If you play a spell targeting an opponent's unit, and they respond by buffing that unit (giving it deflect via Spirit's Refuge), do you have to pay the deflect cost?

Ruling: No, you do not have to pay the deflect cost. Once a spell is finalized, targets are already chosen and costs are already paid. If the opponent buffs their unit in response to a spell already on the chain, it's too late for deflect to matter.

Sequence:

• You finalize your spell (choose targets, pay costs including any deflect)
• Spell goes on the chain
• Opponent can respond by buffing their unit
• The buff happens too late - deflect is only checked during finalization, not resolution

Nuances:

• Falling Star and Icathian Rain are exceptions because they generate reflexive triggers on resolution where targets aren't chosen until after the spell resolves. For these spells, you can respond by buffing a unit and that unit will have deflect for the triggers that are generated.
• Even with Falling Star and Icathian Rain triggers, you can decline to pay deflect and the triggers simply won't finalize.