Most optimal pure hexblade build?

[Vogie]

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Thanks for the answer. What you mean you lose attacks? GFB:"As part of the action used to cast this spell, you must make a melee attack with a weapon" so exactly as with BB I do my first attack with GFB (so at lvl5th I also add 1d8 fire dmg) and then my second attack without cantrip. Am I using it wrong? Sorry I obviously mean melee attack for example with an Halberd just to be clear.

using GFB (or BB) will mean you are taking the "Cast a Spell" action, instead of the "attack" action. So if you use GFB, you will make a melee attack... but only one. You don't get any extra attacks with a feature or invocation, or even a bonus action attack with a light, offhand weapon, because you aren't taking the attack action.

Eldritch Knight Fighters can get around this with War Magic, as they can cast a Cantrip then have a bonus action attack - So, GFB, which includes a single melee attack, and then a second attack as a bonus action. Warlocks don't have such a feature.