As a Swords bard, how do I determine my AC bonus from Defensive Flourish on a critical hit?

Question

Let's say I'm playing a College of Swords bard, and happen to critically strike an enemy with a natural 20. I choose to use my Defensive Flourish at this time, so the dice look like this:

2d6 (Scimitar) + 2d8 (Defensive Flourish)

The Defensive Flourish Blade Flourish option confers this benefit (XGtE, p. 15):

You can expend one use of your Bardic Inspiration to cause the weapon to deal extra damage to the target you hit. The damage equals the number you roll on the Bardic Inspiration die. You also add the number rolled to your AC until the start of your next turn.

Let's say I roll a total of 12 (4 and 8) on my 2d8 from Defensive Flourish on a crit. Do I

• add the total to my AC?
• get to pick which one of the two dice to add? or...
• have to take the first number?

How do I determine my AC bonus from Defensive Flourish on a crit?

Answer 1

Only the initial Bardic Inspiration die applies to the AC boost

According to the rules on critical hits:

When you score a critical hit, you get to roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. Then add any relevant modifiers as normal. To speed up play, you can roll all the damage dice at once. (PHB pg. 196)

The phrasing seems to imply that the extra dice being rolled are only applied to the attack damage, and not to extra effects. As such, only the normal die roll from Bardic Inspiration applies to the non-damage effect of boosting AC.

When it comes to the damage however, you do get to roll the die again and add both rolls to the total damage, just as you normally would on a critical hit.

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While not an official ruling, Mike Mearls seems to be in agreement with this interpretation:

twtrlessJacob: How do critical hits affect non damage related effects from the bardic inspiration dice rolled for Mobile and Defense Flourishes? Are these effects the summation of the double bardic inspiration dice?

Mike Mearls: Crit has no effect on non-damage, unless effect specifically says otherwise