If an Open Hand Monk hits a creature with the shield spell prepared and uses it's Open Hand Technique feature to stop it from using reactions until the end of the Monk's next turn, would the creature be able to cast shield to cancel the attack?
I believe that the reaction kicks in before the attack, but Jeremy will have a definitive ruling
— Mike Mearls (@mikemearls) May 30, 2018
Shield spell—its description says it works against the triggering attack, which means it can cause that attack to miss. If the attack has a special effect that relies on it hitting, that effect doesn't occur if the attack is turned into a miss. #DnD https://t.co/WHZDbQb9vz
— Jeremy Crawford (@JeremyECrawford) May 30, 2018
Open Hand Technique requires you to successfully hit a creature before the effects take hold.
An attack you make doesn’t actually “hit” until the spellcaster either chooses to cast Shield, or decides not to. It’s similar to how an “Interrupt” effect works in Magic: The Gathering.
To put it simply, you make an attack that is about to hit, the spellcaster has a split second to use their reaction on Shield. If they choose to use it, the Shield goes off BEFORE you’ve hit them.
Otherwise, you simply hit them, and only THEN does the Open Hand Technique effect begin to apply.