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Both Booming Blade and Green Flame Blade have the following text:

You make a single melee weapon attack against a creature you can see within the spell's range. If the attack hits...

Reading this question makes me believe that a weapon which has been magically enchanted or enhanced, even temporarily, would overcome this immunity, and that both base weapon damage and cantrip damage would apply on a hit (and double on a critical hit).

Question: What damage, if any, would be generated by attacking a creature (with immunity to damage from non-magical weapons) with a non-magical weapon as part of a Booming Blade or Green Flame Blade attack?

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The base weapon attack that's part of the cantrip doesn't become magical.

Rules designer Jeremy Crawford unofficially addresses this question in a 2017 tweet:

Is the attack made as part of Booming Blade magical? It's delivered as part of a spell, but has its "usual effects" and is a prerequisite for the spell to work.

The booming blade spell isn't intended to make the required weapon attack magical.

So the weapon attack you make as part of the spell is not magical on its own. The two cantrips call for a weapon attack made as part of the cantrip, but the weapon attack on its own is not stated as being modified to become magical; there are simply additional effects to the attack that are granted by the cantrip.

...But the extra damage is magical.

The booming blade spell description says (SCAG, p. 142; 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.

Similarly, green-flame blade says (SCAG, p. 143; 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 green fire leaps from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting ability modifier.

When the caster of either cantrip reaches higher levels, the initial attack does extra damage as well (in addition to boosting the secondary damage).

Per Crawford's unofficial guidance above, the required weapon attack remains nonmagical; the fact that the spell calls for a normal weapon attack doesn't change the properties of the weapon. However, the added damage done by the spell itself is magical, as it's caused directly by the spell. This is true both of the secondary damage of both spells, and of the extra initial damage both spells do at higher levels.

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  • \$\begingroup\$ Also booming blade triggers from effects such as cause fear or dissonant whispers. Things that use the creatures own movement \$\endgroup\$ – Deceptecium Jan 27 at 3:44
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    \$\begingroup\$ @Deceptecium: Booming blade does exactly what the spell says it does. Its secondary damage only triggers on willing movement, nothing else. You are probably thinking of opportunity attacks, which are indeed provoked when a creature is caused to move by something that uses the creature's movement, action, or reaction (such as dissonant whispers). \$\endgroup\$ – V2Blast Jan 27 at 3:51
  • \$\begingroup\$ Huh. My bad I take that back that's odd \$\endgroup\$ – Deceptecium Jan 27 at 5:11
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Only the cantrip's extra damage would damage the creature

As part of both cantrips' effect, extra damage is dealt. If it had said the weapon's damage was changed, the weapon's damage would have been effective.

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