Do players/characters know when a monster uses its Legendary Resistance?

Question

Assuming that players are causing a monster with the Legendary Resistance ability to make enough saving throws that it will use them all up, should the players be aware when that point has come? (As a corollary, should the characters know?)

Monster Manual p.87

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Option 1a. Players and Characters Know, including how many

Player: Elpheba casts Fireball targeting the point closest to the dragon. Make a Dexterity saving throw!

DM: Rolls and checks player's spell save DC. The dragon would have failed the saving throw, but it uses one of its legendary resistances to succeed instead! It twists and turns in the air to avoid the brunt of your fireball's effect. Roll for damage and the dragon takes half.

Player: Drat! But at least we've burned through the last of them... Let your highest-level spells fly, friends!

Option 1b. Players and Characters Know, but not how many

Player: Elpheba casts Fireball targeting the point closest to the dragon. Make a Dexterity saving throw!

DM: Rolls and checks player's spell save DC. The dragon would have failed the saving throw, but it uses one of its legendary resistances to succeed instead! It twists and turns in the air to avoid the brunt of your fireball's effect. Roll for damage and the dragon takes half.

Player: Drat! But at least we've burned through three of them.

Option 2. Players and Characters Don't Know

Player: Elpheba casts Fireball targeting the point closest to the dragon. Make a Dexterity saving throw!

DM: Rolls and checks player's spell save DC. The dragon succeeds on the saving throw as it twists and turns in the air to avoid the brunt of your fireball's effect. Roll for damage and the dragon takes half.

Player: Drat!

Option 3. Players Know, Characters Don't

Player: Elpheba casts Fireball targeting the point closest to the dragon. Make a Dexterity saving throw!

DM: Rolls and checks player's spell save DC. The dragon would have failed the saving throw, but it uses one of its legendary resistances to succeed instead! It twists and turns in the air to avoid the brunt of your fireball's effect. Roll for damage and the dragon takes half.

Player: Drat! But at least we've burned through another of them.

Answer 1

This is left to the DM

Character perspective

Legendary resistance is a game mechanic, like saving throws, hit points, turns and die rolls. The characters do not know anything about game mechanics. They are in the game world and can only observe and know what is perceptible to their senses or given to them by magic or special abilities. So, the characters might not even know if a save was made, unless making or failing has a perceptible effect.

The target (PHB p. 203) might not know:

Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all.

If even the target does not know if there was a spell, then the mechanics of saving throws are not known in game for observers, and legendary resistance, which modifies the outcome of them is neither.

It is not even clear if the caster knows a save was made. The linked question concludes that they either do not know, or it is up to the DM to decide. Jeremy Crawford in an unofficial tweet shares that designer intent is that "A spellcaster doesn't automatically know whether a spell's target succeeded on a saving throw against the spell, but with most spells, the effects are perceivable on the target.". Again, as legendary resistance is a mechanic modifying the outcome if saving throws, if even the throws are unknown, so should the resistance be. All you observe is the outcome.

However, there are effects in the game that you can use when someone makes a successful saving throw. For example, the casting time of silvery barbs is:

1 reaction, which you take when a creature you can see within 60 feet of yourself succeeds on an attack roll, an ability check, or a saving throw

This would not work if the player or character did not know someone succeeded on a saving throw, and many saving throws would not have an immediate observable effect. However, this might be a special exception like for the reaction trigger of the shield spell that also can overrule general treatment of a mechanic.

Because there is no explicit guidance in the rules how characters know about saving throws at all, and because different areas of the rules suggest differently, the DM will have to decide on this.

Player perspective

There rules in chapter 8 of the DMG, Running the Game make it clear that the DM has a lot of liberty about what information they share with players.

For example, even rolling dice is optional, DMs can both just decide if something succeeded (p. 236), or roll secretly. Clearly, there is no obligation to share game mechanics with the players, this is up to the DM and their style.

In my experience, we have played it so that players and characters do not know if legendary resistance was used. All they know was that their spells had no effect. I think this is good, because on higher levels, monsters the have a hard time against PCs if you match them up by CR. Knowledge is power, and the players not knowing this helps to keep them guessing and makes the fight a little harder. For example in one combat against an ancient gold dragon, we never knew why our spells failed. We even did not know he had lair actions that gave him advantage to saves on certain turns. We did not know how many of its legendary resistances we had burned through. That made the dragon a lot more impressive and scary.

Answer 2

The rules don’t say.

Unfortunately, the rules simply don’t address this question. It is left to the DM to choose how to handle the use of Legendary Resistance. At best, I can leave you with some guidance around how I’ve used Legendary Resistance. Marq’s answer here gives a good perspective on legendary resistance that may help realign your perspective of the feature:

Legendary Resistance is a DM tool, not an ability that a monster chooses to use.

I think it's best to treat Legendary Resistance as a game-mechanical way for the DM to make a climactic battle entertaining. A monster with Legendary Resistance doesn't choose whether to use it or not; the DM chooses whether having the monster make a saving throw will make the battle more entertaining or not. It's (presumably) available so that epic monsters that are typically fought solo aren't quickly overcome by a couple of effects that require saving throws.

This is how I’ve used Legendary Resistance in the past. In terms of the options you present in the question, I’ve played this using Option 3, that the DM and players are aware that a Legendary Resistance has been used, but in-character, there is, of course, no such concept as “saving throws” or “legendary resistance”.

Answer 3

Characters - probably not, players - up to the DM

From the character's perspective none of the meta terms mean anything, I believe that includes knowledge about saving throws or legendary resistances. These terms are meaningless to the characters. The spellcaster would just know that there is a chance the enemy might shake off a given spell and possibly have some idea how likely that is. Therefore they would just observe on thing - the spell not working properly (how that manifests depends on what the spell block says about saves, could be nothing at all happening, could be some partial effect but not the full impact).

From the player's perspective, that is 100% dependent on that DM. There are DMs that tend to be as secretive as possible and with these the player would almost certainly not know about it either, and there are DMs that do everything in the open that would tell you some equivalent of "The monster rolled a 9 and failed but uses its legendary resistance to succeed".

From personal experience, I am one of those DMs that do most things out in the open so in my games I tell the players when a monster uses a legendary resistance, including how many they have left, in my experience it tends to temper the disappointment that their spell didn't work with "at least we made it use up a valuable resource". My players are very good about not metagaming though so I feel comfortable telling them this, in a group more prone to using meta knowledge I would roll the save in secret and then just tell them it succeeds without mentioning legendary resistance.