If I cast Green-Flame Blade but my attack misses, and I use an extra action from Haste to attack again, does GFB's effect apply if that attack hits?

Question

Let us assume that my wizard has the Haste spell cast on him, granting him an additional action on each turn. He casts Green-Flame Blade as his first action, attacking as part of the spell, but that attack misses.

If my wizard uses his additional action from haste to make another melee weapon attack, and it hits, does the effect of Green-Flame Blade trigger then?
Or is the magic spent with the first missed attack?

I know the spell's duration is Instantaneous, but the trigger is a successful attack. It makes sense to me that another attack in the same round could possibly trigger it if the first misses.

Answer 1

Green-Flame Blade only "triggers" if using the action to cast the spell

The Green-Flame Blade cantrip has a casting time of 1 action, and its description says (TCoE, p. 107; SCAG, p. 142):

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.

Casting the spell is the action that lets you make a melee attack, not the other way around.

If you miss, then it misses, and that's the end of the action, just like if you were using Fire Bolt.

Now, if you had something silly like Action Surge and the Quickened Spell Metamagic, you could use action 1 to cast Green-Flame Blade, use Action Surge and use action 2 to cast Green-Flame Blade, then use Quickened Spell to use a bonus action to cast Green-Flame Blade – but that's hardly optimal.

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A note about Haste: The Haste spell is limited in which actions can be performed with the additional action it grants you:

That [additional] action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action.

Answer 2

The green-flame blade spell cannot activate after missing with its attack

The spell's duration is "Instantaneous"

Firstly, the duration of green-flame blade is "instantaneous". What this means is that

The spell harms, heals, creates, or alters a creature or an object in a way that can't be dispelled, because its magic exists only for an instant."

Because of this, it would at least be unusual if the spell had an effect after it was cast (though some instantaneous spells do have lasting effects (such as feeblemind) so this isn't conclusive evidence on its own).

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The spell does not indicate that it persists

We can compare the wording of green-flame blade to something like branding smite:

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 next time you hit a creature with a weapon attack before this spell ends [...]

There are a few things to note here. First, branding smite has a duration of 1 minute and requires concentration, while green-flame blade does not. This indicates that the former can last after its initial casting while the latter cannot.

Second, branding smite occurs "the next time you hit a creature" whereas green-flame blade occurs on a hit with the attack that is made as part of the spell's casting. If it were intended to apply to attacks other than the one made when casting the spell, the spell would mention this fact similar to how branding smite does.

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Lacking any text to support that green-flame blade has a lasting duration and lacking any text to support that it can activate on an attack besides the one made as part of the spell, we conclude that green-flame blade cannot activate with the attack from the haste spell.

Answer 3

The effect of Green-Flame Blade does not trigger again

The description of Green-Flame Blade says, in part (SCAG, p. 142; emphasis added):

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 [...]

The effects described are related to the attack that is part of the action casting the spell.

Additional attacks, whether from Haste, or because you are multiclassed with a fighter that has Action Surge or Extra Attack, or any other feature giving you additional attacks, are not part of the action of casting the spell.