Here's what I mean by this question; The spell deals additional thunder damage upon hitting a creature with the weapon and the spell says it lasts 1 round. The extra attack feature from Bladesinger states you can forgo one of the attacks with a cantrip. If you make booming blade your first attack and follow up with the same weapon used in booming blade, does the additional damage to the weapon attack carry over to the additional attack from extra attack or am I reading too much into it?
Unfortunately this is incorrect. Effectively, as a bladesinger, the following happens:
You take the attack action, allowing you to make two attacks. On the first attack, you choose to switch the regular weapon attack for a cantrip, in this case booming blade. Assuming you hit, the target is now under all the effects of the regular spell, including the chance of taking damage if it chooses to move on that turn.
You take your second attack, as granted by the level 6 feature. However, this is a regular weapon attack, and as such it doesn't get any benefits of the booming blade spell you have just cast; only the features that would be expected from this (such as any effects specific to a magic weapon you are using) will apply.
At the end of your regular attack action, you have made two attacks; one using the booming blade cantrip and one as a normal attack. The target is under the effects of the booming blade cantrip once, because you only hit it once with that cantrip.
For example, if you are a level 6 bladesinger attacking with a rapier and no additional spells, your first attack will deal 1d8 piercing (rapier) + 1d8 thunder (booming blade) + your weapon damage modifier. Your second attack will only deal 1d8 piercing (rapier) + your weapon damage modifier. Then before the start of your next turn, the target takes an additional 2d8 thunder damage if it willingly moves, not 4d8.
Here's what I mean by this question;
The spell deals additional thunder damage upon hitting a creature with the weapon and the spell says it lasts 1 round.
The extra attack feature from Bladesinger states you can forgo one of the attacks with a cantrip.
If you make booming blade your first attack and follow up with the same weapon used in booming blade, does the additional damage to the weapon attack carry over to the additional attack from extra attack or am I reading too much into it?
Unfortunately this is incorrect. Effectively, as a bladesinger, the following happens:
At the end of your regular attack action, you have made two attacks; one using the booming blade cantrip and one as a normal attack. The target is under the effects of the booming blade cantrip once, because you only hit it once with that cantrip.
For example, if you are a level 6 bladesinger attacking with a rapier and no additional spells, your first attack will deal 1d8 piercing (rapier) + 1d8 thunder (booming blade) + your weapon damage modifier. Your second attack will only deal 1d8 piercing (rapier) + your weapon damage modifier. Then before the start of your next turn, the target takes an additional 2d8 thunder damage if it willingly moves, not 4d8.
Alrighty thanks for the clarification, I was only interested in the damage upon hitting and got confused on the cantrip lasting 1 round.