Can you react to a hidden card being placed?

Ruling: No, you cannot react to a hidden card being placed. Hiding a card does not create a chain and is not considered playing the card. You can react to a hidden card when it is flipped over/revealed, as that counts as a spell.

Nuances:

• Hiding a card is similar to moving a unit or drawing for turn - these are base speed actions that don't create chains
• The hidden card is only considered "played" when it is revealed
• You can only react to cards if they are spells or if the card triggers an ability
• Units technically go on the chain but resolve immediately, so you can't react to them either
• You cannot react to moving a unit unless it has a "when I move" trigger which would go on the chain