So, I DM'd a fight against a couple of Hydra's. It was fine until after the session when I decided to peer at my player's sheet to find out one of the players has been lying about his spells for a little while.

We use D&DBeyond for rules and making sheets, but use Roll20 for rolling and organizing House Rules. Well, the Hexblade Warlock claimed that Branding Smite did Fire Damage, which I neglected to check due to me running 7+ statblocks between hydras, Yuan-ti, Guards, and NPC's. So I allowed him to kill the first hydra by burning it while it was down. On the second Hydra when he cast it again I noticed the attack dealt Radiant damage, and he claimed that he meant to use Searing Smite which he also had which I allowed to be kind (Due to them being fairly limited on resources after Hydra 1). After which he attempted to Counterspell an enemy (Used Rod of the Pact Keeper for the extra spell slot) which I allowed because Warlocks get them.

Well, I checked after-the-fact and learned that he had neither Searing Smite or Counterspell prepared, meaning that while he was looking at his sheet he decided to lie and said he had both of those spells. This is not the first time this had happened either.

How would you handle a player doing this multiple times? I don't want to add stress but I also do not want this to continue as lying about the sheet takes away from the game as a whole. 

It is unfortunate that you can't trust one of your players.  Might want to have a talk with them. 

When you say he didn't have them "prepared" as a warlock is that he never knew them in the first place, right? 

Anyways, try this: at the start of the game make an announcement that all spell casting character MUST provide you with the list of spells they know or have prepared, both in the case of a wizard.  Furthermore, if any changes are being made to that list after resting, those changes must be declared or be invalidated.  It is their job to keep that list updated as they level up and learn more spells and to keep providing it at the start of each session.  

It may be a slight annoyance to the players, but you gotta do what you gotta do to stomp out cheating.  

You can do the same with item attunement, so players are not swapping through items illegally. 

I'm going to disagree. Don't start making people prove things to you. This is supposed to be a fun experience.

Recognize that the player just wants to feel cool. It's fine. He isn't intending to cheat you or the other players. Either overlook it, or have a chat with the player in private about how strictness with spells is required for the game to be balanced. Don't accuse him of having done something wrong -- just explain why strictness will increase the fun.

This is perhaps the most difficult issue in D&D... someone who cheats at the game. I'm going to break the bad news here, and tell you that this sort of thing almost never ends well. There are a few possible ways to handle it and most of them are going to lead to an unpleasant outcome -- assuming the best outcome is you like the player otherwise, and you want to keep him in the group, but just have him stop cheating. That outcome is very unlikely, and I think you need to know this going in. I'm not trying to be pessimistic here -- I just want you to realize as a DM, that the likelihood of the outcome you want is pretty low... and prepare you for the more likely scenarios.

If you just ignore it, he's going to keep doing it, and sooner or later this is going to do serious damage. First, as you observed, it will make your challenges impossible to prepare for the party, because you won't be able to predict what he's going to claim his character can do. Second, his PC will be the star of the show, because he's allowed to just have the weakness of any monster on the board so that he can "be awesome." As he sees you ignoring/allowing the cheating, he's going to do it more, and it will get worse, since by not stopping it you're kind of giving permission to keep doing it. The other players are going to figure out, eventually, that he's cheating, and worse, that you're letting him do it, and at that point you are going to have a major issue. It may look like you are playing favorites, letting him get away with things and not other people. Or it may look like you don't give a bleep about the rules, in which case they ALL start doing it. Or they may just decide to leave the game over it. Ignoring it is probably not going to work, in other words.

If you confront him on it, no matter how politely you do it, he's going to become defensive... He won't want to admit he cheated. He'll claim it was "just a mistake." But if he's done this before and this is in his makeup as a person, it will happen again. And then you're going to have to call him on it again. Even doing this privately, one-on-one, could become very ugly, and the odds are that he will quit the group over it.  You can try the "Here, you don't seem to understand some of the rules so let me explain them to you" approach which is less adversarial but only works with newer players. If this person has been playing D&D 5e for 3 years or something, then you can't really say that sort of thing with a straight face. It's not like a vet player is going to confuse Radiant with Fire damage, for instance... whereas you could at least pretend that you think a newer player might, which would allow him to "save face." Odds are this won't work either, as unless it is a very new player he'll think instead of accusing him a cheating that you're calling him stupid.

You could try to "watchdog" it as proposed by demanding a list of prepared spells, and personally reading the description of every spell as they are cast to confirm that what the players are telling you about the spells agrees with the text. However, that is a huge amount of work put on the DM, and you are left with the problem that if he is going to cheat, the list might not stop him. If you're playing on DDB or R20 he must know you can look at his character sheet, so the fact that what he's doing can be cross-checked hasn't stopped him yet. So now when he says "I cast counterspell" but doesn't have it, what are you going to do? You are back to point B above, confrontation. But now it's going to have to happen in live pay, at the (virtual) table, right in front of everyone. "You don't have that on your sheet, so you can't cast it" is not going to go well. Trust me, I've seen it happen (not in D&D but Champions -- "Your character takes 11 STUN as END" , "Why?" "Because you are out of END," "How would you know that?" "Because I've been tracking your END...." followed by a huge fight about why the GM was tracking this guy's END in the first place.) Players know what it means when the DM starts monitoring their character sheet, and they will get indignant over it -- especially if they in fact ARE cheating (when they have the least actual right to be indignant).

I wish I had a real solution for you -- "Do it this way," -- but unfortunately I don't. None of the solutions suggested here nor any I have ever seen before can solve cheating in the way we all want, which is to just get the cheating player to stop doing it and play along properly with the rest of us.

It might be a genuine lack of understanding the rules. I know when I first played I was casting hellish rebuke at the wrong level as a tiefling, using spell save DC as AC (why I played a wizard for my first character as a 12 year old, I will never know.)

Gently pull the player aside, or talk to them after a session, and ask if they realised they had cast spells they didn't have prepared/known. If they say no, which is what I'm guessing you'd expect in this scenario, ask for them to make a note of the damage die + type on their sheet next to all their spells, so then at least if they've cast the wrong spell the damage & type should be correct. You could also ask another player to help them out for a session or two, and double check their spells are doing what the player claims they are. If this doesn't work, I'd ask them to roll a new character that doesn't rely on spells, or even ask them to leave the table.

I'm also slightly surprised your other players didn't catch that the player in question didn't have those spells- I'm currently playing with a party of five (three full casters, one half caster) and we all know the majority of each other's spells.

Quote from BioWizard >>

You could try to "watchdog" it as proposed by demanding a list of prepared spells, and personally reading the description of every spell as they are cast to confirm that what the players are telling you about the spells agrees with the text. However, that is a huge amount of work put on the DM, and you are left with the problem that if he is going to cheat, the list might not stop him. If you're playing on DDB or R20 he must know you can look at his character sheet, so the fact that what he's doing can be cross-checked hasn't stopped him yet. So now when he says "I cast counterspell" but doesn't have it, what are you going to do? You are back to point B above, confrontation. But now it's going to have to happen in live pay, at the (virtual) table, right in front of everyone. "You don't have that on your sheet, so you can't cast it" is not going to go well. Trust me, I've seen it happen (not in D&D but Champions -- "Your character takes 11 STUN as END" , "Why?" "Because you are out of END," "How would you know that?" "Because I've been tracking your END...." followed by a huge fight about why the GM was tracking this guy's END in the first place.) Players know what it means when the DM starts monitoring their character sheet, and they will get indignant over it -- especially if they in fact ARE cheating (when they have the least actual right to be indignant).

I wish I had a real solution for you -- "Do it this way," -- but unfortunately I don't. None of the solutions suggested here nor any I have ever seen before can solve cheating in the way we all want, which is to just get the cheating player to stop doing it and play along properly with the rest of us.

 As far as the watchdog method, I think it would stop most would be cheaters from succumbing to dishonesty in the heat of the moment, which I’m assuming is what is happening in this case. It just gives the players an obvious “l’m watching” cue.  

I mean think about it.  You know that the DM has a list of spells your character knows/has prepared.  Are you really going to try and cast one not on that list?  Really?  If a player is that brazen about cheating, doing it knowingly and shamelessly, I would say they fully deserve to be called out right there at the table.  I mean where do we draw the line here, that’s on the same level as a fighter attacking twice or three times when they can’t yet.  

It does require a bit more work on the DMs part, but not too much more I’d argue; you can put most of the burden on the player by requiring them to provide the list, along with a short description of the spells if you don’t know them.  Also I would argue it is productive work, you are becoming familiar with what spells your players know/like to prepare, and if you don’t know the spell already, you are learning what it does.  That is not bad knowledge to have as a DM.

Have all characters use the Display in VTT button when they cast a spell or use an ability. It has to be on the character sheet to do that, and all the details will show up right there on Roll20.

 As far as the watchdog method, I think it would stop most would be cheaters from succumbing to dishonesty in the heat of the moment, which I’m assuming is what is happening in this case. It just gives the players an obvious “l’m watching” cue.  

 You know that the DM has a list of spells your character knows/has prepared.  Are you really going to try and cast one not on that list? 

If he's that dead set on cheating, he will either (a) quit the game, or (b) find another way to cheat that you are not watching. Or else (c) constantly argue the rules with you. None of these are desirable circumstances (assuming, in case a, that the DM doesn't want the player to leave the game).

Someone cheating like this -- it often not a game issue. It is a human issue. There is something going on in the mind of this player causing him to lie about what his spells can do, lie about what spells he has prepared, etc. What's going on is not "the DM isn't watching." It's something deeper in the psyche. The DM watching is not necessarily going to work, in that case.

I walk around my development every nice day. Most days I walk around shortly after or during the Amazon van's deliveries. I see packages sitting on people's front stoops and porches. I sometimes amuse myself by wondering, based on size and orientation, what might be in the box. But I have never once considered stealing one of those boxes, because it is just not in my make-up to do it. Furthermore, it doesn't appear anyone else in my complex entertains such thoughts, because so far in the years I have lived here, I've not heard of anyone falling victim to "porch piracy."

I and the people in my complex don't leave packages on their owners' doorsteps because we'd get into trouble for taking them. We leave them there because it's wrong to take them, and we don't want something that doesn't belong to us.

But we do know that "porch pirates" exist... And we also know that if you put an obvious camera on your porch with a sign saying "this is going to catch you if you steal my stuff," it may make the porch pirate leave your house and the houses on your block alone, but it is NOT going to make them stop being porch pirates. They'll just go somewhere that doesn't have the security system and do it there.

Just yesterday I watched a video by this guy who is some sort of engineer, and he does cool/interesting things and posts them to YouTube. He has been the victim of porch pirates and so he decided to take matters into his own hand. He built this high-tech decoy package that looked super valuable, but it was actually a glitter bomb and sprayed noxious skunk (and other) smells. He also rigged up cameras inside to film people's reactions. Not one of the porch pirates he filmed indicated any sort of remorse about it. They got mad, they cursed, they often expressed the idea that the guy who had protected his package from them was the actual jerk. As if they deserved to have this package they didn't buy or order, just because they wanted it, and the person protecting his package was the jackhole.

My point is -- the engineer guy didn't make one of them change their minds, or realize that what they are doing is wrong, or feel even the slightest bit of remorse or guilt over it. At best, he made them stop porch pirating, but you can bet they just switched to another, safer, way to steal. Because the pirates don't steal the package because it's there -- the package being there affords them an opportunity to engage in this behavior that is already in them before they ever saw that package.

It's the same thing with the RPG cheater (assuming it's not a young kid who is just testing boundaries). The guy who is lying about what his spells can do so he can kill the hydra, or what spells he has prepared so he is able to counterspell the BBEG, is not doing it because the DM didn't notice. The DM not noticing just gives him the opportunity to engage in these behaviors. If the DM watches him on spell prep, he'll cheat on die rolls. If the DM makes him roll in the VTT, he'll find a way to hack the VTT. If the DM prevents that, he'll find another way to cheat.

And if you completely stop him from cheating, odds are (and I saw this happen in my game group years ago) he will just stop playing with you. 

My point here is -- if someone wants to be dishonest for deep-seated emotional or psychological reasons, bird-dogging their character sheet is not going to fix the problem.

Quote from BioWizard >>

 As far as the watchdog method, I think it would stop most would be cheaters from succumbing to dishonesty in the heat of the moment, which I’m assuming is what is happening in this case. It just gives the players an obvious “l’m watching” cue.  

 You know that the DM has a list of spells your character knows/has prepared.  Are you really going to try and cast one not on that list? 

If he's that dead set on cheating, he will either (a) quit the game, or (b) find another way to cheat that you are not watching. Or else (c) constantly argue the rules with you. None of these are desirable circumstances (assuming, in case a, that the DM doesn't want the player to leave the game).

Someone cheating like this -- it often not a game issue. It is a human issue.

Right.  Yes I completely agree with what you are saying, and I did hear that story too.  But that is outside the purview of the DM, to have to be a shrink for his/her cheating players.  

At the end of the day, you simply need to protect your group that's the bottom line.  When this player cheats, they are cheating you and your hard work you spent to prepare this session for everyone, they are cheating the other players who now at a disadvantage power-wise compared to this player who can break the rules and do whatever they want, and ultimately they are cheating themselves- they didn't earn that victory, they cheated.  

I'm not advocating that every DM becomes an anti cheating bloodhound as a matter of course.  But if you are in a situation like Woad Bear where your game is being threatened, there are steps you can take to firstly create a deterrent, one that will easily expose the violation when it happens, and from there you can take progressively harsher disciplinary action against the offending player up to ultimately removing them from the group if need be.   

I'm going to assume here that you want to keep this player in your game.

When it comes to cheating, a lot of people do it because they think no one is paying attention. It doesn't mean they should do it, it just means that they are doing it because they think they won't get caught. So let them know they got caught, but do it nicely. "I noticed that you didn't have Counterspell prepared, last time. I'm sure you just forgot, but to help remind you, I'll be asking if you have spell's prepared when you cast them."

How they react to this will give you a better idea of how to deal with them. If they apologize give them a chance to fix their behavior. If they get defensive and angry, you know you have a problem person on your hand.

Keep their character sheet open in another tab, and when they cast a spell double check if they have it prepared or not. If they don't, simply ask, "Did you remember to prepare that spell?"

Best case, they will realize that you are paying attention and they will stop trying to cheat. Worse case, they may create a big angry scene. If they do I would point out that the reason that spell casters have to choose and prepare spells is to balance the various classes. It's to allow all the players to have a chance to shine and not have a few classes be over powered and dominate the game. Frame it in terms of the other players enjoyment, that way if they do leave your game, the other players will understand you are trying to let everyone have a good time and not one character dominate combat.

The truth is there's a good chance that this won't end well, but you shouldn't give up on the slim chance that it does. On the other hand this player is already causing you stress and hurting your enjoyment. So dealing with it one way or another will allow you to get to a good point for all the players.

I guess i don't get what kind of people you guys are playing with. I can't imagine anyone I know cheating at a game, especially one that's just for fun.

If you go on a hike together, do you have to worry that this person might knock you off a cliff in order to finish first?

Do you need to hire a ref to play a game of pickup basketball?

Are these adults you guys are talking about?

Edit: a bit saltier than I intended, sorry. I'm irritated that you fine folks have to deal with situations like players cheating. If shouldn't be like that.

You in fact are the DM. if you know this person is cheating throw something that he cant beat down and let the game roll. At the end of it you can show mercy or finish them.

The fact that he claimed fire damage on the smite spell shows that he is also pulling up monster stat blocks and is cheating his way into attacking vulnerabilities. Sure, he could have been mistaken, but I would bet that if Hydra's heads were affected by cold, he would have claimed one of his spells did cold damage.

Well... Hydras are pretty well known to be vulnerable after a fashion to fire (not really vulnerable but don't regenerate), much like trolls. I agree it was metagaming... but it's not necessarily the case that he pulled up the stat block. I'd know it without looking at the stat block. (The difference here is, I wouldn't necessarily make my character act on that knowledge.)