If a mage casts a spell within 5 feat of a character with the Mage Slayer feat, does the reaction of attacking the casting character go off first, or does the spell go off first?

The Mage Slayer feat (PHB, p. 168) says:

When a creature within 5 feet of you casts a spell, you can use your reaction to make a melee weapon attack against that creature.

So which happens first?

Another reaction, the shield spell, says that when you are hit by an attack, you get +5 AC against all attacks, including the triggering one - but if the attack didn't hit then the reaction couldn't have been triggered. So I assume both interrupt the triggering action, and are then treated as occurring before the trigger.

  • \$\begingroup\$ Reopened - the question here isn't about whether the Mage Slayer attack can stop the spell from going off, it's about whether the attack happens before or after the spell is cast. \$\endgroup\$ – Miniman Jul 31 '17 at 4:56

No, the Mage Slayer's attack takes place after the spell has been cast. From the DMG, page 252:

If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes, as in the Ready action.

Since the trigger for Mage Slayer is when a creature casts a spell, the reaction will take place after a creature casts a spell, not as creature casts a spell.

Note that Jeremy Crawford, official source of rules interpretations, said the same thing.

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  • \$\begingroup\$ No Jeremy Crawford made a ruling about a bonus action Misty step not a reaction. For a similar reaction refer to Sentinel feat where the opponent doesn't move after sentinel goes off. Bonus action on turn only reaction on a triggering effect. \$\endgroup\$ – Billy the baby goat. Jul 29 '17 at 5:34
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    \$\begingroup\$ @Billythebabygoat. The fact that the spell in question was a bonus action is irrelevant. "the spell is intended to resolve before the Mage Slayer attack" \$\endgroup\$ – Miniman Jul 29 '17 at 6:06
  • \$\begingroup\$ This tweet supports interrupting a verbal spell with a readied silence spell. ie, spellcasting can be interrupted. The only question is, is a Mage Slayer reaction faster/slower than a readied Silence? \$\endgroup\$ – Adeptus Jul 31 '17 at 1:21
  • \$\begingroup\$ @Adeptus Presumably, like Counterspell, such a Ready wpulld have to be triggered by a creature casting. Mage Slayer is triggered by a spell being cast. \$\endgroup\$ – Miniman Jul 31 '17 at 2:49
  • \$\begingroup\$ So your point of difference is "casts a spell" vs "is casting a spell"? My argument would be that the difference is unintentional, and reaction: Mage Slayer attack is equivalent in timing to reaction: cast readied Silence (or for that matter, reaction: cast Counterspell) \$\endgroup\$ – Adeptus Jul 31 '17 at 4:49

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