Determining Surprise

When possible, I prefer to have players make the most important rolls, and I prefer to have their character abilities provide meaningful inputs to those rolls.

Yeah, I know: players aren't their character sheets, spot checks suck, etc. But, I also don't want to ignore when it would make sense for an ability to come into play.

Surprise feels like one of those times when it would make sense. Traditional fantasy RPG rules state that, all other things being equal, each side has a 2-in-6 chance to end up surprised by the other party during a random encounter.

A small humanoid after getting surprised (art by Jeff Easley)

Yet, the stakes here match the criteria of calling for a check or saving throw instead of a purely random chance: the outcomes matter, they are obvious, and they are very much in doubt.

Checking for Surprise

When a random encounter occurs, each player should make a WIS saving throw. On a failure, that player's character is surprised and does not take a turn during the first combat round.

Note that this rule specifically determines whether or not the monsters surprise the PCs. Determine the reverse scenario with the standard 2-in-6 roll. This house rule isn't about verisimilitude or accuracy, it's about giving players the illusion of agency.