Do spells 'fizzle' in Riftbound if their target gains Deflect after the spell is already on the chain? Specifically, if Stupefy targets Fiora, then Fiora gains Deflect before Stupefy resolves, does the caster need to pay the Deflect cost?

Ruling: Spells do not fizzle in Riftbound. If a spell is already on the chain when its target gains Deflect, you do not need to pay the Deflect cost because costs are paid when putting cards/abilities on the chain, not on resolution. The spell resolves normally.

Sequence:

• Player declares target and pays all costs (including any Deflect costs that exist at that moment) to put the spell on the chain
• If the target gains Deflect after the spell is on the chain, this does not change what was already paid
• The spell resolves normally without requiring additional payment
• Spells that fail to complete their effects are still considered "played" (they "whiff" but don't "fizzle")

Nuances:

• Triggered abilities are different: you CAN decline to pay Deflect costs for triggered abilities, which removes the trigger from the chain
• Deflect is a cost modifier, not a targeting requirement - target legality checks do not re-evaluate whether costs were paid
• If a spell is copied (like with Mystic Reversal) and targets a unit with Deflect, you don't pay the Deflect cost because costs were already paid for the original spell
• Nothing in Riftbound works retroactively - once a timing window passes, it's done