Booming Blade says:
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.
Counterspell reads:
You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect.
Does Counterspell interrupt the attack made in conjunction with casting Booming Blade?
Counterspell says:
its spell fails and has no effect.
The effect of booming blade is exactly its spell description:
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 then becomes sheathed in booming energy until the start of your next turn. If the target willingly moves 5 feet or more before then, the target takes 1d8 thunder damage, and the spell ends.
The entire description of booming blade is the spell's effect. This is stated explicitly in the rules for spellcasting:
Each spell description in Chapter 11 begins with a block of information, including the spell's name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell's effect.
Therefore, if booming blade is counterspelled, it has no effect, meaning everything in the spell description does not happen, since the spell description is that effect. The effect of a successfully counterspelled booming blade is:
There is no effect (quote block intentionally left blank).
Originally, the spell description said:
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.
This was changed to read:
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.
Many of the answers the argued against my interpretation here relied on the phrase from the original description, "as part of the action used to cast this spell". This is no longer in the spell description.
Now, the spell states that the weapon is used in the spell's casting, and then you make the melee attack. This definitively designates the melee attack as an effect of the spell, which counterspell nullifies.
Each spell description in Chapter 11 begins with a block of information, including the spell's name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell's effect.
I will use the standard form of propositional logic to pick the last sentence apart. Predicate logic shouldn't be used because RAW predicates nothing in the last sentence. Logicians enjoy, the terms I use and my rhetoric are for you.
The rest of the spell entry describes the spell's effect; or, if it is the rest of the spell entry, then it describes the spell's effect. The rest of the spell entry is a sufficient condition, the rest of the spell entry sufficiently describes the spell's effect. The spell's effect is a necessary condition, the spells effect is necessarily described by the rest of the spell entry. Being described by the rest of the spell entry does not sufficiently mean or imply that it is the spell's effect. It is left open whether things that are not the spell's effect can also be described within "the rest of the spell entry."
This means that while the rest of the spell entry describes the spell's effect, it doesn't necessarily disinclude the description of any other thing or occurrence. This is what concretes the argument Peter Cordez made in his answer, and a pillar which he used, prior to editing it (he just didn't use philosophy of logic rhetoric). Unfortunately he edited his answer with consideration to the errata when the errata doesn't say anything conclusively different in this regard.
This means that regarding RAW, there is absolutely no say whether any specific part contained within "the rest of a spell entry" is describing the spell's effect. It is only conclusive that the spell's effect is contained in that portion of the spell entry.
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.
or the errata version:
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.
The text in booming blade is open ended too, it says that the attack is made as part of the same action used to cast the spell, or in the errata, you brandish and make a melee attack with the weapon you used to cast the spell, but it does not suggest that the attack is made as part of the spell. It also does not say that the attack is not part of the spell. If one were to use the same standard form of (propositional) logic they would find that booming blade makes no conclusion to whether the attack made is part of the spell, or spell's effect, or not.
You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect.
Counterspell causes a spell to fail and have no effect. But neither the rules for spellcasting, nor the description of booming blade, make any conclusion whether the attack made as part of casting booming blade is considered to be a part of the spell or its effect. Therefore, whether counterspell stops the attack from going through or not has no RAW ruling.
An answer is most complete when in includes as much relevant material as possible. Although Jeremy Crawford's rulings are no longer considered official rulings, they were considered as such at one point. His rulings are quite relevant. Here is JC's tweet.
JC ruled that the attack is part of the spell's effect.
See https://media.wizards.com/2020/dnd/downloads/SCAG-Errata.pdf - the "as part of the action used to cast this spell" wording is gone, and with it the main justification for treating the attack as a component, not an effect of the magic (despite where it was placed in the spell description).
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 then becomes sheathed in booming energy until the start of your next turn. . If the target willingly moves 5 feet or more before then, the target takes 1d8 thunder damage, and the spell ends.
... extra damage with levels
(And same first sentence for Green Flame Blade.)
(Not "within range"? So Spell Sniper won't let you benefit from a Reach weapon with it? Notice that range is now "self" (5ft radius), so clearly another intentional change.)
The fact that an erratum specifically changed "as part of the action..." to this more standard phrasing lends additional weight to the interpretation that the design intent is for Counterspell to cancel all of it, including the attack. Even moreso than if it had been phrased this way the entire time.
It now reads more like the attack itself is a result of the magic. Or happens after the casting (weapon used, past tense), not "as part of", in a way that the caster for some reason wouldn't carry through if the magic were disrupted, apparently... You could still make the argument that the attack itself isn't magic and that you'd still do it even if the magic were disrupted, but this is a significantly weaker argument.
The attack's regular effect doesn't include making the weapon or the damage count as magical for overcoming resistance or anything like that, so it's easy to argue the attack still isn't powered by the magic, for whatever that's worth. It's still a delivery mechanism for the magic. But it does look like we're meant to interpret it as an effect.
This section is now obsolete, and was written based on the older wording, which provided most of the justification for treating this as an exception to the usual norm of the description text being all effect:
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. ...
You are of course free to choose how these cantrips work at your table, but the "attack still happens" ruling is much harder to justify as RAW, post 2020. The rest of this answer was written before I was aware of the errata to Green Flame / Booming Blade, based on the original wording (which I think is neat, and an interesting difference from the way other spells work, but unfortunately isn't how the designers wanted them to.)
If you think disrupting the magic shouldn't stop a sword swing from still maybe connecting and hurting, this is how to explain that logic and explain how that's consistent with the rules. The other interpretation is also somewhat compatible with RAW, if you want to interpret words in very specific ways that seem unlikely to have been fully intended, given the phrasing of the cantrips themselves.
The RAW justification for ruling that Counterspell does stop the melee weapon attack from happening (e.g. @ThomasMarkov's answer) is based on interpreting the PHB wording as saying that everything in the spell entry after the headers is all "effects", in the sense that everything in that text is things that only happen if the magic is successful.
Each spell description in Chapter 11 begins with a block of information, including the spell's name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell's effect. The rest of a spell entry describes the spell's effect.
That interpretation doesn't work even for some PHB spells like Burning Hands, where the text describes a somatic component. "As you hold your hands with thumbs touching and fingers spread, ..." Or Magic Stone "You touch one to three pebbles and imbue them with magic". Would CS stop you from even touching some pebbles? Or Vicious Mockery: "You unleash a string of insults laced with subtle enchantments at a creature you can see within range." Would CS shut you up so that not only is the save vs. psychic damage not needed, nobody even hears your insults?
Does "The rest of a spell entry describes the spell's effect" mean that every word of it is effects? No. A describes B doesn't necessarily mean that A contains nothing else. That choice of wording ("effect") is telling you that's where you should look to find out what the spell does, not trying to tell you there's nothing else in that part of a spell entry. It's also part of a summary of the layout, not a taxonomic classification into headers vs. effects.
So RAW, it's very possible to argue that the body of a spell entry isn't strictly limited to the (magical) spell effects that Counterspell would stop. Other effects of the process of attempting to cast the spell still happen.
Counterspell: You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect
The rules are written in common English, with only some words being given specific technical meanings, like "action", "attack", "saving throw", etc. "Effect" / "effects" doesn't seem to be in that category, or if it is, it can also be used with its plain English meaning in other contexts.
"Effect" in this context should be understood as magical effect. I don't have any sources to back up this claim, but CS doesn't freeze your arms in mid-swing to stop you from making the touch part of an attack, or in this case the weapon-attack part of the casting process. The spell description even says you make that as a separate part of the Action you take to cast this spell.
If you do take the view that the entire description text is just "effect", in the same sense that CS is using the word, then you could say that ScAG presents these cantrips in a sloppy way, with non-effects (like the weapon attack as part of the same Action) in the "effect" section. That should have been listed as a component or with the casting time.
Additionally, as @illustro's answer points out, You attempt to interrupt a creature in the process of casting a spell can be read as saying you try to make something happen during the process of someone casting a spell. This will disrupt the magic but not freeze their hands in place or anything.
Booming Blade and GF Blade aren't listed as having a Somatic component1 because apparently there aren't any fiddly hand gestures, just attacking with the weapon. But as the first thing in the description, in the same place we find some kind of gesture descriptions in other spells (Burning Hands hand position, or touching the target for many spells), we find that you make an attack as part of the process of casting the spell. (Not as a result of the magic taking effect).
If you were counterspelled while attempting to deliver a touch spell, you'd normally still end up touching the target.
There's no indication that this weapon attack is a result of the magic working, just a way to channel the magic into the target.
These ScAG cantrips are half melee, half magic. It seems normal that some of the damage can't be prevented with CS; casting one is just saying some magic words before / during making a melee weapon attack. (This is pure narrative interpretation on my part; that's the picture the text paints for me.)
You could argue that having the magic disrupted by CS will distract you from the melee attack so much that it automatically misses, if you want it to work that way instead. (In the unlikely even that someone spends a spell slot on CS on a cantrip, although perhaps they didn't know what spell you were casting.) So it's plausible to have a narrative explanation for how CS would stop the attack from working, other than having the attack powered by the magic (which isn't supported by the spell text).
Footnote 1: Fun fact: Listing the spell as having a Somatic component wouldn't require a free hand here, even without War Caster: the hand used for Somatic components can be the same hand that accesses a material component or focus. In this case, the weapon is the component, so that allows it to be in your hand the entire time you cast the spell, even if it did have a somatic component. Which they don't.
Not listing a Somatic component avoids the need for following this chain of rules logic to reach the conclusion that one can use it with a 2h weapon or sword+board, which is an advantage for players.
CSing one of these cantrips is probably such a rare thing that it has nearly no impact on overall balance of the cantrips, or the classes using them.
The cantrips are situational (less good against an unmoving or solo targets respectively) and not overpowered for most characters (AFAIK?). Having the minor benefit of only being partially stoppable with CS is not unreasonable and doesn't make them too strong.
Balance-wise, spending an Action on one of these cantrips means (at higher levels) you're giving up an Extra Attack. The ScAG cantrips are only really good for characters that are good at normal melee attacks and can cast spells.
Just making that happen already means you're probably not a pure caster class, so you already made compromises / tradeoffs / paid opportunity costs, like multi-classing, taking a feat to get the cantrip, or being an Eldritch Knight2. And of course choosing one of these as one of your cantrips, perhaps giving up Prestidigitation, Mending, or even Fire Bolt.
Ruling this way means that CS is very much not a good defence against them. But even if you rule that it does stop the attack, CS is not a great choice because the resource cost (a 3rd lvl spell slot) is relatively precious compared to the damage prevented, in most situations.
Footnote 2: Nothing against pure Eldritch Knights, just it means you had to choose that instead of other subclasses. They're pretty good. EKs specifically (at lvl 7) get to make a bonus-action weapon attack after using an action on a cantrip, so these cantrips are especially great for EKs from level 7 (war magic) up. (Extra attack x2 at 11th level is the same time you get an extra 1d8 on both damage rolls, so Attack vs. cantrip depends on your weapon expected damage vs. moving or two targets, and against high AC whether more chances for smaller hits are better than one big one...
In November 2020, a Sword Coast Adventurer's Guide Errata was released, changing the spell text in question significantly.
The relevant spell rules section, post-errata, now reads:
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. [...]
This changes the order of operations of the spell from:
to:
As a result of this significant revision to the wording of the spell description rules, Counterspell now, definitively, does prevent the melee attack from resolving.
There are no absolute rules in D&D 5e, beyond the Specific vs General Rule (and Rule 0)
If a specific rules contradicts a general rule, the specific rules wins (PHB > How to Play)
The general rule for spellcasting is described in the Spellcasting section of the PHB (emphasis mine)
Each spell description in Chapter 11 begins with a block of information, including the spell's name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell's effect.
Casting Time, in particular, is described in the PHB as:
Most spells require a single action to cast, but some spells require a bonus action, a reaction, or much more time to cast.
The Booming Blade introduces an exception to the general spellcasting rules, by including a rule in the spell description, to modify the spell casting procedure.
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.
The casting procedure, under normal circumstances, is one of:
Booming Blade modifies this, for this spell, to be:
Counterspell, says:
You attempt to interrupt a creature in the process of casting a spell [...], its spell fails and has no effect.
So, the effect of Counterspell is an attempt to insert magic during the casting of the spell to make it so the spell effects are nullified. Importantly, it does not prevent the resolution of the casting action (ie the spell slot of the original creature, if they used a spell slot, is still wasted, and they have still used up their action), simply, that when the casting action is resolved, the spell has no effect.
Specific vs General requires us to determine which is the more specific rule, in order to determine precedence. In this case, it's relatively straightforward for two reasons:
As Booming Blade's rule for the casting modification is more specific in both circumstances, by the specific vs general rule, it takes precedence.
Thus, the Booming Blade action completes, that is, the spell action is completed, including the melee attack, but if the hit is successful, none of the other effects of the spell resolve, and the defender is only hit by a melee attack.
It's also important to note, that since this is a melee attack, it is also a relevant trigger for other reaction spells like shield.
No, interrupt actually has a few different meanings depending on it's use (specifically on whether it is being used as a transitive verb or intransitive verb).
One meaning of the word interrupt is preventative:
The meaning here is that the main action, the burglary was prevented from being completed by the interruption. This is meaning is also common as a result of computer programming.
The other meaning is a modificative:
In this case, the action (the presentation of a comedy act) is not prevented from completing, but modified, specifically the inclusion of an unanticipated joke exchange between the audience and the performer. The action completes and the performer receives their dues.
When reading counterspell, we need to keep both of these interpretations in mind, as both are valid interpretations of what counterspell is doing. In particular because counterspell doesn't say the casting of the target creature is halted, but that the spell they cast fails and has no effect.
Since spells only do what they say they do, both of these valid interpretations are possible.
Since, by specific vs general, the melee attack is explicitly part of the casting and not part of the effect, the DM is free to use either interpretation. My ruling, as a DM, is that the melee attack resolves, but on a hit, the actual effect of Booming Blade (the target becoming "sheathed in booming energy", the resulting damage on movement).
This is made even more explicit by this line from Booming Blade's rules modifying the casting action:
On a hit, the target suffer's the attack's normal effects [...]
Think about a character that has the Extra Attack feature, like an Eldritch Knight, or a Paladin who has this spell (either via multiclassing or a feat). In order to cast this spell they are giving up quite a lot:
From a balance perspective, for these classes, the effect of preventing the melee attack as well as the additional spell effects, is to make it so they have even less reason to use this spell.
Yes. Due to the two ways in which interrupt can be interpreted, Counterspell itself can be interpreted as actually preventing the casting action from being completed. If the action is magically prevented from being completed, then the attack never resolves and the melee attack is cancelled.
The SCAG errata for Booming Blade reports
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 then becomes sheathed in booming energy until the start of your next turn.
Hence, the attacks is an effect of the spell. If Counterspell (PHB, page 228) succeeds
the creature's spell fails and has no effect.
and then the attack does not take place.
The Booming Blade (SCAG, page 142) description says (emphasis mine)
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.
The PHB explicit says under the Spell Casting section that (emphasis mine)
Each spell description in Chapter 11 begins with a block of information, including the spell's name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell's effect.
The Booming Blade's description presents a sort of contradiction: indeed, per PHB's rules the quoted text describes the effects of the spell, hence the melee attacks is one of this effects. Nonetheless, this very text says that the melee attack is part of the casting.
On the other hand, Counterspell says (emphasis mine)
You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect.
Since Booming Blade is a cantrip, Counterspell automatically succeeds. The highlighted text in the above description tells us that, on success,
Both interpretations lead to say that a counterspelled Booming Blade has no effects at all, both the melee attack and the subsequent thunder damage do not take place.
Moreover, allowing the melee attack even if Booming Blade has been counterspelled is not balanced: indeed, you have the possibility to do an attack even if you already spent your action to cast a spell (invalidated by the enemy's Counterspell).
This reasoning applies also to Green Flame Blade (SCAG, page 143), since it shares the same text.
I seem to be in the camp that counterspell doesn't hold a person, bind their hands, create a barrier, or silence them. Rather it nullifies the spell component itself.
A spell like booming blade requires a successful melee attack in order to work. As much as it is said to be apart of the spell description it is not the spell effect. In this specific instance
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 then
becomes sheathed in booming energy until the start of your next turn. If the target willingly moves 5 feet or more before then, the target takes 1d8 thunder damage, and the spell ends.
Both as the Somatic and Material component, the caster is free to move. Counterspell doesn't prevent such an action. Even the verbal component the words may still come out, the caster isn't magically silenced, we have a spell specifically for that.
It would literally be as though the spell fizzles out, but none the actions leading up to it fizzling out.
if booming blade or green flame blade are counter-spelled, does the attack still go through?
if booming blade or green flame blade are counter-spelled,
You have discovered that your enemy is a fool. Feast upon them.
The economics of this transaction: your enemy traded a level 3 spell slot for a level 0 slot.
You just won that exchange.
You suckered the enemy into wasting a counterspell on a 0 level spell (cantrip) that you can cast again on the next round, for a 3d level spell slot that is a limited resource. And now, next round, the enemy usually can't counterspell the Hold Person that your cleric is going to throw, or some other spell from level 1 through 9, depending on your tier of play and your team's composition.
It is worth noting that the Aeorian Nullifier (Wildemount) has counterspell at will. I didn't find any others, but a DM could customize a monster to have that spell known for an NPC/Monster that has a 3d level casting slot. Most NPCs will use it with a spell slot (or in the "x / day" style).
@Kirt noted in a comment that ...
A powerful NPC in a certain official adventure has counterspell as a third level spell, can cast spells of up to third level at-will, and can use a legendary action to cast any of their at-will spells.
I discovered in reading through the D&D beyond references to counterspell that a variety of NPCs or Monsters have counterspell as a listed spell. The Evoker and Abjurer (VGtM), or the Mage and Arcanaloth (MM), or Githyanki Gish(MtoF) are straight up NPCs and monsters; most of the others are in published adventures so I'll keep them behind a spoiler. Note: any caster with a 3d level spell might have it subbed in for a listed one, if the DM customizes NPCs. (I customize spells known occasionally if I feel the generic NPCs' thematics are 'off' for the story).
Manshoon's Simulacrum, Rishaal, Violence, Animated Statue, Miraj Vizann, Bastian Thermandar, Evoker, Kaevja Cynavern, Marta Moonshadow, Azbara Jos, Vajra Safahr, Sylvira Savikas, Rath Modar, Darribeth Meltimer, Turbulence, Counterflux Blastseeker, Alhoon, Preeta Kreepa, Pow Ming, Shedrak of the Eyes, Left Hand of Manshoon, Berlain Shadowdusk, Biomancer, Manshoon, Skeemo Weirdbottle
