deals combat damage.

Note that damage and toughness reduction are tracked separately (CR 704.5g handles lethal damage). A creature reduced to 0 toughness by a continuous effect dies via CR 704.5f regardless of any damage marked on it, and this is distinct from dying to lethal damage under CR 704.5g.