I dislike non-stacking Dis/Advantage

[grarrrg]

and for an elf... this thing does me little good here'

That ties back into my point actually.

The people in the world should have a -good idea- of what their spells do. But they shouldn't know -EXACTLY- what their spells do, otherwise you break the immersion.

"does little good here" is not the same as "this is completely worthless".

[NichG]

That ties back into my point actually.


"does little good here" is not the same as "this is completely worthless".

That was me role-playing haughtiness, not being precise.

Training wheels help you balance when learning to ride a bike. That doesn't mean they'll be helpful to a professional cyclist.

[RSP]

My question is, WHY is knowing what kind of magic exists, and the fact they Charm you metagame knowledge? There's no reason to consider it metagame knowledge. There are very few spells that allow someone to mind control you that don't Charm you at the same time.

Knowing that a spell helps protect against mind assault is different than knowing it gives you Advantage on Saves. The former is an RP, in game thing; the latter is mechanics. If the PC knows about Advantage on Saves, then they know their actions are dictated by d20 rolls that can be manipulated, which is not how I expect the game to be played (nor how I enjoy it).

Further, per the RAW, Wis Saves from spells and the spells that give Advantage against them are generally not even noticeable:

“Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature’s thoughts, typically goes unnoticed, unless a spell says otherwise.”

So none of the PCs protected by IF know they’re protected (other than the caster saying “you’re protected” or some such). There is no knowledge imparted to the PC of “oh, I’m now getting Advantage on Int/WisCha Saves”. They don’t feel anything, RAW.

Beacon of Hope instills hope and vitality in those effected, but that likewise doesn’t mean the PC is suddenly aware of the Advantage mechanic.

Why wouldn't a character know about the effects of a spell. Yes, they are mechanics, but those mechanics have a tangible effect on the world.

Mechanics always have a tangible effect on the game world, yet characters have no knowledge of them. Why would spells be any different?

A d20 roll determines whether a character gets hit, the ensuing damage roll whether they die. Does that mean, since the character is affected by the mechanics that they’re now aware of them?

Not in my understanding of the game.

Sure, characters won't know everything in the PHB.
But when your party specifically researches a foe that likes to sling enchantments and charms, don't you think it'd make sense that they would learn what a lot of charm spells do, including Dominate Person?

Again, we didn’t learn that they use DP. Our previous experience with mind control was an aboleth which doesn’t use spells at all but a unique ability.

But again, this only matters if you assume Elves know about the Advantage mechanic with Charm, that PCs know the Advantage mechanic is used with DP, and that PCs know the rules the Advantage mechanic not stacking with other instances of the Advantage mechanic.

That’s a really big assumption on meta gaming knowledge being held by the PCs.

And why do you assume an Elf doesn't understand they have a resistance to charm? Hell, I'm a Human, I've never studied advanced biology, but I know that Humans have a natural resistance to a lot of substances that would harm a lot of animals. I also know that there is a limit to that resistance. Why wouldn't an Elf have learned similar things about Elves while growing up?

Do you understand the mechanics and nature of the interactions within your biology or do you understand the general theme of how your body works? Do you innately know, for instance, how your body will respond to a specific germ if that germ were introduced to your body? Do you innately know how your body will react if exposed to a specific bacteria?

I’d imagine not, unless you studied the interaction specifically with yourself (as not all people react the same to all instances of exposures - allergies for instance may play a part in responses).

So why would an Elf know anymore than “elves are generally tougher to charm than other humanoids”? Why would they instantaneously become knowledgeable in the Advantage mechanic and know that DP uses that same mechanic?

That’s a pretty bold conclusion to jump to, in my opinion.

[schm0]

You need to draw a line somewhere.
Unless someone wants to argue that the people in the game have figured out that their world runs on finite numbers and dice-rolls.

The people in the world should have a -good idea- of what their spells do. But they shouldn't know -EXACTLY- what their spells do, otherwise you break the immersion.


Cast charm on a human, it works most of the time, but not all of the time.
Cast charm on an elf, it fails most of the time, but not all of the time.
Cast a resist-charm spell on a human then cast charm, charm fails most of the time, but not all of the time.

Cast a resist-charm spell on an elf, then cast charm.
Charm fails most of the time. Does it fail more often? Or about the same?
Maybe it only had a 10% success rate on elves to begin with. How many times do I need to try to charm a resisted-elf before I'm sure the resist really doesn't do anything?

If I'm an elf that risks getting mind controlled then I'd still want to opt for the resist spell if able, just in case.

Do we have to draw a line in that case? The caster knows what a specific spell does. So let's use a specific example: charm person. The caster knows how to cast the spell, its effects, and that sometimes the target resists the effects of the spell. Whether the caster knows that elves are resistant to charms, and humans are not, is a matter of character background, personal knowledge (i.e. an Intelligence roll, likely Arcana), or the DM may just say this is common knowledge.

And if one to were try to "double up" on the elves natural resistance, well, the mechanics equate to a narrative equivalent in that case (assuming the caster knew about the elf's innate resistance to charms): that casting such a spell would have no effect.

In other words, the fact that dis/advantage doesn't stack translates into PC knowledge and is not metagaming.

[RSP]

In other words, the fact that dis/advantage doesn't stack translates into PC knowledge and is not metagaming.

Only if you’re assuming PCs innately know what is a “doesn’t double up” feature and what is not.

Bless isn’t a “doesn’t double up” feature but Beacon of Hope is. Where is that knowledge coming from? If someone doesn’t know the spell BoH, do they still innately know it’s a “doesn’t double up” spell? This sounds an awful lot like “they understand game mechanics” while trying to call it something else.

[grarrrg]

And if one to were try to "double up" on the elves natural resistance, well, the mechanics equate to a narrative equivalent in that case (assuming the caster knew about the elf's innate resistance to charms): that casting such a spell would have no effect.

"Because the mechanics say it doesn't stack, the (N)PC's automatically know it doesn't stack."
"But _HOW_ do they know it doesn't stack?"
"Because it doesn't."

Perfectly clear reasoning to me

[Elenian]

I mean obviously we can world-build in such a way as to accommodate 5e mechanics and explain them in a way that is available in-character. Eg:

Elves have a distinctive psychophysical property, called "ampendasáma" (lit. "uphill-mindedness") in the high tongue of old, that makes their minds difficult to manipulate. Ampendasáma has been studied extensively by elven loremasters, and is known to be wholly incompatible with (and far superior to) the state of "pahtasáma" (lit. "closed-mindedness"), possessed by certain lesser beings such as Svirfneblin. Magic, such as the spell Intellect Fortress can close the mind of a being - indeed the loremasters theorize that such magic was involved in the ancient creation of the deep gnomes - but only by suppressing their ampendasáma. Elves thus make little use of such magic, reserving it for only those situations (such as direct mental attack rather than subtle manipulation) where the state of ampendasáma is superior.

Or whatever.

But any such explanation is going to be kinda contrived and weird. In our world, different advantages tend to compound, and when we want to do something difficult, we seek out as many different advantages as possible. The fact that it mostly doesn't work that way in dnd-land, if we treat it diegetically as, like, a real feature of dnd physics and not just an abstraction in the game, absolutely can be an obstacle to roleplay and immersion.

[Keltest]

"Because the mechanics say it doesn't stack, the (N)PC's automatically know it doesn't stack."
"But _HOW_ do they know it doesn't stack?"
"Because it doesn't."

Perfectly clear reasoning to me

I mean, yeah? Observable phenomenon don't really need much justification for people knowing it.

[Segev]

"This spell makes you resist mind magics. Elves are already so hard to affect that this spell has no observed effect on their resistance."

Maybe the province of the 'stacking advantage to negate disadvantage' is in a buff of its own?

The Rod of Leverage
Rare magic item; requires attunement
This rod lets its attuned wielder leverage great advantage. If the attuned wielder is affected by more sources of advantage than sources of disadvantage on a relevant roll, he retains his advantage on the roll.

[RSP]

I mean, yeah? Observable phenomenon don't really need much justification for people knowing it.

What is observable in the game world in a character receiving advantage (a game mechanic not known in the game world) on a save that is also not a perceivable in-game event, from a spell that has no perceivable elements?

If a caster Subtle spells DP and the save is made the target of the spell wouldn’t even be aware they were targeted by the spell…how would they therefore know their innate resistance played a part in the spell failing, when they don’t even know a spell was cast or they were targeted and made a save?

What do you think is the “observable phenomenon” here?

"This spell makes you resist mind magics. Elves are already so hard to affect that this spell has no observed effect on their resistance."

It’s not just elves though, Gnomes and Yuan-Ti Pure Bloods would likewise not benefit from such spells, and they seemingly have a completely different “source” for their Advantage.

[Keltest]

What is observable in the game world in a character receiving advantage (a game mechanic not known in the game world) on a save that is also not a perceivable in-game event, from a spell that has no perceivable elements?

If a caster Subtle spells DP and the save is made the target of the spell wouldn’t even be aware they were targeted by the spell…how would they therefore know their innate resistance played a part in the spell failing, when they don’t even know a spell was cast or they were targeted and made a save?

What do you think is the “observable phenomenon” here?

Elves not being particularly susceptible to charms? Like, a charm working is a binary effect, it either works or it doesn't. Thats something you can observe. Do it a bunch, you collect data. From the data, you can reach conclusions.

Like, doing a thing a bunch and watching what happens is some of the basis of science and exploration. I'm not sure what point youre trying to make with your example. Yes, obviously if you go out of your way to hide the observable parts of a spell it will be harder to observe, but thats not at all related to the contention.

ETA: even at a really basic level, people can say "oh, yeah, after thirty or so casts of it i have never seen such and such creature get hit by Disintegrate. It must have a good ability to avoid it."

[JNAProductions]

Again-if magic and/or elves are brand new to the world, then I'd agree with RSP. There hasn't been time to study the effects, no body of knowledge would exist.
But, correct me if I'm wrong, RSP-this adventure is taking place in a relatively standard D&D world, where elves and magic have existed for a very long time.

[schm0]

Elves not being particularly susceptible to charms? Like, a charm working is a binary effect, it either works or it doesn't. Thats something you can observe. Do it a bunch, you collect data. From the data, you can reach conclusions.

Like, doing a thing a bunch and watching what happens is some of the basis of science and exploration. I'm not sure what point youre trying to make with your example. Yes, obviously if you go out of your way to hide the observable parts of a spell it will be harder to observe, but thats not at all related to the contention.

Not to mention, the magical nature of elves is well documented in the lore and is a nod to their fey origins. In almost every fantasy setting, elves are inherently tied to magic in some form or another. The fact that elves are resistant to charm effects is 100% lore-driven, and in turn that lore is expressed as a mechanic.

[Psyren]

Per DMG 239 you can override prevailing conditions and impose advantage or disadvantage if you really want to. So if the player has 20 sources of disadvantage and one source of advantage, you aren't shackled to neutrality if you don't think that makes sense. That's just the default rule to keep things simple. At the end of the day, the system puts the DM in charge, there's nothing that needs to be altered on the page.

[Theodoxus]

Per DMG 239 you can override prevailing conditions and impose advantage or disadvantage if you really want to. So if the player has 20 sources of disadvantage and one source of advantage, you aren't shackled to neutrality if you don't think that makes sense. That's just the default rule to keep things simple. At the end of the day, the system puts the DM in charge, there's nothing that needs to be altered on the page.

True, however I don't know if RSP is coming at this as a DM or a player. A player could certainly make a case that "hey, there's a lot of advantage going on here, most of it magical, any chance we can get a boon for the elf to not be dominated". But it's still up to the DM (who might not fully grasp just how AO-like they actually are and can bend or break the Rules of the Universe (or at least freely utilize the 'rule of cool') to make the game more fun for everyone) to adjudicate.

Having known some pretty combative DMs, even making such a nuanced request (with possibly using the Rules Lawyer technique of #BookAndPage) could backfire. It's definitely a 'know your audience' type deal.

The general advice though is spot on.

[stoutstien]

Per DMG 239 you can override prevailing conditions and impose advantage or disadvantage if you really want to. So if the player has 20 sources of disadvantage and one source of advantage, you aren't shackled to neutrality if you don't think that makes sense. That's just the default rule to keep things simple. At the end of the day, the system puts the DM in charge, there's nothing that needs to be altered on the page.

Aye it's an often missed stop loss for nonsense like 50 advantages getting cancelled by one instance of disadvantage. The point is to be able to eyeball it rather needing a bean counter but old habits die hard.

[Segev]

Per DMG 239 you can override prevailing conditions and impose advantage or disadvantage if you really want to. So if the player has 20 sources of disadvantage and one source of advantage, you aren't shackled to neutrality if you don't think that makes sense. That's just the default rule to keep things simple. At the end of the day, the system puts the DM in charge, there's nothing that needs to be altered on the page.

It actually says the opposite. It points out that if there are multiple conditions going both ways, they cancel out to a normal roll. No option is listed to disregard that. The closest it comes is that a DM could choose to pretend not to see any of the sources of disadvantage in order to grant advantage, or vice-versa. That isn't overriding prevailing conditions, though; that is more akin to 'prosecutorial discretion' in that he can choose to pretend the conditions he wants not to interfere do not exist.

[Psyren]

It actually says the opposite. It points out that if there are multiple conditions going both ways, they cancel out to a normal roll. No option is listed to disregard that.

That's the normal case, but you can still decide otherwise:

"Characters often gain advantage or disadvantage through the use of special abilities, actions, spells, or other features of their classes or backgrounds. In other cases, you decide whether a circumstance influences a roll in one direction or another, and you grant advantage or impose disadvantage as a result."

And of course, there's rule zero on top of that if the DM really feels that the 20 sources of advantage shouldn't be overridable.

[Segev]

That's the normal case, but you can still decide otherwise:

"Characters often gain advantage or disadvantage through the use of special abilities, actions, spells, or other features of their classes or backgrounds. In other cases, you decide whether a circumstance influences a roll in one direction or another, and you grant advantage or impose disadvantage as a result."

And of course, there's rule zero on top of that if the DM really feels that the 20 sources of advantage shouldn't be overridable.

While I agree rule zero is in full force here, I think this is only rule zero you're invoking. The rules explain how to apply the decision of something influencing the roll, and to disregard influences is not really following the rules so much as it is pretending to be unaware so you can bend the rules, akin to refusing to ac knowles a nomination of a candidate you don't want to let people vote on by pretending you didn't hear the nomination.

Rule zero is really what applies to let a DM throw out the neutralization rule if he thinks it is silly in particular case.

[RSP]

Elves not being particularly susceptible to charms? Like, a charm working is a binary effect, it either works or it doesn't. Thats something you can observe. Do it a bunch, you collect data. From the data, you can reach conclusions.

Like, doing a thing a bunch and watching what happens is some of the basis of science and exploration. I'm not sure what point youre trying to make with your example. Yes, obviously if you go out of your way to hide the observable parts of a spell it will be harder to observe, but thats not at all related to the contention.

ETA: even at a really basic level, people can say "oh, yeah, after thirty or so casts of it i have never seen such and such creature get hit by Disintegrate. It must have a good ability to avoid it."

Who is doing this research? I’m not sure anyone does this on a magnitude that it could be considered valid research. So, first off, I’m not aware of any premade world that deals in this kind of stuff. We play in FRs, and though I’m not an expert on the world, I’m unaware of any such “Johnny Wizard released his latest finding on magical research…”

Furthermore, I’m not sure it’s even something that would come up. First off, anyone going around and regularly using Charm to magically affect people probably isn’t advertising that they’re doing so, nor sharing their results.

Secondly, there’s an issue in that anyone pulling this off on the regular probably has a decent DC on their spells, whereas NPC commoners do not have a good Wis Save (+0). So if rolling against, say a 16 DC, your average commoner will resist 25% of the time. An Elf would have about a ~43% chance to resist (feel free to check my math, I’m not sold on it being accurate).

Yes that’s statistically significant, but you would need to run this on hundreds of elves vs a control group of hundreds of non-resistant races to get those numbers, which still only represent a marginal ability to resist Charms: more elves are still succumbing to Charms than resisting it. Plus, if running it elves vs non-elves, you’re data will be less significant as you’ll have gnomes and Halflings throwing off the numbers.

Do you see this kind of research occurring in your fantasy worlds? Do you see this being done in FR or other worlds where it’s common knowledge what the percentages are of what races resist what effects?

Again-if magic and/or elves are brand new to the world, then I'd agree with RSP. There hasn't been time to study the effects, no body of knowledge would exist.
But, correct me if I'm wrong, RSP-this adventure is taking place in a relatively standard D&D world, where elves and magic have existed for a very long time.

Again, what studies? What “body of knowledge” exists that’s saying “Elves are 40% more likely to resist Charms, but still are less likely to resist than succumb to said effects”.

And then, who’s running the follow on study in which they do valid experiments seeing if Beacon of Hope or IF overlap with that natural elven resistance?

Not to mention, the magical nature of elves is well documented in the lore and is a nod to their fey origins. In almost every fantasy setting, elves are inherently tied to magic in some form or another. The fact that elves are resistant to charm effects is 100% lore-driven, and in turn that lore is expressed as a mechanic.

By why are we assuming it’s PC knowledge of the mechanic that defines that “resistance”? Why are we assuming it’s PC knowledge that IF uses a similar mechanic? Why are we assuming it’s PC knowledge that those mechanics don’t overlap?

[JNAProductions]

Did people, before modern-day research principles and knowledge sharing became a thing, just not know anything?

Look, I get it. People are stupid. That includes you and me and everyone else participating in this thread.
But people are ALSO smart! Look at the kinds of things real-life people did in ancient times. Largely architectural, sure, but still amazing.

If you want to roleplay your PCs as being unaware of the world, that's fine. But don't say that people who play their PCs as knowing facts about their world aren't roleplaying.

[RSP]

Did people, before modern-day research principles and knowledge sharing became a thing, just not know anything?

Look, I get it. People are stupid. That includes you and me and everyone else participating in this thread.
But people are ALSO smart! Look at the kinds of things real-life people did in ancient times. Largely architectural, sure, but still amazing.

If you want to roleplay your PCs as being unaware of the world, that's fine. But don't say that people who play their PCs as knowing facts about their world aren't roleplaying.

Except you’re stating they know facts about the metagame mechanics of the game; that is not the same thing as saying the PCs know facts about the game world.

Again, I don’t think 5e presupposes that PCs (or NPCs) have metagame knowledge of the mechanics of the game system.

__________

More over, I think it’s an apt criticism that Advantage is a lazy mechanic that’s arbitrarily used to “simplify” a small portion of the rules, while the system generally allows for plenty of fiddly compiling of enhancements on enhancements, whether numeric or not.

The fact that the in-game characters are oblivious to the which enhancements “stack” and which don’t, only builds off the fact that it’s a lazy, arbitrarily applied mechanic.

[JNAProductions]

The fact that the in-game characters are oblivious to the which enhancements “stack” and which don’t, only builds off the fact that it’s a lazy, arbitrarily applied mechanic.

Why would they be oblivious to it, though?
You are stating that as fact but ignoring any argument to the contrary.

Wood and iron-you can make a sword out of both. Do you need to know the knowledge of molecular structures to understand that iron makes a better sword?

[RSP]

Why would they be oblivious to it, though?
You are stating that as fact but ignoring any argument to the contrary.

Wood and iron-you can make a sword out of both. Do you need to know the knowledge of molecular structures to understand that iron makes a better sword?

Why would they be oblivious to the game mechanics? Is that what you’re asking?

If so, it’s because they’re characters in a make believe world that are assumed not to know about the fact that they’re characters in a made up world…

[NichG]

Except you’re stating they know facts about the metagame mechanics of the game; that is not the same thing as saying the PCs know facts about the game world.

Again, I don’t think 5e presupposes that PCs (or NPCs) have metagame knowledge of the mechanics of the game system.

__________

More over, I think it’s an apt criticism that Advantage is a lazy mechanic that’s arbitrarily used to “simplify” a small portion of the rules, while the system generally allows for plenty of fiddly compiling of enhancements on enhancements, whether numeric or not.

The fact that the in-game characters are oblivious to the which enhancements “stack” and which don’t, only builds off the fact that it’s a lazy, arbitrarily applied mechanic.

It's a fact about the game world that Intellect Fortress provides no defenses to an elf against being charmed that are better than what they have innately. It's also a mechanic. You can look for justifications to not know it, but if you do so, you shouldn't get bent out of shape when not knowing it means you make a wasteful decision in-character. There are plenty of justifications to know it too without 'oh that means that characters have read the rulebooks and know that HP is a thing' extremes.

It's like all of the 'choose differently' things with regards to when RP interferes with things. The books don't say how mages invented Enchantments. You could assume the answer that makes the RP and mechanics conflict the most, or assume the answer that makes the RP and mechanics conflict the least. If you happen to choose the one that makes it conflict more and then complain about losing immersion, well, you had a solution to that - you could have picked the way of imagining the world that explains the thing you found odd, rather than which makes it seem incoherent.

[JNAProductions]

Why would they be oblivious to the game mechanics? Is that what you’re asking?

If so, it’s because they’re characters in a make believe world that are assumed not to know about the fact that they’re characters in a made up world…

Why would they be oblivious to the facts of their world?

No, there wouldn't be an EXACT knowledge of how it works-they don't know that elves roll 2d20b1 for saves against Charms by default, while dwarves roll 1d20 in the same circumstances.
But mages throughout history would've observed that elves are more resistant to Charm effects than dwarves.

[RSP]

It's a fact about the game world that Intellect Fortress provides no defenses to an elf against being charmed that are better than what they have innately.

IF provides protection to an elf, though, in plenty of ways. The only way to know that it doesn’t provide protection to an elf, is if you had those hundreds of characters to specifically test IF on.

Furthermore, what PCs are aware of such tests/results? Does IF come with a stipulation of it not overlapping with Beacon of Home, Gnomish heritage, Elven Ancestry, etc, while stipulating it works in conjunction with Bless, Resistance, Rings of Protection, Cloaks of Protection, etc?

Does it change your perspective that I’m playing a Sorcerer and therefore there is no formal training on the spells he knows, or do you assume he obtains innate knowledge of what does or doesn’t stack with his Elven nature simply by being?

You can create all these things in your game world, but as far as I’m aware, they are not components of FR or any of the pre-made worlds.

And again, outside of a formal list of “here are the magical and mundane effects that do not overlap” (however you create said list in your world), knowledge of the mechanic is needed to know what does or doesn’t stack with Advantage.

[GooeyChewie]

Why would they be oblivious to the game mechanics? Is that what you’re asking?

Characters in-world would be oblivious to the mechanics by which we play the game, that’s true.

But characters in-world would not necessarily be oblivious to the in-world ramifications of those mechanics.

A character knowing that spell gives you similar mental fortitude as an elf - which doesn’t help you at all if you are already an elf - seems like a reasonable thing for a person to know, if that person lives in the world that has such spells and elves in it.

[RSP]

Why would they be oblivious to the facts of their world?

No, there wouldn't be an EXACT knowledge of how it works-they don't know that elves roll 2d20b1 for saves against Charms by default, while dwarves roll 1d20 in the same circumstances.
But mages throughout history would've observed that elves are more resistant to Charm effects than dwarves.

Why are those “mages throughout history” sharing how they charmed people? If Elves were flat out immune, than I’d imagine it comes up more often, but it’s very possible that out of the 50 creatures a certain Wizard Charmed over the course of their lives, only 5 are elves, and all of them were successfully charmed (particularly if they were powerful enough to cast DP). Therefore, they’d not even comment on Elven resistance as it was a non-factor for them. Much less, I’m sure, have those Wizards tried to Charm characters they knew were under the effects of IF, and did so enough to note the probabilities of how often IF actually had an affect on whether their Charm took hold (they would, after all, still be succeeding on Charms more often than not); then they’d still have to compare total numbers of elves Chatmed bs non-Elves (knowing to remove gnomes, Yuanti, Halflings, or anyone under the effect of a spell that grants advantage to charm saves, in order to conclude “oh IF doesn’t work with Elves.

This all also assumes that every creature categorized as an Elf in a given world has Fey Ancestry, which I don’t think the rules actually state is the case.

But no, I don’t think it would be common knowledge that IF doesn’t help Elves against Charm.

[schm0]

By why are we assuming it’s PC knowledge of the mechanic that defines that “resistance”? Why are we assuming it’s PC knowledge that IF uses a similar mechanic? Why are we assuming it’s PC knowledge that those mechanics don’t overlap?

The trait is called fey ancestry, a relic from their eladrin ancestors. It's 100% a lore-based, in-character trait that elves and many others are aware of. If you want to put it up to an Arcana check, that's your choice, but the fact is that resistance to charms is a trait that all elves share and very much within the possibility that something a PC would know.

We can safely assume the PCs that cast spells know how magic works, as well. That includes understanding that certain spell effects do not stack. If the mechanics for casting spells or gaining dis/advantage works differently in your world, then that becomes the knowledge that the PCs know.

What the PCs don't know is meta information. For example, that advantage means you roll two dice and take the highest. Or +5 to your passive score. The numbers on your character sheet. What they do know narrative effects of their actions, such that the advantage mechanic equates to a small boon, allowing them to increase the likelihood that they succeed at something.