Can you respond to your own spell after you cast it before it resolves?

When you cast a spell, it is placed on the stack and then players receive priority in turn order. Crucially, you — as the active player or the player who just took an action — receive priority first (CR 117.3b). This means you get the first chance to cast another instant-speed spell or activate an ability before your original spell resolves.

A spell or ability on the stack resolves only when all players pass priority in succession without taking any action (CR 117.4). So simply casting a spell does not immediately make it resolve; the game pauses and asks everyone (starting with you) if they want to do anything first.

You can use this window to cast instants, activate activated abilities (like tapping a land for mana or using a planeswalker ability, if at instant speed), or trigger any responses you control — all before your original spell resolves.

Concrete example: You cast Lightning Bolt targeting an opponent's creature. Before it resolves, you notice the creature would be lethal to your opponent if it survived. You can immediately cast a second instant — say, Giant Growth — targeting your own creature, or activate an ability. Both spells now sit on the stack; Giant Growth resolves first, then Lightning Bolt.

The only limitation is speed: whatever you cast or activate in this window must be usable at instant speed (CR 117.1, 302.5). Sorceries, for instance, cannot be cast during this window even on your own turn while your spell is on the stack.