When a card effect plays another card (like Harrowing playing Udyr, or Promising Future), can opponents react to the card being played by the effect?

Ruling: You can react to the original spell (like Harrowing or Promising Future) when it is played, but you cannot react to cards that are played as part of that spell's resolution. Cards played by effects are placed as "pending" and resolve as part of the original spell's resolution.

Sequence:

• Player plays a spell that will play other cards (e.g., Harrowing, Promising Future)
• Opponents can react to this spell on the chain
• When the spell resolves, it plays the specified cards as "pending" items
• All pending cards are added to the chain in order
• Permanents played this way resolve immediately when added
• Any "when played" triggers from those permanents go to the end of the chain
• The chain continues to resolve in reverse order
• No reactions can be made to the cards being played by the effect during this resolution

Nuances:

• Cards played from trash (like with Harrowing) do not get banished because they don't need to be held somewhere if they can't be played
• Cards played from deck get banished temporarily because they need to be kept somewhere if they can't be played and can't be returned to their original position in the deck
• When playing cards from trash, the target is chosen when adding to the chain, not during resolution
• "When played" triggers from units played by effects are placed on the chain after the pending cards resolve, but before earlier chain items resolve