I saw questions on several other websites as to how this combo works, and saw several answers. My take, as a player, generally leans towards doing as much as possible with it. So the way I see it, I cast Booming Blade, spend a sorcery point to Twin it, and now I have melee attacks against 2 targets. If 1 or both hit, the recipient is now subject to the potential Movement damage that comes with the 2nd half of Booming Blade.
This cantrip seems to fulfill the requirement of Twinned Spell in that it only targets 1 creature. Or the way I saw Jeremy Crawford somewhat reword this, it only affects 1 creature. I add this part only to head off responses that go something like, "Booming Blade doesn't say the spell targets a creature, it just necessitates a melee attack that targets a creature."
If the answer to this is yes, then I can make my next thread in Tips and Tactics pondering the merits of multiclassing to Sorcerer (from EK). Please don't get lost on this tangent lol
At first I wanted to say you should need to have the Extra Attack feature, two weapon fighting or similar to take advantage, but as I thought about it - the action you are taking that round is casting the spell. As part of that action, the spell necessitates you make a melee attack - this attack is part of casting the spell, it is not an action, bonus action, or reaction in and of itself. Therefore, despite some confusing wording, I think the attack and the sheath of booming energy (and extra damage at higher levels) all happens against 2 separate targets when twinned.
Yep, it clearly specifies the creature against which a weapon attack is made as being "the target", it specifies "one creature", its range is 5', not self, thus it meets all requirements for twinning the spell. Honestly I'm not sure what debate there would be. The melee attack is part of the action which is casting the spell, the spell being twinned would twin the melee attack. Unusual, but specific trumps general.
My issue with allowing BB to be twinned is that it allows for sorcerers to gain an additional melee attack where they wouldn't normally get one. With a regular Sorcerer using a dagger or staff it's probably not an issue, but once you add a strong melee attack (say multiclass Paladin 2), it becomes significantly more powerful.
The RAW argument: a Sorcerer cannot meet the requirement of making 2 melee attacks with their Action, as would be required for twinning BB. From Booming Blade: "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." Thus, if a character cannot make that melee attack with a weapon for whatever reason, the spell fails. Since characters cannot make 2 weapon attacks with an Action, they cannot meet the requirement of Twinning BB. They would need to be able to hit 2 targets with 1 melee attack to meet that requirement- which is all DM fiat.
I would argue that without the Extra Attack feature a Sorcerer cannot meet the requirements of making a second melee attack to twin the spell- although by RAW, you can only use the Extra Attack feature when using the Attack Action, not the Cast a Spell Action. You might want to argue that this logic also applies to touch spells, but touch spells don't include the additional effects of the melee strike. You can't combine monk unarmed attacks and touch spells.
I would rate this as a medium to strong exploit depending on how rests are done and how grouped up foes tend to be on the battlefield. This allows certain builds to outshine other martial builds at the table, which is why I'd rate it as an exploit. Yes, it is limited by Sorcery Points and the two target restriction, so it's not outrageous. Consider a Pal 2 / Sorc X with a great sword and great weapon fighting style. I would argue that by 11th, a Pal 2 / Sorc 9 would would outshine similar power builds of 11th level that don't exploit BB. The 'Blue Dragon Knight' would be able to Twin and possibly Quicken BB for three attacks (on 2 targets) each at 2d6+Str +2d8 +3d8 BB rider +2d8 smite as needed. That's a lot of boom. In addition, they would have access to some nifty spells like Fireball, Haste, and Improved Invisibility. I would suggest allowing it to be Quickened but not Twinned, as this uses a bonus action for the extra attack and costs a more appropriate 2 Sorcery Points. This brings it in line with the War Priest feature from cleric.
My issue with allowing BB to be twinned is that it allows for sorcerers to gain an additional melee attack where they wouldn't normally get one. With a regular Sorcerer using a dagger or staff it's probably not an issue, but once you add a strong melee attack (say multiclass Paladin 2), it becomes significantly more powerful.
The RAW argument: a Sorcerer cannot meet the requirement of making 2 melee attacks with their Action, as would be required for twinning BB. From Booming Blade: "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." Thus, if a character cannot make that melee attack with a weapon for whatever reason, the spell fails. Since characters cannot make 2 weapon attacks with an Action, they cannot meet the requirement of Twinning BB. They would need to be able to hit 2 targets with 1 melee attack to meet that requirement- which is all DM fiat.
I would argue that without the Extra Attack feature a Sorcerer cannot meet the requirements of making a second melee attack to twin the spell- although by RAW, you can only use the Extra Attack feature when using the Attack Action, not the Cast a Spell Action. You might want to argue that this logic also applies to touch spells, but touch spells don't include the additional effects of the melee strike. You can't combine monk unarmed attacks and touch spells.
I would rate this as a medium to strong exploit depending on how rests are done and how grouped up foes tend to be on the battlefield. This allows certain builds to outshine other martial builds at the table, which is why I'd rate it as an exploit. Yes, it is limited by Sorcery Points and the two target restriction, so it's not outrageous. Consider a Pal 2 / Sorc X with a great sword and great weapon fighting style. I would argue that by 11th, a Pal 2 / Sorc 9 would would outshine similar power builds of 11th level that don't exploit BB. The 'Blue Dragon Knight' would be able to Twin and possibly Quicken BB for three attacks (on 2 targets) each at 2d6+Str +2d8 +3d8 BB rider +2d8 smite as needed. That's a lot of boom. In addition, they would have access to some nifty spells like Fireball, Haste, and Improved Invisibility. I would suggest allowing it to be Quickened but not Twinned, as this uses a bonus action for the extra attack and costs a more appropriate 2 Sorcery Points. This brings it in line with the War Priest feature from cleric.
Your "RAW argument" doesn't apply. The sorcerer is not taking the Attack action. They are taking the "cast a spell" action and then twinning the spell since the spell is eligible. Resolution of the spell requires a melee attack against an target within the range of the cantrip. There are no constraints based on the number of attacks the sorcerer could have taken when using the Attack action since they are NOT taking the Attack action.
It works even better if you have spell sniper and use a weapon with a 10' reach :) (whip for example if dex based or polearm if str based or either if cha based).
Personally, I don't consider it an exploit. An 11th level sorlock with hex up could do up to 6d10 + 6d6 + 30 using quickened agonizing-eldritch blast without using any spell resources and just two sorcery points. The pal2/sorcerer9 with a greatsword using twinned and quickened BB does 6d6 + 6d8 + 15 ... divided between two adjacent targets and requires 3 SP ... there is an additional 3d8 of bb rider damage on each target if they decide to move (note that booming blade rider damage does not stack since the spell effects don't stack) ... and of course, like any paladin, they can use smite. A level 11 PAM paladin can attack for 3d10+3d8+15 against a single target. If they are a vengeance paladin with hunters mark up this goes to a potential 3d10+3d8+3d6+15 + smites ... GWM adds 30 damage but increases the chance of missing.
Bottom line is that although booming blade is a good option ... there is no way I can see it being called so good as to be an "exploit" ... and although it is a bit better than some of the other melee options, there are others that are competitive.
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 it becomes sheathed in booming energy until the start of your next turn. If the target willingly moves before then, it immediately takes 1d8 thunder damage, and the spell ends.
Twinned Spell:
When you cast a spell that targets only one creature and doesn’t have a range of self, you can spend a number of sorcery points equal to the spell’s level to target a second creature in range with the same spell (1 sorcery point if the spell is a cantrip).
Twinned spell allows you to select another target with a spell already cast. What it doesn't do is instantly have you cast the spell a second time. Since the melee weapon attack is "part of the action used to cast this spell", twinned spell would not let you make another melee weapon attack. By RAW, the effect on the target (on hit) is that the target "suffers the attack's normal effects, and (move and boom)". So I'd say by RAW twinning a booming blade that hits would double the melee attack damage as well as the thunder damage, but you wouldn't get to make another attack.
In terms of how the spell actually works, I envision a caster doing some magic hocus pocus for a few seconds to "prep" their weapon, and then making an attack with their magically charged weapon. Twinned spell would double the charge, so the weapon would just deal normal slashing/bludgeoning/piercing damage but if the target moves it will take 2d8 instead of 1d8 thunder damage.
It works, but you must target two separate enemies, both of them in melee range - never the same enemy. If the enemies aren't killed on the initial strike (which is barely half the damage potential of Booming Blade) then you either need to extricate yourself from that situation (suffering the likely two opportunity attacks) or just stand there. If you are just standing there then the two enemies have no need to move and can just wail on on you in melee - melee attacks against a spellcaster are the dream of most random henchmen. It's a potent combo to be sure, but no all-powerful silver bullet.
Why couldn't the Twinned BB target two eligible foes with one melee attack? I smack this guy, the guy next to him feels it too. It's magic.
unfortunately, it's because that's what the rule explicity states: When you cast a spell that targets only one creature and doesn’t have a range of self, you can spend a number of sorcery points equal to the spell’s level to target a second creature in range with the same spell (1 sorcery point if the spell is a cantrip).
basically, you are casting the spell once, and hitting two targets when you do so.
That's why you use quicken when you want to hit the same thing twice; you are casting the same spell twice so you can target the same creature with both attacks.
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Reading the Sage Advice about Counterspell being meant to cancel the weapon attack as well as the spell damage, the intent seems to be that both are part of the spell effect.
It's hard to draw the line between RAW and RAI that way, but I don't see any game breaking exploit here. If there are two targets within range of Booming Blade, and you twin it, the effect of the spell should be applied to both.
I also don't see a problem in a Sorcerer making two weapon attack rolls in that case. When you are twinning a spell with a spell attack roll, you also roll twice for the attack.
Further, if a Fighter with access to Booming Blade casts that spell, they are excluded from using their Extra Attack feature. Consequently, if a Sorcerer is expending resources to twin the spell, it should be applied accordingly.
Booming Blade has a range of self. Twinned Spell doesn't work on spells with a range of self. Unfortunately this means you can't get two attacks in using it.
The biggest bummer is that this spell goes to so many classes that get Extra Attack, and so it can essentially become a situational spell as you can't Cast a Spell twice.
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Booming Blade has a range of self. Twinned Spell doesn't work on spells with a range of self. Unfortunately this means you can't get two attacks in using it.
The biggest bummer is that this spell goes to so many classes that get Extra Attack, and so it can essentially become a situational spell as you can't Cast a Spell twice.
No, booming blade and green-flame blade do not have a range of self. They originate from the self, meaning the spellcaster serves as the point of origin for these spells. In the case of the former, it only has one potential target. But the latter spell, potentially, has two targets. This was the problem with these spells before when their range was incorrectly labeled as "5 ft." One could still be twinned, or combined with War Caster, while the other couldn't. They were imbalanced.
That said, the word "Self" does still appear in the description. That, alone, might be enough to stop twinning for the both of them and make them consistent with each another.
Booming Blade has a range of self. Twinned Spell doesn't work on spells with a range of self. Unfortunately this means you can't get two attacks in using it.
The biggest bummer is that this spell goes to so many classes that get Extra Attack, and so it can essentially become a situational spell as you can't Cast a Spell twice.
No, booming blade and green-flame blade do not have a range of self. They originate from the self, meaning the spellcaster serves as the point of origin for these spells. In the case of the former, it only has one potential target. But the latter spell, potentially, has two targets. This was the problem with these spells before when their range was incorrectly labeled as "5 ft." One could still be twinned, or combined with War Caster, while the other couldn't. They were imbalanced.
That said, the word "Self" does still appear in the description. That, alone, might be enough to stop twinning for the both of them and make them consistent with each another.
Since the errata, if you read the stats for both, they each say Range/Area: Self 5ft. If you look at it on the mobile version, it adds a "/" in there, leading it to say Range/Area: Self/5ft. Why this doesn't show in the desktop version is beyond me, but the range is clearly self. The area is 5 ft, meaning if they move 5 ft the bonus damage activates. If we don't read it that way, then both Booming Blade and Green-Flame Bladearen't to be read consistently with all the other spells.
Of course, like you said, it wasn't always like this. It wasn't until an errata change that the range got put to self. I do find it odd in the description however that it doesn't allow for reach weapons, as it says "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."
As far as green-flame blade is concerned, the fact that it can have two potential targets could disqualify it anyway if it didn't already have an explicit range of self, but that depends on if you either take Jeremy Crawford's tweets as law or come to that conclusion yourself.
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Booming Blade has a range of self. Twinned Spell doesn't work on spells with a range of self. Unfortunately this means you can't get two attacks in using it.
The biggest bummer is that this spell goes to so many classes that get Extra Attack, and so it can essentially become a situational spell as you can't Cast a Spell twice.
No, booming blade and green-flame blade do not have a range of self. They originate from the self, meaning the spellcaster serves as the point of origin for these spells. In the case of the former, it only has one potential target. But the latter spell, potentially, has two targets. This was the problem with these spells before when their range was incorrectly labeled as "5 ft." One could still be twinned, or combined with War Caster, while the other couldn't. They were imbalanced.
That said, the word "Self" does still appear in the description. That, alone, might be enough to stop twinning for the both of them and make them consistent with each another.
Since the errata, if you read the stats for both, they each say Range/Area: Self 5ft. If you look at it on the mobile version, it adds a "/" in there, leading it to say Range/Area: Self/5ft. Why this doesn't show in the desktop version is beyond me, but the range is clearly self. The area is 5 ft, meaning if they move 5 ft the bonus damage activates. If we don't read it that way, then both Booming Blade and Green-Flame Bladearen't to be read consistently with all the other spells.
Of course, like you said, it wasn't always like this. It wasn't until an errata change that the range got put to self. I do find it odd in the description however that it doesn't allow for reach weapons, as it says "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."
As far as green-flame blade is concerned, the fact that it can have two potential targets could disqualify it anyway if it didn't already have an explicit range of self, but that depends on if you either take Jeremy Crawford's tweets as law or come to that conclusion yourself.
The range isn't "Self" in the same way it is for, say, expeditious retreat or shield. The printed book (and preceding errata for SCAG) lists the range as "Self (5-foot radius)". This means the spell originates from the spellcaster, who serves as the point of origin. The notation is formatted the same as, say, conjure barrage or thunderwave. The radius is so you know only targets within 5-feet of the spellcaster can be targeted by the spell. The bonus damage for booming blade always triggered as soon as the target willingly moved 5 feet or more.
The spells were subjected to errata both to more accurately reflect how they work and to bring them in line with each other. If a DM wishes to disallow them to be twinned, that's their prerogative. But the spells themselves (a) can be cast with only one target in mind and (b) do not actually have a range of self.
So I just realized that I've been looking at thunderwave wrong this whole time. I was thinking it was centered on you, not you being on the side.
That being said, the word "Self" is still in the range, even if there's a stipulation. Why would they make an errata change to Self (5 ft radius) instead of 5 ft? I can see your argument that the spell stretches out beyond the self, and is therefore able to be Twinned if the DM allows it, but I think part of the reason for the change is, like you said, to make BB and GFB able to be affected by the same features. If GFB can't be Twinned, neither can BB, and I think changing the range tried to cement that idea.
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Thank you for that link, that's actually very helpful!
I want to point out that they discuss booming blade and green-flame blade in particular, with emphasis on booming blade and metamagic at around 12:00 in that video you shared. Jeremy Crawford says that the errata change now "makes [Booming Blade] ineligible for Twinned" Spell as it now has a range of self, and that was part of their intention of changing it to self (5 ft radius).
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I saw questions on several other websites as to how this combo works, and saw several answers. My take, as a player, generally leans towards doing as much as possible with it. So the way I see it, I cast Booming Blade, spend a sorcery point to Twin it, and now I have melee attacks against 2 targets. If 1 or both hit, the recipient is now subject to the potential Movement damage that comes with the 2nd half of Booming Blade.
This cantrip seems to fulfill the requirement of Twinned Spell in that it only targets 1 creature. Or the way I saw Jeremy Crawford somewhat reword this, it only affects 1 creature. I add this part only to head off responses that go something like, "Booming Blade doesn't say the spell targets a creature, it just necessitates a melee attack that targets a creature."
If the answer to this is yes, then I can make my next thread in Tips and Tactics pondering the merits of multiclassing to Sorcerer (from EK). Please don't get lost on this tangent lol
At first I wanted to say you should need to have the Extra Attack feature, two weapon fighting or similar to take advantage, but as I thought about it - the action you are taking that round is casting the spell. As part of that action, the spell necessitates you make a melee attack - this attack is part of casting the spell, it is not an action, bonus action, or reaction in and of itself. Therefore, despite some confusing wording, I think the attack and the sheath of booming energy (and extra damage at higher levels) all happens against 2 separate targets when twinned.
Also there is this sage advice, which is about Green flame blade being counterspelled and what happens to the attack - Crawford's response seems 100% applicable to this situation as well. https://www.sageadvice.eu/2016/05/22/if-my-green-flame-blade-is-counterspelled-do-i-still-get-to-make-a-normal-melee-attack/
I remember reading this debate on another forum, and the answer is: You are able to twin the spell, you do hit 2 creatures with it.
While I'm not a fan of the "Crawford says" resolution to debates, in this case I must deign to iterate those words...
Yep, it clearly specifies the creature against which a weapon attack is made as being "the target", it specifies "one creature", its range is 5', not self, thus it meets all requirements for twinning the spell. Honestly I'm not sure what debate there would be. The melee attack is part of the action which is casting the spell, the spell being twinned would twin the melee attack. Unusual, but specific trumps general.
My issue with allowing BB to be twinned is that it allows for sorcerers to gain an additional melee attack where they wouldn't normally get one. With a regular Sorcerer using a dagger or staff it's probably not an issue, but once you add a strong melee attack (say multiclass Paladin 2), it becomes significantly more powerful.
The RAW argument: a Sorcerer cannot meet the requirement of making 2 melee attacks with their Action, as would be required for twinning BB. From Booming Blade: "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." Thus, if a character cannot make that melee attack with a weapon for whatever reason, the spell fails. Since characters cannot make 2 weapon attacks with an Action, they cannot meet the requirement of Twinning BB. They would need to be able to hit 2 targets with 1 melee attack to meet that requirement- which is all DM fiat.
I would argue that without the Extra Attack feature a Sorcerer cannot meet the requirements of making a second melee attack to twin the spell- although by RAW, you can only use the Extra Attack feature when using the Attack Action, not the Cast a Spell Action. You might want to argue that this logic also applies to touch spells, but touch spells don't include the additional effects of the melee strike. You can't combine monk unarmed attacks and touch spells.
I would rate this as a medium to strong exploit depending on how rests are done and how grouped up foes tend to be on the battlefield. This allows certain builds to outshine other martial builds at the table, which is why I'd rate it as an exploit. Yes, it is limited by Sorcery Points and the two target restriction, so it's not outrageous. Consider a Pal 2 / Sorc X with a great sword and great weapon fighting style. I would argue that by 11th, a Pal 2 / Sorc 9 would would outshine similar power builds of 11th level that don't exploit BB. The 'Blue Dragon Knight' would be able to Twin and possibly Quicken BB for three attacks (on 2 targets) each at 2d6+Str +2d8 +3d8 BB rider +2d8 smite as needed. That's a lot of boom. In addition, they would have access to some nifty spells like Fireball, Haste, and Improved Invisibility. I would suggest allowing it to be Quickened but not Twinned, as this uses a bonus action for the extra attack and costs a more appropriate 2 Sorcery Points. This brings it in line with the War Priest feature from cleric.
Your "RAW argument" doesn't apply. The sorcerer is not taking the Attack action. They are taking the "cast a spell" action and then twinning the spell since the spell is eligible. Resolution of the spell requires a melee attack against an target within the range of the cantrip. There are no constraints based on the number of attacks the sorcerer could have taken when using the Attack action since they are NOT taking the Attack action.
It works even better if you have spell sniper and use a weapon with a 10' reach :) (whip for example if dex based or polearm if str based or either if cha based).
Personally, I don't consider it an exploit. An 11th level sorlock with hex up could do up to 6d10 + 6d6 + 30 using quickened agonizing-eldritch blast without using any spell resources and just two sorcery points. The pal2/sorcerer9 with a greatsword using twinned and quickened BB does 6d6 + 6d8 + 15 ... divided between two adjacent targets and requires 3 SP ... there is an additional 3d8 of bb rider damage on each target if they decide to move (note that booming blade rider damage does not stack since the spell effects don't stack) ... and of course, like any paladin, they can use smite. A level 11 PAM paladin can attack for 3d10+3d8+15 against a single target. If they are a vengeance paladin with hunters mark up this goes to a potential 3d10+3d8+3d6+15 + smites ... GWM adds 30 damage but increases the chance of missing.
Bottom line is that although booming blade is a good option ... there is no way I can see it being called so good as to be an "exploit" ... and although it is a bit better than some of the other melee options, there are others that are competitive.
Booming Blade:
Twinned Spell:
Twinned spell allows you to select another target with a spell already cast. What it doesn't do is instantly have you cast the spell a second time. Since the melee weapon attack is "part of the action used to cast this spell", twinned spell would not let you make another melee weapon attack. By RAW, the effect on the target (on hit) is that the target "suffers the attack's normal effects, and (move and boom)". So I'd say by RAW twinning a booming blade that hits would double the melee attack damage as well as the thunder damage, but you wouldn't get to make another attack.
In terms of how the spell actually works, I envision a caster doing some magic hocus pocus for a few seconds to "prep" their weapon, and then making an attack with their magically charged weapon. Twinned spell would double the charge, so the weapon would just deal normal slashing/bludgeoning/piercing damage but if the target moves it will take 2d8 instead of 1d8 thunder damage.
It works, but you must target two separate enemies, both of them in melee range - never the same enemy. If the enemies aren't killed on the initial strike (which is barely half the damage potential of Booming Blade) then you either need to extricate yourself from that situation (suffering the likely two opportunity attacks) or just stand there. If you are just standing there then the two enemies have no need to move and can just wail on on you in melee - melee attacks against a spellcaster are the dream of most random henchmen. It's a potent combo to be sure, but no all-powerful silver bullet.
Why couldn't the Twinned BB target two eligible foes with one melee attack? I smack this guy, the guy next to him feels it too. It's magic.
I'd allow it in my game, the player is finding creative ways to use their resources without breaking anything.
I would rule that the spell does not target a creature. Not all metamagics work with all spells.
unfortunately, it's because that's what the rule explicity states: When you cast a spell that targets only one creature and doesn’t have a range of self, you can spend a number of sorcery points equal to the spell’s level to target a second creature in range with the same spell (1 sorcery point if the spell is a cantrip).
basically, you are casting the spell once, and hitting two targets when you do so.
That's why you use quicken when you want to hit the same thing twice; you are casting the same spell twice so you can target the same creature with both attacks.
Any time an unfathomably powerful entity sweeps in and offers godlike rewards in return for just a few teensy favors, it’s a scam. Unless it’s me. I’d never lie to you, reader dearest.
Tasha
Very interesting situation.
Reading the Sage Advice about Counterspell being meant to cancel the weapon attack as well as the spell damage, the intent seems to be that both are part of the spell effect.
It's hard to draw the line between RAW and RAI that way, but I don't see any game breaking exploit here. If there are two targets within range of Booming Blade, and you twin it, the effect of the spell should be applied to both.
I also don't see a problem in a Sorcerer making two weapon attack rolls in that case. When you are twinning a spell with a spell attack roll, you also roll twice for the attack.
Further, if a Fighter with access to Booming Blade casts that spell, they are excluded from using their Extra Attack feature. Consequently, if a Sorcerer is expending resources to twin the spell, it should be applied accordingly.
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Booming Blade has a range of self. Twinned Spell doesn't work on spells with a range of self. Unfortunately this means you can't get two attacks in using it.
The biggest bummer is that this spell goes to so many classes that get Extra Attack, and so it can essentially become a situational spell as you can't Cast a Spell twice.
Reality is more amazing than we are often led to believe.
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No, booming blade and green-flame blade do not have a range of self. They originate from the self, meaning the spellcaster serves as the point of origin for these spells. In the case of the former, it only has one potential target. But the latter spell, potentially, has two targets. This was the problem with these spells before when their range was incorrectly labeled as "5 ft." One could still be twinned, or combined with War Caster, while the other couldn't. They were imbalanced.
That said, the word "Self" does still appear in the description. That, alone, might be enough to stop twinning for the both of them and make them consistent with each another.
Since the errata, if you read the stats for both, they each say Range/Area: Self 5ft. If you look at it on the mobile version, it adds a "/" in there, leading it to say Range/Area: Self/5ft. Why this doesn't show in the desktop version is beyond me, but the range is clearly self. The area is 5 ft, meaning if they move 5 ft the bonus damage activates. If we don't read it that way, then both Booming Blade and Green-Flame Blade aren't to be read consistently with all the other spells.
Of course, like you said, it wasn't always like this. It wasn't until an errata change that the range got put to self. I do find it odd in the description however that it doesn't allow for reach weapons, as it says "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."
As far as green-flame blade is concerned, the fact that it can have two potential targets could disqualify it anyway if it didn't already have an explicit range of self, but that depends on if you either take Jeremy Crawford's tweets as law or come to that conclusion yourself.
Reality is more amazing than we are often led to believe.
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The range isn't "Self" in the same way it is for, say, expeditious retreat or shield. The printed book (and preceding errata for SCAG) lists the range as "Self (5-foot radius)". This means the spell originates from the spellcaster, who serves as the point of origin. The notation is formatted the same as, say, conjure barrage or thunderwave. The radius is so you know only targets within 5-feet of the spellcaster can be targeted by the spell. The bonus damage for booming blade always triggered as soon as the target willingly moved 5 feet or more.
The spells were subjected to errata both to more accurately reflect how they work and to bring them in line with each other. If a DM wishes to disallow them to be twinned, that's their prerogative. But the spells themselves (a) can be cast with only one target in mind and (b) do not actually have a range of self.
So I just realized that I've been looking at thunderwave wrong this whole time. I was thinking it was centered on you, not you being on the side.
That being said, the word "Self" is still in the range, even if there's a stipulation. Why would they make an errata change to Self (5 ft radius) instead of 5 ft? I can see your argument that the spell stretches out beyond the self, and is therefore able to be Twinned if the DM allows it, but I think part of the reason for the change is, like you said, to make BB and GFB able to be affected by the same features. If GFB can't be Twinned, neither can BB, and I think changing the range tried to cement that idea.
Reality is more amazing than we are often led to believe.
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Crawford's given an interview on why they changed the spell descriptions
https://www.youtube.com/watch?v=xUOaQ_XY7wE&ab_channel=Dungeons&Dragons.
Thank you for that link, that's actually very helpful!
I want to point out that they discuss booming blade and green-flame blade in particular, with emphasis on booming blade and metamagic at around 12:00 in that video you shared. Jeremy Crawford says that the errata change now "makes [Booming Blade] ineligible for Twinned" Spell as it now has a range of self, and that was part of their intention of changing it to self (5 ft radius).
Reality is more amazing than we are often led to believe.
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