How does Not So Fast work with targeting spells like Void Seeker and Hex Tech Ray - can you react immediately when the spell is played, or only after targeting is declared?

Ruling: Most spells target when added to the chain before reactions can be played, so you would play Not So Fast after your opponent has chosen their target and passed priority.

Sequence:

• Opponent plays the targeting spell (e.g., Void Seeker)
• Opponent chooses the target for that spell
• Opponent may add any reactions they wish to the chain while they have priority
• Opponent passes priority
• You can now play Not So Fast as a reaction

Nuances:

• When a player has priority, they may add as many reactions as they wish before choosing to pass priority to their opponent