Can I play a reaction when an enemy plays a Carnivorous Snapvine to their base targeting a unit at a battlefield I control?

Question

Accepted Answer

Yes. You may play a Reaction in response to an opponent playing Carnivorous Snapvine. If your Reaction makes the targeted unit no longer a legal target (for example by moving it off that battlefield to base), then Carnivorous Snapvine will not affect that unit when it resolves.

You can play Reactions in response to Carnivorous Snapvine being played. Reactions are permitted while the play is on the chain [813]. If your Reaction causes the targeted unit to no longer meet the Snapvine’s targeting restriction (for example it is moved off the battlefield to base), that target is no longer legal for the Snapvine’s resolution and it will be unaffected by the damage instruction [359.3.e.4] [359.3.e.5] [FAQ:unleashed#22].

Opponent plays Carnivorous Snapvine to their base and during the play declares the chosen enemy unit at your battlefield as the target; Carnivorous Snapvine is now a pending chain item. While that chain item exists, you are permitted to play any legal card or activate any legal ability with the [Reaction] keyword because Reaction allows play/activation during Closed States on any player's turn [813]. If your Reaction changes the targeted unit so that it no longer meets the Snapvine's targeting restriction at resolution (for example, you move it off that battlefield to your base), then at resolution that target is no longer a legal target and will be unaffected by the Snapvine's damage instruction [359.3.e.4] [359.3.e.5]. If your Reaction only changes numeric values like Might before resolution, those changed values will be what matters when the Snapvine resolves (subject to usual timing/legal checks) because Reaction is only a permission and does not change how the effect itself functions [813.3].

If your Reaction moves the unit to a Non-Board Zone (e.g., returns it to hand or banishes it), the game object is no longer the same and any targeting relationship is severed; the Snapvine will not be able to target it even if it later returns [359.3.e.4]. If you use a Reaction that only adjusts Might (for example, Defiant Dance giving -2 Might), the damage Snapvine deals will be calculated using the Mights at resolution; making the unit lose enough Might does not by itself stop it from being a legal target unless the targeting restriction depends on Might or location [359.3.e.12] [813.3]. If you intend to use a Reaction that is an ability that Adds resources, note some printed abilities/cards restrict reacting with Add abilities (see the card text on Kai'Sa - Daughter of the Void/Diana - Scorn of the Moon). Also adding is a Limited Action and has special timing rules for when Add Reactions can be used to pay costs [429].

Short answer: Yes — you can play a Reaction when your opponent plays Carnivorous Snapvine targeting a unit at your battlefield, because Reaction grants the permission to be played during Closed States on any player’s turn [813].

Why: When an opponent plays Carnivorous Snapvine, its play effect ("When you play me, choose an enemy unit at a battlefield…") becomes a pending chain item. While that chain item is on the chain, players may respond with Reactions because the Reaction keyword explicitly allows play/activation during Closed States on any player's turn [813]. That means you can play a Reaction card or activate a Reaction ability before the Snapvine resolves.

What happens if you do: If your Reaction makes the chosen unit no longer meet the Snapvine’s targeting restriction at the moment the Snapvine resolves (for example, you react with a spell that moves the unit from that battlefield to your base), then that unit is not a legal target for the Snapvine when it resolves and will not be dealt damage by it [359.3.e.4] [359.3.e.5]. Moving a unit to another board location (base vs battlefield) does not sever the targeting relationship while the object remains on the board, but if the object no longer meets the targeting restriction at resolution it will be unaffected [FAQ:unleashed#22] [359.3.e.5]. If you instead change the unit's Might (for example with Defiant Dance), that new Might will be used when resolving damage calculations; changing Might does not inherently stop the Snapvine unless the Snapvine’s targeting depends on Might or location [359.3.e.12] [813.3].

Notes and special cases: If the Reaction you want to use is an ability that Adds resources, be careful — adding resources has its own timing rules and some printed abilities include restrictions like "Abilities that add resources can't be reacted to." Also adding is a Limited Action and Add Reactions finalize immediately when used to pay costs during play [429]. If a target goes to a Non-Board Zone (hand, banish, etc.), the object is no longer the same and any prior targeting relationship is severed entirely [359.3.e.4].

If you want a ruling for a specific Reaction card or ability (for example, whether Kai'Sa - Daughter of the Void's or Diana - Scorn of the Moon's Add-Energy reactions can be used here), give the exact card/ability and the resources you have and I will apply the same rules to that exact case.