I’m confused. Both Shield Mastery and Two Weapon Fighting use identical precondition language (“when you take the attack action”). You said you have to finish ALL attacks with the attack action first for Shield Mastery…don’t you have to do that with two weapon fighting as well? They don't have the same wording. Shield Master refers to the Attack action, whereas two-weapon fighting refers to making an attack with the Attack action.
— Jeremy Crawford (@JeremyECrawford) May 17, 2018