Can you use Sneak Attack after using the Scout Rogue's Sudden Strike feature?

Question

The Scout Rogue's Sudden Strike feature (XGtE, p. 47) states:

Starting at 17th level, you can strike with deadly speed. If you take the Attack action on your turn, you can make one additional attack as a bonus action. This attack can benefit from your Sneak Attack even if you have already used it this turn, but you can't use your Sneak Attack against the same target more than once in a turn.

And Sneak Attack states:

Beginning at 1st level, you know how to strike subtly and exploit a foe’s distraction. Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll [...]

I'm wondering what happens in the following scenario:

1. You take the Attack action (fully) but never proc Sneak Attack
2. You use your bonus action from Sudden Strike and proc Sneak Attack
3. You make another attack (such as through an opportunity attack or Action Surge)

Can your attack in step 3 apply Sneak Attack even though you've already applied it during that turn using Sudden Strike?

Answer 1

No. Once you Sudden Strike, you may no longer use Sneak Attack that turn.

Sneak Attack is usable once per turn:

Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack [...]

That is always true. But Sudden Strike grants you a bonus action attack and let's you ignore the once-per-turn limitation for that attack only:

This attack can benefit from your Sneak Attack even if you have already used it this turn [...]

Outside of the bonus action attack granted by Sudden strike, Sneak Attack can only be used once per turn. So a rogue that fails to sneak attack before using the Sudden Strike bonus attack is effectively depriving themselves of a potential second Sneak Attack.

Answer 2

Sudden Strike allows you to apply Sneak Attack if you've already used it.

Nothing in Sudden Strike says using it for a Sneak Attack doesn't count as using Sneak Attack.

So in the case you've outlined, any attacks subsequent to your Sudden Strike would see that you already used Sneak Attack that round and would not be eligible for further sneak attacks.

You mentioned opportunity attacks -- just to note it, sneak attack is once per turn, so even if you use it on your turn, you can still sneak attack with an opportunity attack during somebody else's turn. You couldn't sneak attack if you somehow got an opportunity attack during your own turn, but that would be an exceedingly unusual situation.