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I am a new GM playing D&D 4e. I have a question regarding the Alertness feat. In the Compendium it is described as the following:

Benefit: You cannot be surprised. Also, you gain a +2 feat bonus to Perception checks.

My understanding is that the character gets a boost (the +2 bonus) on Perception, not "Passive Perception". I assumed he has to roll on passive perception if he notices an ambush, is that right?

So how does this translate to the game in regards to not being able to be surprised? E.g., if a character has a passive perception of 5 + the roll (let's say 3), his result would be 8, which might still not be enough to notice the ambush (DC for this would be 10). Is he the surprised, or not?

Or does not being able to be surprised mean then the attacker can't get a surprise bonus on his attack roll? Anyway, the attacker would still get the first attack if his target missed the passive perception roll, right?

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This is not an answer, but worth mentioning: you never roll passive Perception. Your passive Perception is just your ordinary perception - with an assumed "take 10" as the die roll. It's noted down precisely to avoid having to make actual die rolls. So anything that boosts the skill boosts the passive as well; there's no difference. – Tynam Apr 23 '14 at 22:10
@Tynam That's about half an answer, though. And the one existing answer is the matching half: why not suggest this to them as an edit, to make a happy whole? :) – SevenSidedDie Apr 23 '14 at 22:53
Damn, i did not notice before that the passive Perception/Insight are 10 above the regular. I usually asked my players to roll on top of passive skill check.. well, from now on not anymore.. – pet Apr 27 '14 at 2:27
up vote 16 down vote accepted

4e authors' policy is that fluff, i.e. the small snippets of descriptive texts you often encounter, are fully separated from the rules' text.
This means that when the rules say "you cannot be surprised", it's not descriptive: it's prescriptive.

So, whenever you would be surprised, you aren't instead. Even if the character didn't notice an ambush, he still acts during the surprise round and enemies gain no combat advantage against him.
The character is immune to the surprised condition.

Passive Perception is a special case. While being a fixed value (and one could think it doesn't benefit from the feat because it's not a check), it's what you'd get on your Perception Check had you rolled a 10. Consequently, Alertness' benefit applies to Passive Perception too.

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I see, i used Perception and Insight wrongly. :( – pet Apr 27 '14 at 2:28

Surprised is referring to the "surprised" condition. Normally in an ambush, if the rolls to ambush succeed, the ambushers get a surprise round in which they can each take 1 action and the ambushed characters can't act. The ambushers also gain combat advantage against the ambushed. If a character has alertness, they don't grant combat advantage in that situation, and they can act during the surprise round.

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