Can you react to anything?

Whether you can react to a game action depends on whether that action creates a chain and whether you have priority.

General Rule for Reactions

You can only play a Reaction when you have priority and there is an active chain (a "Closed State"), or if the card you are playing has the Reaction keyword, which permits it to be played during a Closed State.

Can you react to playing a card?

• Spells: Yes. When a player plays a spell, it is added to the chain as a pending item. Once it finalizes, it becomes a finalized chain item, at which point players receive priority to play Reactions.
• Permanents (Units/Gear): Generally, no. Permanents do not create a priority window for opponents when they are played. They enter the chain, finalize immediately, and leave the chain to become game objects. Because they leave the chain during finalization, there is no finalized chain item for an opponent to respond to.
  • Exception: If a permanent has a "When you play me" triggered ability, that ability is added to the chain. You can react to that triggered ability.

Can you react to "Hidden" cards?

• Hiding a card: No. Hiding a card (placing it facedown) is a discretionary action that does not create a chain. It finalizes immediately, and no priority window is created.
• Playing from Hidden: When you flip a card face-up to play it from a hidden zone, it is treated as playing a card.
  • If the card is a spell, you can react to the spell itself.
  • If the card is a permanent (unit/gear), you cannot react to the card being played, but you can react to any "When you play me" triggered abilities that the card creates upon entering play.

Summary Table

*Note: You can still react to any triggered abilities generated by the permanent.