The wording of Booming Blade seems ambiguous to me. Can someone confirm whether the "melee attack" uses the casting ability, such as Intelligence, because it is a cantrip, or else uses Strength or Dexterity because it is a weapon.

You use whatever ability you use to make a weapon attack (usually strength or dexterity). It would would say spell attack if it wanted you to use you spell casting ability.

Quote from Wragged >>

You use whatever ability you use to make a weapon attack (usually strength or dexterity). It would would say spell attack if it wanted you to use you spell casting ability.

 I get that. But.

The wording of the spell turns this weapon into a spell component. Then says you "brandish the weapon used in the spells casting", which can read that it is a spell attack. Moreover, in the previous edition, Booming Blade and Green Flame Blade explicitly use Intelligence to make the attack.

For the rules lawyers, here is the wording that seems ambiguous. The same wording is present in both Booming Blade and Green Flame Blade, so it might have mechanical intention.

Booming Blade
...
Components: S, M (a melee weapon worth at least 1 sp)
...
You brandish the weapon used in the spell’s casting and make a melee attack against one creature. ...  On a hit, the target suffers the weapon’s attacks normal effects and then becomes sheathed in booming energy. ...

So it does seem to be a "weapon attack", yet it is delivered via the spell casting as part of the material component (weapon) and the somatic component (brandish). So it is a specific kind of spell attack?

The spells says you're making the attack with a weapon. In the absence of language to the contrary, an attack with a weapon is always a weapon attack.

Quote from SagaTympana >>

The spells says you're making the attack with a weapon. In the absence of language to the contrary, an attack with a weapon is always a weapon attack.

 What it says is, you "brandish the weapon AND make a melee attack". The attack itself is explicitly part of the cantrip, unlike say a Paladin Thunderous Smite spell, where the weapon is not part of the spellcasting.

Quote from Gvarayi >>

Quote from SagaTympana >>

The spells says you're making the attack with a weapon. In the absence of language to the contrary, an attack with a weapon is always a weapon attack.

 What it says is, you "brandish the weapon AND make a melee attack". The attack itself is explicitly part of the cantrip, unlike say a Paladin Thunderous Smite spell that is cast AFTER "a successful hit".

None of that contradicts what I said. Again, it's explicitly an attack with a weapon.

Quote from SagaTympana >>

Quote from Gvarayi >>

Quote from SagaTympana >>

The spells says you're making the attack with a weapon. In the absence of language to the contrary, an attack with a weapon is always a weapon attack.

 What it says is, you "brandish the weapon AND make a melee attack". The attack itself is explicitly part of the cantrip, unlike say a Paladin Thunderous Smite spell that is cast AFTER "a successful hit".

 

None of that contradicts what I said. Again, it's explicitly an attack with a weapon.

 But it is a spell attack that uses a weapon as its spell component.

Compare the Warlock Eldritch Blast cantrip. Its description specifies it is a "ranged spell attack". Leaving open the possibility that Booming Blade is a "melee spell attack", that intentionally delivers normal weapon damage (as 4e describes it, with Intelligence attack that deals [W] damage).

Weapon attacks might use Strength or Dexterity, or even Charisma. So the term "weapon attack" might not preclude the casting ability.

The Druid Shillelach [ʃɪ.le.li] cantrip transforms the weapon itself, but explicitly allows the casting ability to substitute while using the weapon normally.

It is a melee weapon attack. It uses whatever ability modifier you would use for a melee weapon attack. It does not use your spell attack modifier because it isn’t a spell attack.

This precise question is answered in the Sage Advice Compendium, quoted below, which you can find here. And yes, the SA compendium is a source of official rulings.

First, each of [booming blade and green-flame blade] involves a normal melee weapon attack, not a spell attack, so you use whatever ability modifier you normally use with the weapon. (A spell tells you if it includes a spell attack, and neither of these spells do.) For example, if you use a longsword with green-flame blade, you use your Strength modifier for the weapon’s attack and damage rolls.

Quote from Emerald101 >>

This precise question is answered in the Sage Advice Compendium, quoted below, which you can find here. And yes, the SA compendium is a source of official rulings.

First, each of [booming blade and green-flame blade] involves a normal melee weapon attack, not a spell attack, so you use whatever ability modifier you normally use with the weapon. (A spell tells you if it includes a spell attack, and neither of these spells do.) For example, if you use a longsword with green-flame blade, you use your Strength modifier for the weapon’s attack and damage rolls.

Thanks for the official clarification. Heh, it seems I wasnt the only one who found the wording ambiguous. :) 

Quote from dumbBunny >>

Thanks for asking the question and posting the replies; I'm a dense 5e noob, with two follow up questions. These are related to the meaning of Extra Attack, which is a bit ambiguous to me. 

1. If a character has leveled up so they can take (2) attacks per action, how does this work with Booming Blade? Do they make both attacks with Booming Blade, or one attack with it - one without, or, as some say, since you are using BB, you can take only one attack. That last scenario, is based on classifying BB as a spell attack, which in reading the answer in Sage Advice, seems wrong. I would assume this means only the first of the two attacks gets BB, but I have no basis to justify this.

2. If you are doing two weapons fighting, does taking BB prevent attacking with your bonus action? RAW, "When you take the Attack action and attack with a light melee weapon that you're holding in one hand, you can use a bonus action to attack with a different light melee weapon that you're holding in the other hand." I.E.if you don't make an attack with your action, you can't take an attack with you Bonus Action. I think SA answers this one - Using Booming Blade is an attack, not a spell attack.

 1. Extra attack does not work with booming blade normally. When casting booming blade, you are using the 'cast a spell' action which happens to involve a weapon attack, you are NOT taking the attack action. Unless you're a bladesinger who can explicitly replace one of their attacks with a cantrip, you cannot use extra attack with booming blade. 

2. Booming blade does not work with two weapon fighting, two weapon fighting requirse that you took the attack action. Booming blade is not using the attack action, it is using the cast a spell action. And that spell just happens to involve a weapon attack. 

Yet on the Character Class tab of D&DB Generator, it reflects an amount for the attack modifier using the spellcaster's stat, not Str or Dex

That's a limitation of the DDB Generator.

I also wonder do you still need a hand free for the somatic component.  So left hand makes a mystical pass, right hand holding weapon strikes. So unable to hold a shield or a second weapon, or a two handed weapon. (could maybe get away with a versatile) Unless you are an artificer with an infused weapon which can act as a focus, or ruby of war mage weapon to make it an arcane focus etc.

or is the sword etc being the material component, considered held in a free hand.

Quote from OldOneTooth >>

or is the sword etc being the material component, considered held in a free hand.

The weapon is explicitly a material component. Therefore the hand wielding it can also perform somatic components.