Can you respond to a spell before targets are chosen?

Ruling: No, you cannot respond to spells before targets are chosen. All spells and abilities must choose their targets to be finalized onto the chain first. Only after the spell is finalized onto the chain do reaction windows open.

Sequence:

• Opponent plays the spell and chooses targets (spell is not yet finalized)
• Spell is finalized onto the chain with targets chosen
• Reaction windows open and you can now play reactions

Nuances:

• The state closes when a player begins to play a card, but you cannot interrupt the playing of the card itself
• Target selection happens as part of finalizing the spell, before reactions can be played