Question about Metagaming

[animorte]

Obviously, you can never erase what's inside a players head to enforce them acting only on character knowledge (muahahah. That would be an evil genius thing to create though).

You're onto something here. I wish I could put on this here Headband of Immersion every once in a while so I could appreciate my PC and the environment better.

For many people it would honestly probably make much better decisions (naturally there are always exceptions). Anybody ever put themselves in that survival mindset? This could be playing any type of game and think, "What would I really do if put in this situation and legitimately trying to stay alive." Makes me think of Jumanji.

[Jay R]

"Put that book down and make a relevant Knowledge check."

[Witty Username]

Last night I was describing a monster that the player characters had never seen before. One of the players, who recognized it based on the description, immediately pulled out the monster manual and passed it around for the other players to read its entry.

I am curious, what would you have done in that situation?

This during combat?
I think I would be more miffed about the play grinding to a stop depending on how long they took.

My playgroup tends to prefer looking at book after successful Int checks. And uses a decent chuck of homebrew monsters, so it doesn't come up as much.

[Talakeal]

Ok, so now that I have some responses:

What I did was say to the players "Guys, please cut the meta-gaming," in as mild and neutral a voice as I could.

They put the book away and didn't bring it up again, but they gave me some pretty dirty looks as they did so.

[Curbludgeon]

Leaning into Knowledge checks for monster identification, tweaking numbers, and reminders about the separation between player and character knowledge is one thing, while performatively confiscating books to return after class is another. Unless one is playing with one's own children the GM is not the parent of the players; defining the power dynamic at the table in such a way is not only patronizing, but is frequently indicative of an ulterior motive best removed from the playspace.

[animorte]

Leaning into Knowledge checks for monster identification, tweaking numbers, and reminders about the separation between player and character knowledge is one thing, while performatively confiscating books to return after class is another. Unless one is playing with one's own children the GM is not the parent of the players; defining the power dynamic at the table in such a way is not only patronizing, but is frequently indicative of an ulterior motive best removed from the playspace.

Shorthand: They can’t sift through books if books are not present.

[kyoryu]

So the problem around metagaming (when it comes to creature stats) comes down to a simple thing: It's harder to fight creatures if you don't know what they do.

In some cases the difference is minor - an orc is an orc, knowing its exact AC doesn't impact that much. In some cases, the difference is major - at the extreme end you get creatures that are basically invulnerable unless you know their weakness, at which point they become trivial.

So as a GM you have two options, really: You balance the encounters assuming the players do know the stats, or you balance them knowing that the players don't know the stats.

I prefer balancing the game presuming they know the stats. I'll pretty much die on this hill, for a few reasons:

1. Some players will know the stats from having seen the creature before. Presuming they know the stats makes this a non-issue. If you presume they don't know, then you either let the encounter be trivialized or you play the weird game of "do enough dumb things to get permission to do smart things", which isn't really that much fun for most people.

2. If you presume that the players do know the stats, then encounter design becomes more about how to deal with it, which starts to get into interesting tactics, timing, etc., rather than "guess the weakness". I personally prefer this style of game.

3. If players don't know the stats, it's pretty easy to give them sufficient descriptions of the stats - plus knowledge checks, etc. are a thing.

Not playing the "hidden information" game with monster stats just solves so many problems.

[Thrudd]

So the problem around metagaming (when it comes to creature stats) comes down to a simple thing: It's harder to fight creatures if you don't know what they do.

In some cases the difference is minor - an orc is an orc, knowing its exact AC doesn't impact that much. In some cases, the difference is major - at the extreme end you get creatures that are basically invulnerable unless you know their weakness, at which point they become trivial.

So as a GM you have two options, really: You balance the encounters assuming the players do know the stats, or you balance them knowing that the players don't know the stats.

I prefer balancing the game presuming they know the stats. I'll pretty much die on this hill, for a few reasons:

1. Some players will know the stats from having seen the creature before. Presuming they know the stats makes this a non-issue. If you presume they don't know, then you either let the encounter be trivialized or you play the weird game of "do enough dumb things to get permission to do smart things", which isn't really that much fun for most people.

2. If you presume that the players do know the stats, then encounter design becomes more about how to deal with it, which starts to get into interesting tactics, timing, etc., rather than "guess the weakness". I personally prefer this style of game.

3. If players don't know the stats, it's pretty easy to give them sufficient descriptions of the stats - plus knowledge checks, etc. are a thing.

Not playing the "hidden information" game with monster stats just solves so many problems.

There's a place for "hidden information" encounters, it can be fun for an initial encounter, but it can't be a permanent situation - unless your game is structured as a "monster of the week" type thing, where there's a new custom monster to learn about and fight in each session. The first time they run into a new creature, it's scary, they're figuring out how it works. Subsequent encounters with the creature type involve strategizing how best to deal with its strengths and exploit weaknesses.

I wouldn't expect this to work for any creature that's published in a book, however, unless it's unique to a module that I've confirmed nobody at the table has looked at (which is the only way I'd run a module in the first place). Using published information means someone at the table probably knows what it is, and whatever they remember about it is fine, and their characters' knowledge skills can probably fill in the rest of the gaps if it's really important. I agree that it is stupid and pointless to expect players to pretend they don't know how a monster works, risking the lives of their characters until some arbitrary point when they think they've RP'd their ignorance long enough and just happen to "discover" it's weakness.

If you want some mystery, you need to create a custom creature so you know nobody at the table has seen it before. Finding out information about the creature could be a fun element of the game that will interact with skills the characters possess and also allow real-time problem solving and experimentation (so long as it isn't so powerful/horrible that you murder them all immediately if they guess wrong the first time).

[Quertus]

So the problem around metagaming (when it comes to creature stats) comes down to a simple thing: It's harder to fight creatures if you don't know what they do.

In some cases the difference is minor - an orc is an orc, knowing its exact AC doesn't impact that much. In some cases, the difference is major - at the extreme end you get creatures that are basically invulnerable unless you know their weakness, at which point they become trivial.

So as a GM you have two options, really: You balance the encounters assuming the players do know the stats, or you balance them knowing that the players don't know the stats.

I prefer balancing the game presuming they know the stats. I'll pretty much die on this hill, for a few reasons:

1. Some players will know the stats from having seen the creature before. Presuming they know the stats makes this a non-issue. If you presume they don't know, then you either let the encounter be trivialized or you play the weird game of "do enough dumb things to get permission to do smart things", which isn't really that much fun for most people.

2. If you presume that the players do know the stats, then encounter design becomes more about how to deal with it, which starts to get into interesting tactics, timing, etc., rather than "guess the weakness". I personally prefer this style of game.

3. If players don't know the stats, it's pretty easy to give them sufficient descriptions of the stats - plus knowledge checks, etc. are a thing.

Not playing the "hidden information" game with monster stats just solves so many problems.

As much as I agree with you, and as much as I think that this is absolutely the correct answer for the OP, I still have to point out that it’s a false dichotomy, in that you can also, for example, not plan either way / not care either way. Which (other than trying to discourage new players from learning and metagaming *everything* (so that they can experience Exploration of the Unknown, to form an educated opinion on their preferences of Fun)) is pretty much the way I play the game. With things that should definitely feel familiar to the characters being pulled from common media, things that definitely should feel unknown being pulled from my ***, and the huge bulk of everything else getting a big shrug as they’re pulled straight from the bestiary.

And then, because I’m a mad scientist (praise Tzeentch), I’ll meddle with Far too much, and make gelatinous werewolves and two-headed vampires with rolled stats and rolled HP, or a Necron ship with a catapult tied to it, because Orcs overheard someone talking about “catapulting” a ship into the warp without its crew, believed it, and therefore did it. Or whatever other bit of crazy strikes my fancy at the time while being true to the Simulation.

[Jay R]

I warn my players in advance that they cannot completely trust the Monster Manual. Here is the relevant paragraph from my Introduction to the game:

Spoiler: Excerpt from "Introduction to Gaea and Creation of PCs"

Show

DO NOT assume that you know everything about fantasy creatures. I will re-write some monsters and races, introduce some not in D&D, and eliminate some. What’s in the Monster Manual is what most people believe about these monsters, and most of it is true. The purpose is to make the world strange and mysterious. It will allow (require) PCs to learn, by trial and error, what works. Most of these changes I will not tell you in advance. Here are a couple, just to give you some idea what I mean.

1. Dragons are not color-coded for the benefit of the PCs.
2. Of elves, dwarves, gnomes, halflings, kobolds, goblins, and orcs, at least one does not exist, at least one is slightly different from the books, and at least one is wildly different.
3. No sentient race has only one alignment; free will is not a human-only trait. The alignment given in the books will be the most common for that race near where you are, but the presence of Neutral Evil goblins near your village does not mean that there aren’t perfectly Lawful goblin farm communities far to the south, or an advanced city of Good goblins on another continent. Most specifically, babies aren’t Evil, regardless of race.
4. The name of an Undead will not automatically tell you what will or won’t hurt it. I won’t change most Undead; knowledge has value. But I reserve the right to include a few surprises.
5. The first time you see a member of a humanoid race, I will describe it as a “vaguely man-shaped creature.” This could be a kobold, an elf, or an Umber Hulk unless you make a Knowledge roll or otherwise learn what they are.

This description is intentionally written to make them believe that things are changed more than they are. Most monsters aren’t changed at all. Dragons aren’t color-coded; elves don’t exist (yet); goblins are very different (semi-bestial creatures who fight with abandon and poor tactics, and cannot work metal). And I won't use ordinary skeleton rules when I bring in my triceratops skeleton mini. Very few other changes have been made.

But the fact that they had this description, and that the first non-humans they fought were different from the description (goblins raiding the village smithy looking for metal weapons) keeps them from believing in the books too much.

I don't expect these players to do that anyway. They are enjoying the game as it is. But this passage in my introduction would give me free rein to change the monsters’ abilities if they started reading the books during combat.

[kyoryu]

As much as I agree with you, and as much as I think that this is absolutely the correct answer for the OP, I still have to point out that it’s a false dichotomy, in that you can also, for example, not plan either way / not care either way. Which (other than trying to discourage new players from learning and metagaming *everything* (so that they can experience Exploration of the Unknown, to form an educated opinion on their preferences of Fun)) is pretty much the way I play the game. With things that should definitely feel familiar to the characters being pulled from common media, things that definitely should feel unknown being pulled from my ***, and the huge bulk of everything else getting a big shrug as they’re pulled straight from the bestiary.

I think if you are running a more "naturalistic" game where you're not tailoring encounters, you're probably already fairly well on the "assume they know" side of things, because generally those types of games tend to have more monster re-use anyway.

Even if you want to have players have some level of discovery, I still think it's better to presume that they either will have or quickly get the knowledge. Figuring out how to use your tools to deal with the specialness of the critter is generally more interesting than discovering the specialness. And if it's still a fun encounter if you know the gimmick (which is really kinda the core idea here, if you think about it), then it'll still be a fun encounter if they have to figure it out. It's a win/win.

[deadman1204]

I would call the player out on that, then immediately change the stats of the monster. However not simply make them different but bothersome. Like an unexpected resist (ice monster that is immune to fire for example), or if the party is mostly melee, give it extra reach and free disengage. Teach them that cheating has consequences.

The game loses alot of flavor when players know all the monsters.

[gbaji]

I would call the player out on that, then immediately change the stats of the monster. However not simply make them different but bothersome. Like an unexpected resist (ice monster that is immune to fire for example), or if the party is mostly melee, give it extra reach and free disengage. Teach them that cheating has consequences.

The game loses alot of flavor when players know all the monsters.

Eh. I don't know if changing the monster stats/abilities after such an event is a good choice as a GM though. It's going to feel punitive (which it is), and it may lead to the players not trusting the GM over time as a result (ie: If I do something as a player that the GM doesn't like, he'll punish my character for it). Responding to metagaming with more metagaming (which is what you'd be effectively doing) isn't going to be received well. And what is this teaching the players with regard to regular player knowledge? You really don't want a whole table actively disguising their decisions and reasons for those decisions out of fear that if they "get caught" they'll be punished in some way. Some players might take this as a metagame challenge to try to learn and memorize as much of the MM as possible, and then pretend not to know things "just enough" to "get away with it". As a GM you want to avoid making the game about "the players vs the GM" to as great a degree as possible.

I'd just let them know that's not cool to do, and that they are to rely on their character knowledge with regard to creature stats/abilities, but let it slide and move on. Hopefully, assuming I'm sitting at the same table, I spotted the reach for and flipping through the book and stopped them before they got too far anyway. If not, it's still a better approach because you're presenting "rules at my table" to be followed. There's no "me vs your characters". It's "these rules exist to ensure fair play for everyone". Which I think most players will respect. Once the players learn to not just respect you, but to trust you as a GM to be fair and honest with them, you'll find that you just don't have these problems. They'll learn that you will provide them with the information they need for the encounter based on actual character knowledge and abilities *and* with an eye towards a good, fun, well balanced game. They'll learn that you aren't just there to trick them into making mistakes (yes, I've seen GMs who think this is a load of laughs, which is just sad). And ultimately, your game will run smoothly and the players will have a great time.

[Talakeal]

For the record, my OP was not about a combat encounter.

[Quertus]

For the record, my OP was not about a combat encounter.

If it was something out of the Monster Manual, and therefore clearly a monster and worth XP, and it wasn’t a combat encounter, then your murderhobos are clearly defective. You should get them looked at.

And if it wasn’t a Combat encounter, all the more reason to give them the time to read all they want, rather than being a killjoy to what they find fun.

[Talakeal]

If it was something out of the Monster Manual, and therefore clearly a monster and worth XP, and it wasn’t a combat encounter, then your murderhobos are clearly defective. You should get them looked at.

And if it wasn’t a Combat encounter, all the more reason to give them the time to read all they want, rather than being a killjoy to what they find fun.

Forgive me for saying essentially the same thing in two threads at the same time, but couldn't you use this logic to denounce any sort of limitation on player behavior?

Like right now one of my players is blatantly cheating, and all the other players are aware of it and roll their eyes at it. Is it ruining their fun for me to say something about it? How many players have to be in on the cheating before I have the right to say something? If it is unanimous, am I still ruining their fun if I stop trying to make mechanically balanced or challenging scenarios because they are pointless in the face of cheating?

To me, the scenario in the OP is essentially posting spoilers on the internet, it is ruining the surprise for the rest of the group, and makes me wonder why I put all the effort into crafting the mystery in the first place.

[NichG]

Again though, if you want the properties of a monster to be mysterious, the book of monsters that experienced gamers will have seen in a dozen forms over decades isn't generally going to be a good source for that. Make something up that is completely new and not printed anywhere, and its not even possible to 'cheat' this way.

[Talakeal]

Again though, if you want the properties of a monster to be mysterious, the book of monsters that experienced gamers will have seen in a dozen forms over decades isn't generally going to be a good source for that. Make something up that is completely new and not printed anywhere, and its not even possible to 'cheat' this way.

Yeah. But my current group is 2/3rds new players, and it would be nice to teach them about the game world organically without the veterans blurting out spoilers.

[NichG]

Yeah. But my current group is 2/3rds new players, and it would be nice to teach them about the game world organically without the veterans blurting out spoilers.

Well again, if the game world isn't just about the stuff in the Monster Manual, you can absolutely do this. And you can use the Monster Manual when you want to create the experience of 'I don't know what this is, but other people seem to be familiar with it', which is also a certain kind of feel that can be interesting to evoke. It just seems easy enough to control on the DM side of things that it isn't really worth getting bent out of shape about it on the player side of things.

[Mechalich]

Yeah. But my current group is 2/3rds new players, and it would be nice to teach them about the game world organically without the veterans blurting out spoilers.

Hmm, so one thing, with regard to new players (or even experienced players encountering new obscure monsters) is that they are often not satisfied with verbal descriptions and want to see an image of the monster. This leads to an impulse to open the MM to reveal the artwork, which regrettably segues into staring at the entry. Personally, I always printed out images of the monsters to be used in a session so that the players would have access to pictures completely sans stats to avoid this sort of temptation.

[Leon]

Depends if the players are able to differentiate Player knowledge with Character knowledge. If you suspect that they cant or wont then you alter it to suit your needs.

I know for a fact that the Dragon we fought in the finale of the first 5e game i was in had something that was giving it resist fire because it didn't blink when several Fireballs were thrown at it but it went hell for leather after those who were using Lightning on it (after i mentioned from 3e that Whites were weak to fire and the DM cleared that one up and obs decided to further add to it with a effect)

Same campaign i almost caused a TPK because of assumed knowledge being different between editions, turns out that WotC thought it would be neat for Lycanthropes to be flat out immune to damage instead of just very resistant if you didn't have the ways around it and not many low level groups would be expected to have those means given costs and scarcity of items, necklace of fireballs saved us (and the virtue of the DM to have put a quality magic item in instead of the joke item that was normally meant to be where it was) As it was the TPK almost happened less from assumed knowledge and more from wandering off alone and finding something too big to handle and having chase my character back to all lycanthropes were and thus making them angry.

Current game has us up against a Werewolf and while we are of a nature that it wont be a problem from a gear perspective even before we found out its weaknesses in character one player was haranguing the local church for silver weapons, im fully expecting this werewolf to be nonstandard even without that bit of metagaming

[Vahnavoi]

I agree that it is stupid and pointless to expect players to pretend they don't know how a monster works, risking the lives of their characters until some arbitrary point when they think they've RP'd their ignorance long enough and just happen to "discover" it's weakness.

Several different posters have tripped on this, so it's worth saying something about:

It is neither stupid nor pointless. Being able to pretend you don't know something is very basics of acting, and hence also roleplaying. Being able to understand that someone is pretending to be ignorant is also key and basic. People need to be able to do these things if they ever want to portray characters that aren't 1:1 with themselves.

If there are new players at a table, this is something you want to explain to them: that sometimes, your role is of someone who knows less than you do, which entails sometimes the correct action to take in the game is something you would not do normally. Even if it means taking greater risks or outright failing a game objective (such as keeping a character alive). The role where you play to upper limit of your own knowledge is specific and not fit for every kind of game.

Not everyone can do this. For example, small children with weak theory of mind can't make a distinction between what they know and what others know. For a similar reason some people struggle with the notion that, say, a movie character might not know everything the audience does, or that an actor might be different from a character they're portraying. If you're playing with such people, then expecting them to pretend is unrealistic. With everyone else, though? You should explain it, expect it AND enforce it so that the players get the hint.

[GloatingSwine]

Several different posters have tripped on this, so it's worth saying something about:

It is neither stupid nor pointless. Being able to pretend you don't know something is very basics of acting, and hence also roleplaying. Being able to understand that someone is pretending to be ignorant is also key and basic. People need to be able to do these things if they ever want to portray characters that aren't 1:1 with themselves.

Yeah, but this also means that they have to metagame even harder.

Because they have to figure out N plausible but wrong actions (where N is "a number that will satisfy the DM so they don't complain about metagaming") before they do the right action. So they still have to use their player knowledge, they just have to use it in a way that ensures they avoid certain actions because those actions are "correct".

Which is much more annoying to actually play.

And can be avoided by simply not treating published monsters as puzzle monsters. If it's a puzzle monster make it up for the purposes of the puzzle (as long as your group is primed to accept that sort of encounter which history suggests Takaleal's is not).

[Vahnavoi]

Yeah, but this also means that they have to metagame even harder.

And? This isn't the problem. It's the solution.

Because they have to figure out N plausible but wrong actions (where N is "a number that will satisfy the DM so they don't complain about metagaming") before they do the right action. So they still have to use their player knowledge, they just have to use it in a way that ensures they avoid certain actions because those actions are "correct".

You missed the point, hard.

This is a basic theory-of-mind test. It is homologous to playing Sally in a roleplaying game. Looking for the ball in the basket first is the right action. It shows you can keep track of and empathize with someone different from yourself, allowing you to solve puzzles like this.

Figuring out what actions Sally would plausibly take and avoiding actions that are implausible isn't something extraneous to roleplaying, it IS roleplaying. I already explained this in my first post to the thread, but it clearly bears repeating.

Which is much more annoying to actually play.

It's only more annoying to play in the same sense that anything that requires more thought and skill from you is. As with many other challenges, it's possible to scale it based on capacity of the players to reach a desired level of engagement.

And can be avoided by simply not treating published monsters as puzzle monsters. If it's a puzzle monster make it up for the purposes of the puzzle (as long as your group is primed to accept that sort of encounter which history suggests Takaleal's is not).

You make two errors here. The first is presuming that having players use more of their brains to play their characters is something to be avoided. The second is presuming the puzzle is defeating the monster, rather than figuring out what your character would do, how and why.

The second is the really fatal one, and the most pervasive throughout this entire thread.

[Thrudd]

Several different posters have tripped on this, so it's worth saying something about:

It is neither stupid nor pointless. Being able to pretend you don't know something is very basics of acting, and hence also roleplaying. Being able to understand that someone is pretending to be ignorant is also key and basic. People need to be able to do these things if they ever want to portray characters that aren't 1:1 with themselves.

If there are new players at a table, this is something you want to explain to them: that sometimes, your role is of someone who knows less than you do, which entails sometimes the correct action to take in the game is something you would not do normally. Even if it means taking greater risks or outright failing a game objective (such as keeping a character alive). The role where you play to upper limit of your own knowledge is specific and not fit for every kind of game.

Not everyone can do this. For example, small children with weak theory of mind can't make a distinction between what they know and what others know. For a similar reason some people struggle with the notion that, say, a movie character might not know everything the audience does, or that an actor might be different from a character they're portraying. If you're playing with such people, then expecting them to pretend is unrealistic. With everyone else, though? You should explain it, expect it AND enforce it so that the players get the hint.

It depends on the game. If there is an element of player challenge to the game and character death is always on the table, as there usually is in D&D, I feel it's neither fair nor appropriate to ask players to knowingly act in ways that are likely to get their character killed, even in the name of acting. Perhaps the only exception is when you have veteran players with some brand new players - in this case, it might be appropriate for the GM to ask the veterans to take a back seat and give the new players the opportunity to discover things the same way they did when they were new- of course, in this case, the GM is probably also probably toning down the challenge to the level of experience, and the experienced player will understand that.

Actually discovering new things and experimenting with the fictional world and creatures is something that I like to have in my games - not that I discourage acting, but I also like to actually surprise and challenge players and give them the chance to actually discover things and solve problems through play. It is very unsatisfying if the entire game is just me or the players acting like they are discovering a world which they actually already know like the back of their hands. There's no reason that players must always play characters who begin ignorant of the world they live in- when everyone is an experienced player, I think it's reasonable to skip the stage where they act like they've never heard of the classic monsters (if you're using the classic monsters).

Yes, you should emphasize players separating their player and character knowledge as much as possible, and sometimes you have those acting scenes, where you communicate that the character's aren't really in danger so that the players can act through the process of discovering something they already know about their deadly enemies (or just do it through research/knowledge skills outside of combat). But if this is happening too often, I question the GM's creativity. Instead of asking players to act through rediscovering the same stuff they've been playing with for years, maybe create some new stuff.

[GloatingSwine]

It's only more annoying to play in the same sense that anything that requires more thought and skill from you is. As with many other challenges, it's possible to scale it based on capacity of the players to reach a desired level of engagement.

Except it doesn't require "more thought and skill" because it's measuring up to an arbitrary and unknowable standard (the point at which your DM will no longer throw a strop).

This is not a player genuinely attempting to inhabit the life of their character and choose what they would do, this is a player attempting to inhabit their DM's mind and trying to guess how many hoops they're supposed to jump through.

It's a persistent reminder that you are not in the game and acting on the gameworld.

[Easy e]

If knowing the "stats" of a foe ruins a combat, I am questioning why the combat exists in the first place?

That sounds like a combat with little or no stakes to it. Just, fight to fight.

[Thrudd]

If knowing the "stats" of a foe ruins a combat, I am questioning why the combat exists in the first place?

That sounds like a combat with little or no stakes to it. Just, fight to fight.

It doesn't ruin combat, it just ruins some moments where the players are surprised by something and have to figure something out with their brains. They don't always need to not know the stats- they will know the stats after a little while, once they've figured them out. Combat exists for all sorts of reasons, but one of the big ones is to challenge the players. Absent anything else, stakes for combat are at the very least "don't die".
You can have lots of fights against perfectly known enemies, but sometimes you want something new that is mysterious and dangerous and that requires some figuring out. Recommend not using things from books that your players have read, for those situations. This isn't a claim that "every fight needs to be a mystery with hidden stats"- but that sometimes you want some mystery, and players pouring over GM resources to try to figure out what you're doing instead of trying to figure it out by role playing and interacting with the fictional world is sort of antithetical to the spirit of the game.

[kyoryu]

Except it doesn't require "more thought and skill" because it's measuring up to an arbitrary and unknowable standard (the point at which your DM will no longer throw a strop).

This is not a player genuinely attempting to inhabit the life of their character and choose what they would do, this is a player attempting to inhabit their DM's mind and trying to guess how many hoops they're supposed to jump through.

It's a persistent reminder that you are not in the game and acting on the gameworld.

To add on to what you're saying (IOW, not arguing with you):

Literally everybody in this thread understands the basic theory-of-mind of "monster has a vulnerability. I know what it is but my PC doesn't. My PC would not use the vulnerability initially." That's the entire premise.

However, at that point "what the PC would do" would change to "figure out the weakness." We can over-simplify this as "there are N possible vulnerabilities the PC could try." The question at that point becomes "how many do they try before we let them try the right one?" And even this is not really accurate because, hey, we do know the correct solution, so we also have to answer the question "would they ever learn it?"

So, they're going to try stuff, right? Might they get lucky and try the right thing the first time? Maybe, but unlikely. If there are 10 possibilities we can think of, how many do they need to go through?

Understanding that they don't know the correct strategy up front is trivial. (And referring to the Sally-Anne test is kind of implying you think the other people are autistic, so I'd tread carefully there). Modeling the learning process to figure out when they "should" or "would" learn the correct strategy is much, much harder. See also the difficulty of creating riddles/puzzles for other people (ultimately if you want a good one you have to actually test it with real people).

It doesn't ruin combat

It can, in extreme cases. If you have nasty monster that is invulnerable to everything, but just dies if you touch it with the right object, knowing that does invalidate combat (presuming the object is readily available). That's a super extreme example, of course.

The argument is basically "don't do that". Make encounters that are still fun when you know the catch, and then you can also be more free with the info.

[Anymage]

It is neither stupid nor pointless. Being able to pretend you don't know something is very basics of acting, and hence also roleplaying. Being able to understand that someone is pretending to be ignorant is also key and basic. People need to be able to do these things if they ever want to portray characters that aren't 1:1 with themselves.

It still takes mental effort to sequester what you know from what you don't. D&D combat is already asking you to keep track of a lot, so asking you to also devote brainpower to remembering what your character might or might not know on a round-by-round basis is asking a lot. With famous monsters, it's okay to assume a certain level of assumed knowledge. The DM can use tweaking or refluffing if he absolutely wants trick monsters. Although players should have some way of getting a heads up beyond being gotcha'd mid-combat, since presumably other people have encountered these monsters before and would be able to share their information.

If knowing the "stats" of a foe ruins a combat, I am questioning why the combat exists in the first place?

That sounds like a combat with little or no stakes to it. Just, fight to fight.

Having a rough idea what's coming is okay. The characters live in the world, they can generally get a fair idea of threat assessment and know how much more they can take. Plus, given how key to resource management is to D&D, it's okay if the players err on the knowledgeable side of how resource-intensive future encounters might be.

Pulling out the book when you meet a monster will do certain things to a table's pacing and tone. Pacing because flipping through the book and reading the exact stats takes time. Tone, because assuming perfect knowledge makes it easy to see monsters as basically video game mobs. And one person doing so can set precedent for the whole table. (Cheating is similar. If Alice always coincidentally rolls natural 20s on her key rolls she isn't just cheating herself out of risk and excitement. You'll also have Bob asking himself why his character shouldn't be similarly lucky, and it can quickly become a tablewide norm.) It's a good idea to try to head these sorts of things off before they become entrenched in how the whole group plays.