Is Sneak Attack damage maximized on critical hits? Short answer: yes.
For the long answer, first read what the Players Handbook says about critical hits on pg. 278:
Maximum Damage: Rather than roll damage, determine the maximum damage you can roll with your attack. This is your critical damage. (Attacks that don’t deal damage still don’t deal damage on a critical hit.)
Extra Damage: Magic weapons and implements, as well as high crit weapons, can increase the damage you deal when you score a critical hit. If this extra damage is a die roll, it’s not automatically maximum damage; you add the result of the roll.
Furthermore, according to this FAQ:
10. Which dice do I maximize when scoring a critical hit?
Only the dice you would normally roll to calculate damage are maximized. If another bonus (like from a weapon or feat) causes you to roll extra damage dice when scoring a critical hit, those dice are rolled as normal.
Given all this, I think that Sneak Attack damage is maximized on critical hits. The main point being that the definition of “Extra Damage” in this context refers to damage that occurs as a result of the critical; Sneak Attack occurs whether the attack is critical or not, therefore it is maximized.
The rules are clear, but I decided to write up this explicit answer given the confusion I’ve seen in various forums about this question.
Sorry.. but NO.
The only dice you maximize with a critical is the primary power you use.
Your description for Critical hit is amputated. you forgot this.
“When an attack roll against a target gets a natural 20, THE POWER not only automatically hits the target, but also scores a critical hit..”
THE POWER… that you used to make the attack, is maximized. Sneak attack is NOT a power or an attack, and they actually trigger AFTER you hit.
I recently got into a debate with a player of mine about this topic… so I emailed Wizards. here is their response.
“Unfortunately, any extra damage that would add additional die rolls, such as Sneak Attack, does not get maximized on critical hits. As always, however, these rules may very well be ignored or modified per the DMs discretion to fit the overall need of the game and I suggest bringing this up with your games’s Dungeon Master for a final ruling.”
I could point out several other places where your logic is flawed (like on the wording of EXTRA DAMAGE) but I shouldn’t have to. My Email from Wizards is enough to prove it. And if you go back an re-read the rules keeping in mind the ruling.. you should understand.
Cheers!