A quick question. A creature moved out of melee range causing an opportunity attack. Our rogue used Booming Blade with his short sword for the opportunity attack. The question, can the rogue use Booming Blade with this opportunity attack?
No, they can't, unless they have some special feature that allows them to cast a spell in place of an opportunity attack. The most common example of this is the War Caster feat.
In the absence of a feature like that, an opportunity attack allows you to make one melee attack or unarmed strike. It doesn't allow you to cast a spell (even one like Booming Blade that involves making an attack).
Can you use True Strike with Extra Attack, Opportunity Attack, Sneak Attack, and other weapon attack options?
True Strike doesn’t work with Extra Attack or any other feature that requires the Attack action. Like other spells with a casting time of an action, casting True Strike requires you to take the Magic action, not the Attack action. Similarly, unless a special feature allows you to do so, you can’t cast True Strike when making an Opportunity Attack.
However, an attack made as part of True Strike works with Sneak Attack so long as it fills the normal requirements for that feature. For example, if you have the Sneak Attack feature and cast True Strike with a Finesse weapon, you can deal Sneak Attack damage to the target of the attack if you have Advantage on the attack roll and hit.
War Caster doesn't allow for the casting of Booming Blade since the spell in question must be cast at the target creature and Booming Blade has a range of self.
War Caster doesn't allow for the casting of Booming Blade since the spell in question must be cast at the target creature and Booming Blade has a range of self.
This is neither clear from the rulebooks nor anything like universally agreed on. (And there's a very solid intent-of-the-rules argument that they should.)
The rulebooks are actually quite clear on this and of course when it comes to the RAW it really doesn't matter if people agree or not, it only matters what the text actually says.
Both the 2014 version and the 2024 version of the War Caster feat explicitly require that the spell is cast at the provoking creature. This is thematically consistent with the 5e concept of what an opportunity attack represents in the game which is when a creature lashes out at another creature in response to that other creature attempting to hastily retreat away from a melee combat engagement. As such, the wording appears to be extremely intentional.
The idea is that the War Caster creature is meant to directly target the provoking creature with a spell such as Fire Bolt (at disadvantage) or Shocking Grasp or Charm Person or Sacred Flame or Chill Touch or Chromatic Orb or even something like Witch Bolt, for example.
Something like the Read Thoughts effect of the Detect Thoughts spell (pretend for a moment that this is the spell's only possible effect) would not qualify even though the provoking creature is being "targeted" in the 5e Rules Glossary sense. The reason is that the spell is not being cast at that creature, it is being cast at the spellcaster. If you could somehow cast Scrying as an action this also would not qualify for similar reasons.
But probably the best two examples of spells from the 2024 PHB which are disqualified from this interaction for the same reason as Booming Blade would be True Strike and Vampiric Touch. In both of those cases, a single creature is being "targeted", but the spell is not being cast at that creature. Instead, those spells have a range of self, meaning, they are spells which create effects that buff the spellcaster in a particular way. Note that True Strike is actually a Divination spell -- the concept is that the spell mentally empowers the spellcaster while performing a specific activity.
War Caster is already powerful enough, especially when people begin trying to claim that these opportunity attacks can be made against their allies, which is a whole separate rabbit hole. There's no need to make it even more powerful by ignoring some of the restrictions that the text is providing.
War Caster doesn't allow for the casting of Booming Blade since the spell in question must be cast at the target creature and Booming Blade has a range of self.
War Caster doesn't allow for the casting of Booming Blade since the spell in question must be cast at the target creature and Booming Blade has a range of self.
It's unclear what that thread has to do with this one.
The concept of the Range of a spell is extremely straightforward in this game. What is it about that concept that you feel would need to be "explained"?
If an enemy creature is standing 90 feet away from the spellcaster and the spellcaster wants to cast a spell that has a Range of 30 feet, then by rule he cannot cast that spell at that enemy creature. Likewise, if that enemy creature is still standing 90 feet away from the spellcaster and the spellcaster now wants to cast a spell that has a Range of self, then by rule he still cannot cast that spell at that enemy creature.
This one is a slam dunk. There are not any other reasonable interpretations.
The concept of the Range of a spell is extremely straightforward in this game. What is it about that concept that you feel would need to be "explained"?
The concept of "Range" and the concept of "Target" are distinct, but you keep mixing the two. That is the essence of this argument. A range of self does not dictate a target of self.
"A spell’s range indicates how far from the spellcaster the spell’s effect can originate, and the spell’s description specifies which part of the effect is limited by the range." Some self spells do, indeed, target the caster, but not all.
True Strike / Booming Blade / Green-Flame Blade target the target of the attack, but all have a range of self, because the caster's self is the origin, not the target.
War Caster doesn't allow for the casting of Booming Blade since the spell in question must be cast at the target creature and Booming Blade has a range of self.
It's unclear what that thread has to do with this one.
The concept of the Range of a spell is extremely straightforward in this game. What is it about that concept that you feel would need to be "explained"?
It's pretty straightforward. Range "indicates how far from the spellcaster the spell's effect can originate", but it doesn't define the targets. Several people explained that in the thread. You can read their explanations there.
The only reference to see the spell I can find is a 2014 sword coast book.
What is booming blade?
It first appeared in SCAG and was updated in Tasha's Cauldron of Everything to fix the Range, Components, and wording.
SCAG:
Casting Time: 1 action Range: 5 feet Components: V, M (a weapon) Duration: 1 round
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. [...]
TCoE:
Casting Time: 1 action Range: Self (5-foot radius) Components: S, M (a melee weapon worth at least 1 sp) Duration: 1 round
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. [...]
Character77006 I shared this transcription some time ago. It includes the reasons for the changes. It also explains how Range and Self should be understood. (EDIT: for clarity)
You may or may not agree with Jeremy Crawford's explanation, but he clearly states the intent behind changing the spell's range to Self (5 ft radius), the importance of reading the spell entry to understand who the target is, and the discussion about the War Caster feat.
(emphasis mine)
timestamp: the reason why some of these spells have this self parentheses range is we are signalling to the reader this point of origin cannot move
timestamp: in Tasha's Cauldron of Everything we have some spells that actually appeared in an earlier DnD book (Sword Coast Adventurer's Guide) that now reappear and their range changed and that change has naturally caused some questions. Their ranges originally were that simple X ft meaning it could have the point of origin of the spell [...] over there. So these spells were spells like Booming Blade and Green-Flame Blade which when you then read them you see: I'm actually making a weapon attack with these spells and these were actually unusual spells when we wrote them because we had not yet created at that point spells that incorporated into themselves an attack. We had spells like the Paladin's smite spells in the PHB that enhanced attacks but did not include within them the weapon attack itself. Here we experimented with that and to be totally frank the original range entries were wrong[...]
timestamp: we were looking at the spells and realized these range entries are simply wrong because like Burning Hands like Color Spray like Lightning Bolt you swinging your weapon can never originate any farther away than yourself. Saying range X feet was simply an error and so we fixed it. So now they have a range of self with a parentheses 5 ft radius and then inside the spell you read: okay you brandish the weapon you used as a part of the spell casting and you target somebody within this radius [...]
timestamp: Question: How does this interact with metamagic specifically twinning of spells. Answer: This does make the spell ineligible for being twinned because a twin spell does not allow you to twin a spell with a range of Self. Because these ranges have the word Self in them they are not twinnable and that's by design. Honestly these never were meant to be twinnable because again their range was was actually always incorrect [...]
timestamp: people also have wondered how did these interact with War Caster. A feat that allows you to make an opportunity attack with a spell as long as you target only one thing with that opportunity attack. And so then the question is "Can you use booming blade with its new range of self parentheses to make that opportunity attack as defined by War Caster?". The answer is yes and the reason why it goes back to what I was saying about our rules on Range where you'll notice that as soon as we get to the Self parentheses part we don't talk about you targeting yourself because spells in this category you have to look at the spell to see what exactly are you targeting because all Self parentheses tells us some magic is extending out from me [and] we'll see who are, what it's targeting, and in the case of Booming Blade who or what's being targeted is the person you attack with it [...]
Who said anything about targeting? I'm talking about what the spell can be cast at, which is restricted by the Range parameter, by rule. That's actually the entire and only function of that Range parameter of a spell -- to restrict where a spell is allowed to originate. The spell is cast from the spellcaster's location and he casts the spell at (aims it at) the point of origin (along a clear path) in order for the spell to take effect there. Which part of this process is troubling for people?
Who said anything about targeting? I'm talking about what the spell can be cast at, which is restricted by the Range parameter, by rule. That's actually the entire and only function of that Range parameter of a spell -- to restrict where a spell is allowed to originate. The spell is cast from the spellcaster's location and he casts the spell at (aims it at) the point of origin (along a clear path) in order for the spell to take effect there. Which part of this process is troubling for people?
Because "target" is what Warcaster cares about. "The spell must have a casting time of one action and must target only that creature." It doesn't care about the origin.
War Caster doesn't allow for the casting of Booming Blade since the spell in question must be cast at the target creature and Booming Blade has a range of self.
Also, War Caster:
Reactive Spell.When a creature provokes an Opportunity Attack from you by leaving your reach, you can take a Reaction to cast a spell at the creature rather than making an Opportunity Attack. The spell must have a casting time of one action and must target only that creature.
"Cast at" is not a defined term of art here, and it can certainly be read that a single-target spell is "cast at" the target.
(And before you break into the "who does Booming Blade target?" argument, I will freely acknowledge that the weapon-attack cantrips are just plain weird, and a very poor fit for the mechanics built around spellcasting. This not infrequently leaves their interactions with the rules and other abilities ambiguous.)
I wasn't really talking about targeting there. I used the term "target creature" there to just refer to the "provoking" creature. The creature that is provoking the opportunity attack and therefore the creature who becomes the target of the retaliation.
I agree that the 2024 rules dictate that that particular creature is "targeted" by the Booming Blade spell. No one is arguing about that. Now, whether or not that creature is the only target of that spell is another question entirely, but I have not even made that argument since it is not necessary.
Instead, I am talking about War Caster's requirement that the spell must be cast at that creature.
The Reactive Spell feature of the War Caster feat has 4 requirements for its use once a qualifying trigger is acted upon:
-- You must use your Reaction. If you don't have a Reaction available, then you cannot do this.
-- You must cast a spell at the creature. If your spell is not being cast at the creature, then you cannot do this.
-- The spell in question must have a casting time of 1 action. If it has some other casting time, then you cannot do this.
-- The spell in question must target only that creature. If the spell targets a different creature or no creatures or additional creatures, then you cannot do this.
All 4 of those requirements must be met. In the case of Booming Blade, it fails to meet the second criteria because the spell has a Range of self and therefore the spell is cast at the spellcaster instead of at the target creature. Yes, that creature is affected by the resulting spell effect which makes it a target of the spell under 2024 rules, but that spell wasn't cast at that creature because that's not where the spell effect originated.
There always seems to be particular confusion within this community about the Range of Self for spellcasting. I think that it has to do with the fact that we all know and understand that the spellcaster is the one that is casting the spell but then that seems to often be conflated with the concept of the spell "originating" with the spellcaster. I think that some people are picturing it like, the spellcaster casts it, so that's where it comes from and from there the spell affects other creatures and therefore the spell had a Range of Self. NO!!! That is NOT what a Range of Self means! The Range of a spell determines the destination of the spellcasting process. When a spell has a Range of Self, it means that the spell is cast FROM the spellcaster TO the spellcaster! This happens along a clear path. In this particular case the clear path is obviously infinitely small, but it's crucially important to understand that the spell is being cast FROM the spellcaster and is then directed AT the spellcaster as its final destination for where the spell effect will originate.
Once the spell effect has actually originated at its point of origin, that spell effect might then affect (target) other creatures in accordance with the spell description, but that's not what the spell was being cast at. The spell is cast at (directed towards / aimed at / propelled to) the place where it will erupt into existence. At that point, the spellcaster's job is done. He can no longer direct the spell "at" anything. The spell effect already exists at that point. From there, whether or not that spell effect might interact with and/or affect a creature is pretty much out of the spellcaster's control. The spell effect does whatever it says in the description.
Many spells are cast directly at the target creature such as Fire Bolt or Sacred Flame or Shocking Grasp. Others are cast at an object. Many others are cast at a point in space, such as Fireball. In the case of Fireball, creatures within the resulting AoE are considered to be targets of the spell, but that doesn't mean that the spell was cast at any of them. The spell was cast at a point in space. Then, there are the Range of Self spells. Those spells are cast at the spellcaster. Some of those target the spellcaster and some target other creatures instead -- many Emanation spells work like that such as Thunderclap.
For some reason a lot of people skip over this requirement of War Caster and instead focus only on what the spell is capable of targeting. But the feat does have this requirement -- again, it is thematically consistent -- the spellcaster must actually be directing the spell at the provoking creature in response to that creature attempting to retreat.
This should not have required so much explanation. The text for the feat is painfully clear.
Once the spell effect has actually originated at its point of origin, that spell effect might then affect (target) other creatures in accordance with the spell description, but that's not what the spell was being cast at. The spell is cast at (directed towards / aimed at / propelled to) the place where it will erupt into existence. At that point, the spellcaster's job is done. He can no longer direct the spell "at" anything. The spell effect already exists at that point. From there, whether or not that spell effect might interact with and/or affect a creature is pretty much out of the spellcaster's control. The spell effect does whatever it says in the description.
Many spells are cast directly at the target creature such as Fire Bolt or Sacred Flame or Shocking Grasp. Others are cast at an object. Many others are cast at a point in space, such as Fireball. In the case of Fireball, creatures within the resulting AoE are considered to be targets of the spell, but that doesn't mean that the spell was cast at any of them. The spell was cast at a point in space. Then, there are the Range of Self spells. Those spells are cast at the spellcaster. Some of those target the spellcaster and some target other creatures instead -- many Emanation spells work like that such as Thunderclap.
This is the same argument I use for 2014 Twinned Spell when people say Dragon's Breath (the spell) cannot be Twinned because "it forces more than one target to make a Saving Throw", when there is just one single target: the target creature you are granting Dragon's Breath to. Totally Twinnable by RAW (in 2014 rules).
As such, this is exactly true for Booming Blade and War Caster. If you could cast Booming Blade and say "well the effect of the spell only effects the creature that provoked the OA", then you could also cast Fireball as long as only the only target in the area of effect was the triggering creature. I think that's pretty well understood to not be the case.
I wasn't really talking about targeting there. I used the term "target creature" there to just refer to the "provoking" creature. The creature that is provoking the opportunity attack and therefore the creature who becomes the target of the retaliation.
I agree that the 2024 rules dictate that that particular creature is "targeted" by the Booming Blade spell. No one is arguing about that. Now, whether or not that creature is the only target of that spell is another question entirely, but I have not even made that argument since it is not necessary.
Instead, I am talking about War Caster's requirement that the spell must be cast at that creature.
The Reactive Spell feature of the War Caster feat has 4 requirements for its use once a qualifying trigger is acted upon:
-- You must use your Reaction. If you don't have a Reaction available, then you cannot do this.
-- You must cast a spell at the creature. If your spell is not being cast at the creature, then you cannot do this.
-- The spell in question must have a casting time of 1 action. If it has some other casting time, then you cannot do this.
-- The spell in question must target only that creature. If the spell targets a different creature or no creatures or additional creatures, then you cannot do this.
All 4 of those requirements must be met. In the case of Booming Blade, it fails to meet the second criteria because the spell has a Range of self and therefore the spell is cast at the spellcaster instead of at the target creature.
Is it? "Cast at" still remains not officially defined. A spell can just as easily be considered to be cast at its targets(s). It could be considered to be cast at whatever it is you, the caster chooses, be it targets, and area, or a point in space. There's no RAW definition. Your particular definition isn't supported by the text:
A spell’s range indicates how far from the spellcaster the spell’s effect can originate, and the spell’s description specifies which part of the effect is limited by the range.
[...]
Self. The spell is cast on the spellcaster or emanates from them, as specified in the spell.
("Emanates from them" is very clearly not just Emanations. Lightning bolt is a range-self spell.)
So, you go to the description of the spell, and it has you picking another creature. That creature is presumptively the one the spell is cast at. All "Range: Self" means here is that you cannot launch the attack that the spell makes from a point that is not you.
So, while "cast at" still lacks formal definition, I have to conclude that there's a significantly stronger RAW case that you absolutely can cast the attack cantrips off War Caster.
A spell’s range indicates how far from the spellcaster the spell’s effect can originate, and the spell’s description specifies which part of the effect is limited by the range.
[...]
Self. The spell is cast on the spellcaster or emanates from them, as specified in the spell.
("Emanates from them" is very clearly not just Emanations. Lightning bolt is a range-self spell.)
So, you go to the description of the spell, and it has you picking another creature. That creature is presumptively the one the spell is cast at. All "Range: Self" means here is that you cannot launch the attack that the spell makes from a point that is not you.
So, while "cast at" still lacks formal definition, I have to conclude that there's a significantly stronger RAW case that you absolutely can cast the attack cantrips off War Caster.
Do you consider a martial character's regular attacks to be "emanating" from them?
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A quick question. A creature moved out of melee range causing an opportunity attack. Our rogue used Booming Blade with his short sword for the opportunity attack. The question, can the rogue use Booming Blade with this opportunity attack?
No, they can't, unless they have some special feature that allows them to cast a spell in place of an opportunity attack. The most common example of this is the War Caster feat.
In the absence of a feature like that, an opportunity attack allows you to make one melee attack or unarmed strike. It doesn't allow you to cast a spell (even one like Booming Blade that involves making an attack).
pronouns: he/she/they
The SAC has this answer. It's using True Strike for the explanation, but IMO the cantrip is similar to Booming Blade (or Green-Flame Blade and Vengeful Blade).
That "special feature" might be War Caster.
EDIT: Since the question is about Booming Blade , this is the 2014 SAC answer that mentions it: Can you use green-flame blade and booming blade with Extra Attack, opportunity attacks, Sneak Attack, and other weapon attack options?
War Caster doesn't allow for the casting of Booming Blade since the spell in question must be cast at the target creature and Booming Blade has a range of self.
This is neither clear from the rulebooks nor anything like universally agreed on. (And there's a very solid intent-of-the-rules argument that they should.)
Ask your DM.
The rulebooks are actually quite clear on this and of course when it comes to the RAW it really doesn't matter if people agree or not, it only matters what the text actually says.
Both the 2014 version and the 2024 version of the War Caster feat explicitly require that the spell is cast at the provoking creature. This is thematically consistent with the 5e concept of what an opportunity attack represents in the game which is when a creature lashes out at another creature in response to that other creature attempting to hastily retreat away from a melee combat engagement. As such, the wording appears to be extremely intentional.
The idea is that the War Caster creature is meant to directly target the provoking creature with a spell such as Fire Bolt (at disadvantage) or Shocking Grasp or Charm Person or Sacred Flame or Chill Touch or Chromatic Orb or even something like Witch Bolt, for example.
Something like the Read Thoughts effect of the Detect Thoughts spell (pretend for a moment that this is the spell's only possible effect) would not qualify even though the provoking creature is being "targeted" in the 5e Rules Glossary sense. The reason is that the spell is not being cast at that creature, it is being cast at the spellcaster. If you could somehow cast Scrying as an action this also would not qualify for similar reasons.
But probably the best two examples of spells from the 2024 PHB which are disqualified from this interaction for the same reason as Booming Blade would be True Strike and Vampiric Touch. In both of those cases, a single creature is being "targeted", but the spell is not being cast at that creature. Instead, those spells have a range of self, meaning, they are spells which create effects that buff the spellcaster in a particular way. Note that True Strike is actually a Divination spell -- the concept is that the spell mentally empowers the spellcaster while performing a specific activity.
War Caster is already powerful enough, especially when people begin trying to claim that these opportunity attacks can be made against their allies, which is a whole separate rabbit hole. There's no need to make it even more powerful by ignoring some of the restrictions that the text is providing.
I don't agree.
I'm not going to rehash the same arguments here, but what Range means was recently explained in True strike and overthinking it. - Rules & Game Mechanics. War Caster was also mentioned there.
It's unclear what that thread has to do with this one.
The concept of the Range of a spell is extremely straightforward in this game. What is it about that concept that you feel would need to be "explained"?
If an enemy creature is standing 90 feet away from the spellcaster and the spellcaster wants to cast a spell that has a Range of 30 feet, then by rule he cannot cast that spell at that enemy creature. Likewise, if that enemy creature is still standing 90 feet away from the spellcaster and the spellcaster now wants to cast a spell that has a Range of self, then by rule he still cannot cast that spell at that enemy creature.
This one is a slam dunk. There are not any other reasonable interpretations.
The concept of "Range" and the concept of "Target" are distinct, but you keep mixing the two. That is the essence of this argument. A range of self does not dictate a target of self.
"A spell’s range indicates how far from the spellcaster the spell’s effect can originate, and the spell’s description specifies which part of the effect is limited by the range." Some self spells do, indeed, target the caster, but not all.
True Strike / Booming Blade / Green-Flame Blade target the target of the attack, but all have a range of self, because the caster's self is the origin, not the target.
It's pretty straightforward. Range "indicates how far from the spellcaster the spell's effect can originate", but it doesn't define the targets. Several people explained that in the thread. You can read their explanations there.
EDIT: ninja'd by kenclary!
Is booming blade a 2014 spell?
The only reference to see the spell I can find is a 2014 sword coast book.
What is booming blade?
It first appeared in SCAG and was updated in Tasha's Cauldron of Everything to fix the Range, Components, and wording.
SCAG:
TCoE:
Character77006 I shared this transcription some time ago. It includes the reasons for the changes. It also explains how Range and Self should be understood. (EDIT: for clarity)
Who said anything about targeting? I'm talking about what the spell can be cast at, which is restricted by the Range parameter, by rule. That's actually the entire and only function of that Range parameter of a spell -- to restrict where a spell is allowed to originate. The spell is cast from the spellcaster's location and he casts the spell at (aims it at) the point of origin (along a clear path) in order for the spell to take effect there. Which part of this process is troubling for people?
Because "target" is what Warcaster cares about. "The spell must have a casting time of one action and must target only that creature." It doesn't care about the origin.
You:
Also, War Caster:
"Cast at" is not a defined term of art here, and it can certainly be read that a single-target spell is "cast at" the target.
(And before you break into the "who does Booming Blade target?" argument, I will freely acknowledge that the weapon-attack cantrips are just plain weird, and a very poor fit for the mechanics built around spellcasting. This not infrequently leaves their interactions with the rules and other abilities ambiguous.)
I wasn't really talking about targeting there. I used the term "target creature" there to just refer to the "provoking" creature. The creature that is provoking the opportunity attack and therefore the creature who becomes the target of the retaliation.
I agree that the 2024 rules dictate that that particular creature is "targeted" by the Booming Blade spell. No one is arguing about that. Now, whether or not that creature is the only target of that spell is another question entirely, but I have not even made that argument since it is not necessary.
Instead, I am talking about War Caster's requirement that the spell must be cast at that creature.
The Reactive Spell feature of the War Caster feat has 4 requirements for its use once a qualifying trigger is acted upon:
-- You must use your Reaction. If you don't have a Reaction available, then you cannot do this.
-- You must cast a spell at the creature. If your spell is not being cast at the creature, then you cannot do this.
-- The spell in question must have a casting time of 1 action. If it has some other casting time, then you cannot do this.
-- The spell in question must target only that creature. If the spell targets a different creature or no creatures or additional creatures, then you cannot do this.
All 4 of those requirements must be met. In the case of Booming Blade, it fails to meet the second criteria because the spell has a Range of self and therefore the spell is cast at the spellcaster instead of at the target creature. Yes, that creature is affected by the resulting spell effect which makes it a target of the spell under 2024 rules, but that spell wasn't cast at that creature because that's not where the spell effect originated.
There always seems to be particular confusion within this community about the Range of Self for spellcasting. I think that it has to do with the fact that we all know and understand that the spellcaster is the one that is casting the spell but then that seems to often be conflated with the concept of the spell "originating" with the spellcaster. I think that some people are picturing it like, the spellcaster casts it, so that's where it comes from and from there the spell affects other creatures and therefore the spell had a Range of Self. NO!!! That is NOT what a Range of Self means! The Range of a spell determines the destination of the spellcasting process. When a spell has a Range of Self, it means that the spell is cast FROM the spellcaster TO the spellcaster! This happens along a clear path. In this particular case the clear path is obviously infinitely small, but it's crucially important to understand that the spell is being cast FROM the spellcaster and is then directed AT the spellcaster as its final destination for where the spell effect will originate.
Once the spell effect has actually originated at its point of origin, that spell effect might then affect (target) other creatures in accordance with the spell description, but that's not what the spell was being cast at. The spell is cast at (directed towards / aimed at / propelled to) the place where it will erupt into existence. At that point, the spellcaster's job is done. He can no longer direct the spell "at" anything. The spell effect already exists at that point. From there, whether or not that spell effect might interact with and/or affect a creature is pretty much out of the spellcaster's control. The spell effect does whatever it says in the description.
Many spells are cast directly at the target creature such as Fire Bolt or Sacred Flame or Shocking Grasp. Others are cast at an object. Many others are cast at a point in space, such as Fireball. In the case of Fireball, creatures within the resulting AoE are considered to be targets of the spell, but that doesn't mean that the spell was cast at any of them. The spell was cast at a point in space. Then, there are the Range of Self spells. Those spells are cast at the spellcaster. Some of those target the spellcaster and some target other creatures instead -- many Emanation spells work like that such as Thunderclap.
For some reason a lot of people skip over this requirement of War Caster and instead focus only on what the spell is capable of targeting. But the feat does have this requirement -- again, it is thematically consistent -- the spellcaster must actually be directing the spell at the provoking creature in response to that creature attempting to retreat.
This should not have required so much explanation. The text for the feat is painfully clear.
This is the same argument I use for 2014 Twinned Spell when people say Dragon's Breath (the spell) cannot be Twinned because "it forces more than one target to make a Saving Throw", when there is just one single target: the target creature you are granting Dragon's Breath to. Totally Twinnable by RAW (in 2014 rules).
As such, this is exactly true for Booming Blade and War Caster. If you could cast Booming Blade and say "well the effect of the spell only effects the creature that provoked the OA", then you could also cast Fireball as long as only the only target in the area of effect was the triggering creature. I think that's pretty well understood to not be the case.
Is it? "Cast at" still remains not officially defined. A spell can just as easily be considered to be cast at its targets(s). It could be considered to be cast at whatever it is you, the caster chooses, be it targets, and area, or a point in space. There's no RAW definition. Your particular definition isn't supported by the text:
("Emanates from them" is very clearly not just Emanations. Lightning bolt is a range-self spell.)
So, you go to the description of the spell, and it has you picking another creature. That creature is presumptively the one the spell is cast at. All "Range: Self" means here is that you cannot launch the attack that the spell makes from a point that is not you.
So, while "cast at" still lacks formal definition, I have to conclude that there's a significantly stronger RAW case that you absolutely can cast the attack cantrips off War Caster.
Do you consider a martial character's regular attacks to be "emanating" from them?