D&D 5E XGE rules on using thieves tool proficiency for finding traps

[jgsugden]

If a rogue wants to be good at Perception and Investigation, they get lots of skills, and two expertise at first level.

In AD&D, rogues were not good at these things until high level.
In 2nd edition, they had to choose which things they'd be good at - an finding traps was often one of the last things selected to advance.
In 3rd edition they had to take a bunch of skills to be able to do the role effectively.
In 4th edition, you pretty much obtained all of the skill suite in one skill pick, but 4E was really a different game.

The books could be clearer on this, but they are laid out in the DMG:

Detecting and Disabling a Trap
Usually, some element of a trap is visible to careful inspection. Characters might notice an uneven flagstone that conceals a pressure plate, spot the gleam of light off a trip wire, notice small holes in the walls from which jets of flame will erupt, or otherwise detect something that points to a trap’s presence.

A trap’s description specifies the checks and DCs needed to detect it, disable it, or both. A character actively looking for a trap can attempt a Wisdom (Perception) check against the trap’s DC. You can also compare the DC to detect the trap with each character’s passive Wisdom (Perception) score to determine whether anyone in the party notices the trap in passing. If the adventurers detect a trap before triggering it, they might be able to disarm it, either permanently or long enough to move past it. You might call for an Intelligence (Investigation) check for a character to deduce what needs to be done, followed by a Dexterity check using thieves’ tools to perform the necessary sabotage.

Perception to notice. Investigation to deduce what needs to be done. Thieves tools to disarm. Any of those might be optional based upon whether the underlying task is obvious (and thus passed by a passive score).

 

[iserith]

Perception to notice. Investigation to deduce what needs to be done. Thieves tools to disarm. Any of those might be optional based upon whether the underlying task is obvious (and thus passed by a passive score).

To be fair, it could be Perception or Investigation on some traps to resolve tasks related to noticing them. But without expertise in Investigation, you may as well have the wizard do this task instead. It's even safer to have the cleric do it, really, since Perception can be used to resolve tasks to notice a wider range of traps than Investigation.

 

[jgsugden]

I’m curious, could a player at your table skip a step, if they were willing to accept the risk inherent in trying to disable a trap they haven’t deduced the function of yet?

I can't answer for iserith, but that would depend upon the nature of the trap (in my game).

Naturally coming up with an approach to try and disarm a is going to be easier if you understand how it works, but if the party was in a hurry, or didn’t want to risk three wandering monster checks, would it be a valid move to skip Investigating a trap and just try something to disarm it? I’m imagining something like spotting a trip wire and deciding to just cut the wire (at the risk that doing so might actually spring the trap rather than disable it) without taking the time to try and figure out how it works. But maybe that doesn’t really resemble what play looks like at your table.

Great example. Walking it through:

DM creates a trip wire trap. The visible part is a thin wire (hard to see - Perception 17) that disappears into two tiny holes on the sides of the passage. Inside those walls is a mechanical mechanism. When the mechanism is triggered by a change in pressure on the line, it will break a glass vial, dropping a liquid into another liquid which will then explode outwards in a billowing cloud of poisonous gas.

One PC (in the front of the party) has a Passive perception of 17 which allows the PCs to detect the trap automatically. What they see is a wire extending across the passage at ankle height that disappears into two tiny holes, as well as note the presence of a bunch of tiny holes in the ceiling (from which the gas would appear). Their options at this point?

1.) Roll a perception check. Why do it? They might find more information. They might be able to see inside the hole and get a description, without conclusions, of what is inside. That might change how hard it is for them to deduce what it does.

2.) Roll investigation. Why do it? They want to know what the trap does if sprung, in case it springs when they decide to deactivate it, or someone else might trip it at a later date. The DC to deduce what it does should change based upon how much information they have. If they just see the wires and the holes, the DC should be on the harder side, but if they had a great perception roll and saw the mechanism and vials, the DC should be reduced - it is easier to deduce an answer when you have more information. The higher the roll, the more information the PCs get about the meaning of what they can see.

3.) Disable roll. Why do it? You never know when you'll have to fight your way back from a location. Why leave a trap active behind you that someone could set off while you're fleeing over it? The DC should start out at a certain point, but be lower the more you know about it (or, alternatively, you could allow advantage on a fixed DC).

4.) Step over it and leave it in tact. Unless the PCs had a truly horrible acrobatics, I would allow this with no chance of failure. In other words, DC 10 with passive acrobatics allowed. Unless they have a dexterity penalty, they can do it with no risk. With a penalty, they just need someone to help to be guaranteed to make it.

All of this is going to be subject to the DM's design decisions and the DM's interpretation, of course. The DM might decide the mechanism in the wall is simple, and easy to deduce, or that it is disguised and required a higher DC - and because of the disguise, a narrow failure will result in misinformation that will make the disarm DC harder.

Regardless, just as in real life: See it. Figure it out. Deal with it. 3 steps.

I have the sneaky suspicion that this makes a lot more sense to people that make wide use of passive Intelligence, Wisdom and Charisma skills. Unless there are unusual circumstances ongoing (such as intense time pressure due to combat, etc..), I consider the passive scores to always be applied to a situation in which they could be useful, and they also serve as a floor for any roll the PCs make. [Reliable Talent, for a rogue, then is useful for proficient Dexterity and Strength checks, as well as Charisma, Wisdom and Intelligence checks with unusual circumstances.]

 

[Charlaquin]

I can't answer for iserith, but that would depend upon the nature of the trap (in my game).
Great example. Walking it through:

DM creates a trip wire trap. The visible part is a thin wire (hard to see - Perception 17) that disappears into two tiny holes on the sides of the passage. Inside those walls is a mechanical mechanism. When the mechanism is triggered by a change in pressure on the line, it will break a glass vial, dropping a liquid into another liquid which will then explode outwards in a billowing cloud of poisonous gas.

One PC (in the front of the party) has a Passive perception of 17 which allows the PCs to detect the trap automatically. What they see is a wire extending across the passage at ankle height that disappears into two tiny holes, as well as note the presence of a bunch of tiny holes in the ceiling (from which the gas would appear). Their options at this point?

1.) Roll a perception check. Why do it? They might find more information. They might be able to see inside the hole and get a description, without conclusions, of what is inside. That might change how hard it is for them to deduce what it does.

2.) Roll investigation. Why do it? They want to know what the trap does if sprung, in case it springs when they decide to deactivate it, or someone else might trip it at a later date. The DC to deduce what it does should change based upon how much information they have. If they just see the wires and the holes, the DC should be on the harder side, but if they had a great perception roll and saw the mechanism and vials, the DC should be reduced - it is easier to deduce an answer when you have more information. The higher the roll, the more information the PCs get about the meaning of what they can see.

3.) Disable roll. Why do it? You never know when you'll have to fight your way back from a location. Why leave a trap active behind you that someone could set off while you're fleeing over it? The DC should start out at a certain point, but be lower the more you know about it (or, alternatively, you could allow advantage on a fixed DC).

For my part, I don’t generally allow sort of “naked” checks. The player might want to more closely examine something about the parts of the trap they can see, but they couldn’t just make a Perception or Investigation roll and hand wave the details of how they were examining it. Depending on what they do though, they might succeed at figuring out whatever they’re looking for without need of a check. That said, I think the general range of options you present here makes sense. Look to see if there are any visible parts you’ve missed; try to figure out how the trap works based on what you can see; try to disable the trap with only the information you have available to you.

4.) Step over it and leave it in tact. Unless the PCs had a truly horrible acrobatics, I would allow this with no chance of failure. In other words, DC 10 with passive acrobatics allowed. Unless they have a dexterity penalty, they can do it with no risk. With a penalty, they just need someone to help to be guaranteed to make it.

Yeah, a trip wire was maybe not the best example, since it’s pretty trivial to avoid once you’ve spotted it. I really only used it as an example because it presents a very obvious possible course of action to try and disarm it (cutting the wire), but that you wouldn’t be able to know if it would work without further investigation (maybe the trap only triggers on increased tension on the wire, or maybe it triggers with any change in tension on the wire.)

 

[jgsugden]

To be fair, it could be Perception or Investigation on some traps to resolve tasks related to noticing them. But without expertise in Investigation, you may as well have the wizard do this task instead. It's even safer to have the cleric do it, really, since Perception can be used to resolve tasks to notice a wider range of traps than Investigation.

I think that might depend upon what you mean. Perception allows you to notice things. Investigation allows you to draw information from what you have seen.

Perception allows you to identify the existence of something.

Investigation allows you to determine whether something is a clue as to the nature of a situation, or to deduce the overall significance of the raw information you've acquired.

Taking a gross example into play - Bob is a rolled 17th level PC. He rolled a 3 for Wisdom, and now has a 28 Intelligence due to several Tomes of Clear Thought. He also has the Observant Feat, and Expertise in Investigation.

His Perception is -4 (giving him a passive perception of 6). He is currently in dim light, so he has disadvantage on perception rolls and a Passive Perception of 1 for sight based situations. However, his Investigation is +21, and his Passive Investigation is 36. This allows him to deduce the nearly impossible. Essentially, he hardly sees a thing, but whatever information he comes across he can do superhuman levels of deduction around. This would be a PC where I'd give them a lot of information about the world not because they spotted it, but because they could see something else that was brutally obvious, and then deduce the existence of other things. They might not spot the trip wire - but they'd realize that the hallway was the perfect for one in the eyes of the designer given their other aesthetic choices.

 

[jgsugden]

For my part, I don’t generally allow sort of “naked” checks. The player might want to more closely examine something about the parts of the trap they can see, but they couldn’t just make a Perception or Investigation roll and hand wave the details of how they were examining it. Depending on what they do though, they might succeed at figuring out whatever they’re looking for without need of a check...
...

So you don't use passive perception? That would be a naked check, where the player does not specify where their attention is focused.

Putting it another way - a player is walking down a hall and there is a pressure plate trap. It has a base DC of 20 to notice. Do you not allow the PCs any chance to spot it unless they say they are searching the floor? They don't just 'notice' something unusual about a situation?

 

[Charlaquin]

So you don't use passive perception? That would be a naked check, where the player does not specify where their attention is focused.

Putting it another way - a player is walking down a hall and there is a pressure plate trap. It has a base DC of 20 to notice. Do you not allow the PCs any chance to spot it unless they say they are searching the floor? They don't just 'notice' something unusual about a situation?

Passive checks represent the average result of a task performed repeatedly over time. Keeping watch for danger is probably the most common example of an action a character might be performing continuously, so a passive Wisdom (Perception) check would be an appropriate way to resolve that. If a player declared that they were keeping watch for danger, I would compare their passive Wisdom (Perception) to the DC to find any hidden threats*, including traps, until they started performing a different activity. I wouldn’t describe that as a “naked check” though, since there is a clear goal and approach.

*sort of. I generally don’t pre-set DCs in the absence of specific actions being taken by the players, so there aren’t really “DCs to find threats” out there waiting to be compared to PCs’ passive Wisdom (Perception) scores. But, if the players do come across a trap and someone in the group is continually looking for danger, I’ll decide if the trap would be easy, moderate, or hard to find, and assign it a DC of 10, 15, or 20 respectively at that point.

 

[iserith]

I think that might depend upon what you mean. Perception allows you to notice things. Investigation allows you to draw information from what you have seen.

Perception allows you to identify the existence of something.

Investigation allows you to determine whether something is a clue as to the nature of a situation, or to deduce the overall significance of the raw information you've acquired.

Taking a gross example into play - Bob is a rolled 17th level PC. He rolled a 3 for Wisdom, and now has a 28 Intelligence due to several Tomes of Clear Thought. He also has the Observant Feat, and Expertise in Investigation.

His Perception is -4 (giving him a passive perception of 6). He is currently in dim light, so he has disadvantage on perception rolls and a Passive Perception of 1 for sight based situations. However, his Investigation is +21, and his Passive Investigation is 36. This allows him to deduce the nearly impossible. Essentially, he hardly sees a thing, but whatever information he comes across he can do superhuman levels of deduction around. This would be a PC where I'd give them a lot of information about the world not because they spotted it, but because they could see something else that was brutally obvious, and then deduce the existence of other things. They might not spot the trip wire - but they'd realize that the hallway was the perfect for one in the eyes of the designer given their other aesthetic choices.

So if you take a look at the various traps in the DMG (and maybe XGtE, but I don't have this open right now), for some traps, it basically boils down to an either/or Perception/Investigation to notice the trap. If it's an either/or, you are correct in that Investigation is about noticing some kind of clue that points to the trap. But then, that's effectively what Perception is resolving too, though to be fair, it's often about noticing different things (e.g. holes in the wall with Perception or presence of pressure plates with Investigation in the poison dart trap).

But this isn't all traps, so to optimize here, the party needs to put forward the character with the best Perception to garner the best chance of success at spotting traps. Then, if the DM requires them to figure it out before disabling it, put forward the character with the best Investigation. Then finally the character with thieves' tools proficiency. This could be one character, but that's a pretty specific build.

 

[TheAlkaizer]

The way I've done it since 5E is that certain tasks related to an ability require tools to be made with proficiency.

So, for example, a rogue that's proficient with dexterity ability checks that would try to snap something off someone's belt would do so with his ability modifier and proficiency modifier.

But the same rogue that would try to disarm a trap with an dexterity ability check would not get his proficiency bonus if he doesn't have tools; but would get it if he had the tools.

I never thought of using advantage in that situation though, I might try it.