Can you cast an instant during the declare blockers step before damage is assigned?

After blockers are declared, the game goes through a defined process: the active player (the attacking player) receives priority first in the Declare Blockers Step. Both players may then cast instants, activate abilities, and respond to each other before the game moves to the Combat Damage Step where damage is actually assigned and dealt.

This is governed by CR 509.4 and the general priority rules in CR 117.3b, which state that after blockers are declared and any triggered abilities are put on the stack, the active player receives priority. The step does not end until both players pass priority with an empty stack (CR 117.4).

This is a crucial window for many common plays. Tricks like pumping your creatures, giving them trample or first strike, removing blockers with targeted removal, or saving your creatures with protection effects all happen here — before any combat damage is calculated or assigned.

Concrete example: Your 2/2 creature attacks and your opponent blocks with their 3/3. Before combat damage is dealt, you cast Giant Growth as an instant, making your creature a 5/5. Now when combat damage is assigned, your 5/5 deals 5 damage to the 3/3, destroying it, and survives the 3 damage in return.

Note that the Combat Damage Step itself also allows a priority window before damage is actually dealt (CR 510.1–510.2), giving yet another opportunity to cast instants even after blockers and damage assignment order are locked in.