Can you react to a hidden card being played?

Ruling: You cannot react to a card being hidden, but you can normally react to a card played from hidden. A hidden card being played is treated like any other card being played.

Nuances:

• For spells played from hidden, you can react to the spell itself
• For units or other card types played from hidden, you can only react to their play effects (not to the card entering play itself)