Is there a way to turn cantrips into reactions?

I'm trying to find a way to make blade ward (PHB, p. 218) more useful than it is, because it fits my planned playstyle. (For reference, I'm planning to play Sorcerer then multi-class into a Fighter until I get Action Surge.)

  • \$\begingroup\$ Are you trying to cast it as a reaction to a particular trigger? Or are you just wondering about casting action spells/cantrips as reactions in general? Also, is your question just about blade ward, or cantrips in general? \$\endgroup\$ – V2Blast Oct 2 '18 at 0:01

Prepare Blade Ward using the Ready Action

This is how it would work RAW:

You: I use the Ready Action to cast the Blade Ward cantrip. If someone attempts to attack me, I'll activate the spell. My turn is now over.

The obvious downside to this is that you cannot do this while also attacking normally on your turn. To my knowledge, there is no way, short of using Action Surge or the Quickened Spell metamagic, to both attack and Ready a spell (Blade Ward or otherwise) in the same turn. As a result, there's no real benefit to doing this: Blade Ward lasts until the beginning of your next turn anyways, so casting like this gives the same effect, while also consuming your Reaction, making you unable to use your Reaction for anything else.

Consider the spell Shield instead

The 1st level spell Shield seems more suited to what you're trying to do. Its effects are different than Blade Ward (Shield gives +5AC, whereas Blade Ward gives resistance to Physical Damage), but both have the effect of mitigating damage, and the difference with Shield is that it is explicitly cast using a casting time of 1 Reaction. So it's possible to attack in your turn and then still react by casting Shield, should you get attacked.

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  • \$\begingroup\$ Another option for the general question as currently written is War Caster, though it wouldn't work for blade ward. \$\endgroup\$ – V2Blast Oct 2 '18 at 0:02

No, this is not possible

There are only two official ways to modify the casting time of spells. One is the Quickened Spell metamagic for sorcerers:

When you cast a spell that has a casting time of 1 action, you can spend 2 sorcery points to change the casting time to 1 bonus action for this casting.

The other method is the Epic Boon called Boon of Quick Casting:

Choose one of your spells of 1st through 3rd level that has a casting time of 1 action. That spell’s casting time is now 1 bonus action for you.

Neither of these features change the casting time to a reaction and Boon of Quick Casting doesn't even work on cantrips.

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  • 1
    \$\begingroup\$ Although not actually changing the casting time the Ready action would allow it to be used as a Reaction. \$\endgroup\$ – Slagmoth Oct 1 '18 at 16:35

The Eldritch Knight fighter archetype is the closest thing you're going to find.

At 7th level, the Fighter with the Eldritch Knight archetype gains War Magic, which allows them to make a weapon attack as a bonus action after they cast a cantrip. It's not really making the cantrip into a reaction or anything, but it might be the nearest equivalent.

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