Does the secondary damage of the Green-Flame Blade cantrip damage the caster if no other targets are available?

Question

The description of the green-flame blade cantrip (SCAG, p. 143) reads:

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell’s range, otherwise the spell fails. On a hit, the target suffers the attack’s normal effects, and green fire leaps from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting modifier.

Since the caster fits the bolded criteria, does this mean he must target himself if no other creatures are available?

Answer 1

As of November 2020, SCAG has received errata to fix this inconsistency. The description of the green-flame blade spell now says that you "can" do it (emphasis mine):

You brandish the weapon used in the spell’s casting and make a melee attack with it against one creature within 5 feet of you. On a hit, the target suffers the weapon attack’s normal effects, and you can cause green fire to leap from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting ability modifier.

Answer 2

No, you don't have to cause the green fire to leap to another creature, as of the 2020 errata

The green-flame blade cantrip was updated when it was reprinted in Tasha's Cauldron of Everything (p. 107), and the version in the Sword Coast Adventurer's Guide (p. 143) received errata to match.

The relevant line of the green-flame blade spell description now reads:

On a hit, the target suffers the weapon attack’s normal effects, and you can cause green fire to leap from the target to a different creature of your choice that you can see within 5 feet of it.

The use of the word "can" indicates that the green fire does not have to leap to a second creature. Thus, if the only other creatures within 5 feet of the initial target are the caster and their allies (for instance), the caster can choose not to have the green fire leap at all after the damage to the initial target.

Even pre-errata, designer intent indicated that the caster didn't have to target another creature

Pre-errata, it was not directly stated in the spell description whether you had to target yourself with the secondary damage if no other creature was in range, or if you could choose not to have the green fire leap to anybody at all.

However, rules designer Jeremy Crawford unofficially answered this question in a tweet from November 2015:

What does the leap effect of green-flame blade do if there are no hostile targets nearby? Does it jump to allies?

The intent is that you can choose no one. If you can't see, you can't choose anyway, and the flame halts

While the wording might have suggested that the spell has to target another creature, Crawford clearly stated that that was not the intent; you could choose not to target anyone if there was no desired secondary target in range. The errata has brought this aspect of the spell in line with the stated intent.

This makes sense to me. Given that if the attack hits, the caster could choose what creature the green fire leapt to, it makes sense that the caster could use the same control over targeting to prevent the green fire from jumping to any creature at all (e.g. by having it fizzle harmlessly against the ground, or extinguishing itself).

Answer 3

Mike Mearls leaves the choice to the caster.

As always, every DM can rule differently.

Please note that, while they could make their way into erratas, tweets should not be treated as RAW.

Answer 4

...and green fire leaps from the target to a different creature of your choice that you can see within 5 feet of it.

Emphasis mine.

Even if your DM rules that it will jump back and hit you against your will, just close your eyes at the right moment so you're unable to see a target; The spell subsequently fails.

Answer 5

It's up to the DM, there is no official ruling

As noted in Sent_'s answer, Mike Mearls tells us that you needn't target yourself.

While Mike Mearls is a lead designer of DnD 5e -- as nitsua60 reminds us -- he is not the authority of rules and rulings. Mike Mearls concedes that there is no printed rule on his ruling. Thus, his ruling is simply that: A ruling. It carries as much weight as your own ruling as far as rules-as-written goes.

Until there is an official ruling, this is up to your DM.

Answer 6

I would say no. PHB p. 204

Targeting Yourself

If a spell targets a creature of your choice, you can choose yourself, unless the creature must be hostile or specifically a creature other than you. If you are in the area of effect of a spell you cast, you can target yourself.

It seems to me that Green-Flame Blade is by definition used against hostile creatures.

Further, a caster decides whether he is affected by his own spells when he is in the area of effect. So even if the flame was coming back at him, he would not take the damage if he chose not to.