How to handle stealth before combat starts?

Question

Say some goblins set an ambush. They are actively hiding in the hopes of surprising unassuming travelers.

The path of our adventures just so happens to cross this ambush. If they take no active action (from what I've seen around), you make the goblins roll Stealth against the passive perception of the party. Anyone whose passive perception is lower is now surprised when combat starts.

But what if the party has some indication that there might be an ambush. Maybe the ranger has dealt with goblin ambushes before. Perhaps the goblins didn't clean up very well after their previous ambush.

So one the players declares "I think there might be an ambush, can I actively search for enemies?".

What is the "correct" ruling on this?

You could make the (relevant) goblins make a stealth check. But would you have to roll another check when combat starts, to determine surprise, or do you reuse the previous one?

Say all but one goblin gets spotted, would the party still get surprised from the last one, rendering the active check useless? Or the opposite if only one goblin of 5 gets spotted, does it nullify the surprise of all other goblins?

Answer 1

Don't roll for the ambushers individually

The very first example of an ambush in the Lost Mine of Phandelver suggests a single roll for all the goblins, not a per-goblin roll. This removes the issue with seeing some goblins and not others. If the group has mixed stealth modifiers, you can use the lowest of the group, the average, or something else (the rules for PC group checks are more complicated, but it's overkill for NPCs, as the official published adventures imply).

Unless there is a clear reason for an active search at a specific moment in time, use the passive Perception

Passive Perception covers "repeated searches over time", so use that unless there is a real reason for them to be extra-suspicious approaching the ambush site. "Dealt with goblin ambushes before" might give advantage (a +5) on the passive check if they're really specialists in that sort of thing, but it's still passive. "Perhaps the goblins didn't clean up very well after their previous ambush" would be how you might describe a poor roll by the goblins, or you could impose disadvantage if you want the ambush to fail, but it's still passive (of course, a poor roll by the goblins means most folks with even modest passive Perception will spot the ambush, which makes sense if they set it up poorly).

If there is an immediate reason to search now, replace the passive roll with an active roll if it's higher

If you're narrating scene by scene, and some cue in your description makes the ranger suspect an ambush and spend the extra effort to look for one at that moment, not merely looking all along a miles long trail, give them a roll, and if the Perception roll exceeds their passive Perception, use that instead to determine if they're surprised (outside of high pressure combat situations, passive Perception provides a floor for the roll).

Answer 2

System agnostic advice

The accepted answer by ShadowRanger is excellent advice for using the specific rules of 5e in regards to stealth, but I though it would be best to accompany that with some more general advice on stealth as well.

Namely, it's very important that you don't let stealth become too abstract. Your goblins are not just making stealth checks - they're actively hiding and sneaking around. Your first priority when planning this encounter is to figure out how they're doing that. This may seem obvious, but it's important for a few reasons.

First, it helps determine when an active check is needed.

When your party is just ho-humming along, you can assume a basic level of awareness from them. Even if they say they are looking out for danger, that doesn't really change anything - they're just strolling by with their ears and their eyeballs, and no amount of "I'll keep a lookout" should modify that. They get passive perception and that's it, maybe with a bonus if they intentionally slow down to look harder.

The exception, of course, is if the party actually declares some specific course of action. If you've already decided that your goblins are hanging out in the treetops, anyone who specifically says "I'll keep an eye out for the treetops to see if I can spot anyone waiting in ambush" is entitled a check to notice the ambush. However, if you've decided that your goblins are hiding near the ground in the bushes, or if you're not using goblins at all but rather a big pit of quicksand, the character searching the treetops forfeits his passive perception to notice that (or takes a big penalty, if you're feeling generous).

(This can also answer your question of splitting up surprise. If there are goblin archers in the trees and goblin pikers in the bushes, and only the party member searching in the trees passes his check, then only the archers lose the benefit of surprise.)

This may even give characters the ability to circumvent the need to roll altogether. If the party decides to meticulously travel at a snail's pace, with the wizard's familiar scouting through the trees, the fighter stabbing his halberd through every bush they pass, and the druid tapping the ground before them with her staff, then the goblins don't even get a chance to hide at all unless they change their strategy.

Second, and I hinted at this with that last paragraph, it's more fair.

If you don't know where your ambushing monsters actually are, and instead you're just relying on the abstraction of the system to handle those details, you may run the risk of doing something that breaks common sense. Since common sense is what players ultimately rely on (more than they should rely on the rules, in fact), this leads to broken expectations.

Suppose, for instance, you did have someone intentionally stabbing every bush they pass. If you just resolve this as advantage on a perception check, that player is going to be unhappy when his low roll fails to notice a bunch of goblins jumping out of the bushes. This should have been the player's time to shine, but a desire to stick to the rules and the dice may turn this objectively correct solution into a very disappointing roll. No matter how well something is hidden, someone looking in exactly the spot it's at should find it.

FInally, it makes stealth interesting.

Once you know where your threats are and you encourage players to take specific countermeasures, the game opens up tremendously. If you prompt players on what it is that they are specifically looking out for when they say they're "keeping an eye out," you suddenly activate the probelm solving lobe of that player's brain. In my experience, this quickly turns into a whole-party planning session of how to most effectively tackle the problem. One player will suggest a way to search the ground, another will pitch in a spell or two, yet another will try to rally the group to keep up the pace. Instant strategy meeting, and all you did was put some goblins in a tree.

In short, make sure you know with every roll what your characters are actually doing. It doesn't matter if the player wants to keep an eye out - you need to turn that into an actual in-world action. As soon as you know what actually goes into that perception roll, you'll know immeditately whether or not to roll it.

Answer 3

The PCs are already actively looking for danger when traveling

You say,

The path of our adventures just so happens to cross this ambush. If they take no active action (from what I've seen around), you make the goblins roll Stealth against the passive perception of the party.

The problem with what you've 'seen around', is that most people simply don't understand how the 5e rules work for active vs. Passive Perception checks, and thus get it wrong. The wording "active" and "passive" is confusing¹, so unless you look closely at what the rules actually say, you might also come to the mistaken conclusion that active perception involves a PC 'trying' to look for threats and Passive Perception is just some innate danger sense they have without intentionally focusing on threats. This is just wrong.

Instead, the rules assume that PCs are always actively looking for threats (unless what they are doing is so demanding of their attention that they cannot²) but that they are doing so with Passive Perception checks. In your case, the PCs are moving overland and are already alert for potential ambushes, so if a player says they suspect an ambush and want to 'actively search' for enemies, your answer should be, "You already are".

The rules for this are in the PHB Chapter 8 (Adventuring), under Movement (Noticing Threats). As a DM, if the PCs are traveling, you are expected to compare their Passive Perception scores against the Stealth scores of the goblins to see whether the ambushers are noticed. The Perception of the characters can be lowered by the players stating that they are moving at a fast travel pace, but it is not increased by a player saying that their character is specifically looking for danger³, because it is assumed that they already are.

As you describe the party's overland travel, you might bring up various features of the landscape that could be ambush points. If any of the players say that they want to check for hidden foes, again, the answer is "you already are".

Surprise happens before you can Search

At some point, however, you will decide that the PCs are not just traveling across empty terrain, but have come upon the goblin ambush. The "Noticing Threats" rules calls this "Encountering Creatures"⁴, and since the goblins have hostile intent, we now need the rules for "Surprising Foes"⁵. This marks an important narrative and rules shift from Chapter 8 (Adventuring) to Chapter 9 (Combat).

Once your PCs are in Combat, your player's desire to "actively search" for Hidden foes is relevant - it is accomplished by their PC taking the Search action on their turn, since Searching is one of the Actions in Combat. If a PC Searches, they do make an active Perception check by rolling their Perception against the goblins' Stealth. However, a PC can only Search on their turn. Before the PC can start their first turn, the DM has to initiate the Combat by checking for Surprise and calling for all participants to roll Initiative.

The rules for Surprise⁶ mark the transition from Traveling to Combat. Importantly, a PC starts the Combat either surprised or not, based on their Passive Perception score⁶. They were already actively looking for danger, but if their Passive Perception score was not good enough to spot the worst-Hidden goblin, then they will be surprised when the Combat starts and they will not be able to do anything on their first turn, regardless of their Initiative.

If, however, the PC's Passive Perception score was good enough to notice at least one goblin, then they are not surprised by any of the goblins and start the Combat aware of the ambush. When their turn comes up in Initiative order, they will already be aware of the presence of at least one goblin and possibly more⁷. When the non-surprised PC's turn arrives, they can take the Search action, allowing them an active Perception check by rolling their Perception. If they do roll higher than their Passive Perception score, they may then become aware of any goblins that were previously Hidden to them at the start of their turn.

However, even if they don't take the Search action, they are still actively looking for danger⁸, and their Passive Perception score remains in play, potentially alerting them to the presence of goblins who have not taken their turns yet (or others who already took their turns and then Hid again).

In short, the player's desire for an active check can only be met on their own turn, but they don't have a turn until the combat starts, and by the time the combat starts they are already either surprised or not based on their Passive score.

What should you use for the Goblins' initial Stealth scores?

If just a few goblins are involved in the ambush, you can roll Stealth for each of them, when they Hide in advance of the characters' arrival. However, the more goblins there are, the more likely it is that the party will notice the ambush and not be surprised. To some extent this is realistic, because presumably a few goblins would select the best hiding spots first, while if more and more of them were involved the options for hiding places would get worse. Also, the party has only to notice the worst-hidden goblin, and in a larger group there is more likely to be a goblin with poor skill or bad luck. However, what is not realistic is the extent to which the success of the goblin ambush will steeply decline with additional goblins. This is a result of the way active Stealth is determined in the rules, as a d20 roll (representing luck) modified by the Stealth bonus of the goblins (+6, representing skill). Notice that usually the impact of the luck portion is far greater than the impact of the skill portion of the check. Whenever this is true, and when only the worst roll among many counts, we have a situation called "rolling to failure". Given enough goblins, one of them will make an unlucky Stealth roll, with a result below the Passive Perception of the PCs, who will then be alerted to all the goblins in the ambush. Against a PC with a Passive Perception of 12, a single goblin will roll a Stealth of 13 or better 70% of the time - but the chance of ten goblins all beating the Passive Perception of that PC is less than 3%. Again, if any of the goblins roll poorly, even a moderately Perceptive PC will not be surprised by any of them.

In order to handle the goblin ambush with more verisimilitude, and not use it as an exercise in "rolling to failure", you have a number of options. You could permit the goblins to to collectively make a group check for Stealth, you could allow the goblins to each use their Passive Stealth Scores of 16 as the baseline for their ambush (and only require them to start rolling Stealth actively once in combat), or, if the goblins have had at least a minute to prepare for the party and know they are coming, you could permit them "automatic success" and begin the ambush with a Stealth of 26.

---

Footnotes and Citations

¹ 'Active' and 'passive' describe the mechanics of the Check, not the kind of Perception. 'Active' and 'Passive' do not refer to what the PCs are doing. The PCs are assumed to be actively looking for danger in either case. An active Perception check is one in which the player actively makes a roll, while a Passive check is one in which the DM uses the Passive Perception score of the PC without a roll being made.

² Examples of such activities are given as navigating, drawing a map, tracking, and foraging.

³ The assumption that the PCs are already actively looking for danger when traveling, even when the DM is using Passive Perception scores, as stated can be found in the Noticing Threats section of the PHB, which says (emphases mine):

Use the passive Wisdom (Perception) scores of the characters to determine whether anyone in the group notices a hidden threat. The DM might decide that a threat can be noticed only by characters in a particular rank. For example, as the characters are exploring a maze of tunnels, the DM might decide that only those characters in the back rank have a chance to hear or spot a stealthy creature following the group, while characters in the front and middle ranks cannot.
While traveling at a fast pace, characters take a –5 penalty to their passive Wisdom (Perception) scores to notice hidden threats.
[...]
Other Activities
Characters who turn their attention to other tasks as the group travels are not focused on watching for danger. These characters don’t contribute their passive Wisdom (Perception) scores to the group’s chance of noticing hidden threats.

⁴

Encountering Creatures. If the DM determines that the adventurers encounter other creatures while they’re traveling, it’s up to both groups to decide what happens next. Either group might decide to attack, initiate a conversation, run away, or wait to see what the other group does.

⁵

Surprising Foes. If the adventurers encounter a hostile creature or group, the DM determines whether the adventurers or their foes might be surprised when combat erupts. See the Combat section for more about surprise.

⁶

The DM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side. Any character or monster that doesn't notice a threat is surprised at the start of the encounter.
If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends. A member of a group can be surprised even if the other members aren't.

⁷ The PC's Passive Perception score might have been better than the Stealth of multiple goblins at the start of the ambush, and other goblins may have revealed themselves since the start of the combat, since a goblin that started the combat Hidden from the non-surprised PC but had a higher Initiative could have attacked before the PC's turn, forcing it out of Hiding.

⁸ In the Using Ability Scores section, in the sidebar on Hiding, the rules describe the procedure for hiding in combat. Even if a PCs action for the round is not Search, it still gets a Passive Perception check to see Hidden enemies (emphases mine):

When you try to hide, make a Dexterity (Stealth) check. Until you are discovered or you stop hiding, that check's total is contested by the Wisdom (Perception) check of any creature that actively searches for signs of your presence...
In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you. However, under certain circumstances, the DM might allow you to stay hidden as you approach a creature that is distracted, allowing you to gain advantage on an attack roll before you are seen.
Passive Perception. When you hide, there's a chance someone will notice you even if they aren't searching. To determine whether such a creature notices you, the DM compares your Dexterity (Stealth) check with that creature's passive Wisdom (Perception) score, which equals 10 + the creature's Wisdom modifier, as well as any other bonuses or penalties. If the creature has advantage, add 5. For disadvantage, subtract 5. For example, if a 1st-level character (with a proficiency bonus of +2) has a Wisdom of 15 (a +2 modifier) and proficiency in Perception, he or she has a passive Wisdom (Perception) of 14.