Given that silvery barbs has a casting time that says:

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

I want to know how this spell interacts with a monster with the Legendary Resistance trait:

If [monster] fails a saving throw, it can choose to succeed instead.

succeeding a saving throw due to using their Legendary Resistance.

If I, as the DM, announce to the players that the monster succeeds a saving throw, but not how the monster succeeded (i.e. due to using a Legendary Resistance), and then a player announces they want to use silvery barbs, what happens?

1. Does the spell make the creature roll again, despite having succeeded only due to Legendary Resistance (and if they fail the roll again and have a use of Legendary Resistance left, they may choose to succeed again, thus using two uses)?
2. Does the spell happen but effectively do nothing, because Legendary Resistance already declared that "it chooses to succeed instead"?
3. Is it not possible to cast the spell because they technically failed the roll, even if the resolution in the end was a succeeded saving throw?

Dale M's answer to this question touches on this, but I was wanting to understand this interaction in more depth.

Given that silvery barbs has a casting time of:

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

I want to know how this spell interacts with a monster with the Legendary Resistance trait succeeding on a saving throw due to using their Legendary Resistance:

If [monster] fails a saving throw, it can choose to succeed instead.

If I, as the DM, announce to the players that the monster succeeded on a saving throw, but not how the monster succeeded (i.e. due to using a Legendary Resistance), and then a player announces they want to use silvery barbs, what happens?

1. Does the spell make the creature roll again, despite having succeeded only due to Legendary Resistance (and if they fail the roll again and have a use of Legendary Resistance left, they may choose to succeed again, thus using their trait twice)?

2. Does the spell happen but effectively do nothing, because Legendary Resistance already declared that "it chooses to succeed instead"?

3. Is it not possible to cast the spell because the monster technically failed the roll, even if the resolution in the end was a succeeded saving throw?

Dale M's answer to this question touches on this, but I am wanting to understand this interaction in more depth.

Given that silvery barbs has the casting time that says:

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

So what happens if a monster with Legendary Resistance:

If [monster] fails a saving throw, it can choose to succeed instead.

succeeds due to said Legendary Resistance.

If I, as the DM, announce to the players that the monster succeeds a saving throw, but not how the monster succeeded (i.e. due to using a Legendary Resistance), and then a player announces they want to use silvery barbs, what happens?

1. Does the spell make the creature roll again, despite having succeeded only due to Legendary Resistance (and if they fail the roll again and have a use of Legendary Resistance left, they may choose to succeed again, thus using two uses)?
2. Does the spell happen but effectively do nothing, because Legendary Resistance already declared that "it chooses to succeed instead"?
3. Is it not possible to cast the spell because they technically failed the roll, even if the resolution in the end was a succeeded saving throw?

Dale M's answer to this question touches on this, but I was wanting to understand this interaction in more depth.

Given that silvery barbs has a casting time that says:

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

I want to know how this spell interacts with a monster with the Legendary Resistance trait:

If [monster] fails a saving throw, it can choose to succeed instead.

succeeding a saving throw due to using their Legendary Resistance.

If I, as the DM, announce to the players that the monster succeeds a saving throw, but not how the monster succeeded (i.e. due to using a Legendary Resistance), and then a player announces they want to use silvery barbs, what happens?

1. Does the spell make the creature roll again, despite having succeeded only due to Legendary Resistance (and if they fail the roll again and have a use of Legendary Resistance left, they may choose to succeed again, thus using two uses)?
2. Does the spell happen but effectively do nothing, because Legendary Resistance already declared that "it chooses to succeed instead"?
3. Is it not possible to cast the spell because they technically failed the roll, even if the resolution in the end was a succeeded saving throw?

Dale M's answer to this question touches on this, but I was wanting to understand this interaction in more depth.

Given that silvery barbs has the casting time that says:

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

So what happens if a monster with Legendary Resistance:

If [monster] fails a saving throw, it can choose to succeed instead.

succeeds due to said Legendary Resistance.

If I, as the DM, announce to the players that the monster succeeds a saving throw, but not how the monster succeeded (i.e. due to using a Legendary Resistance), and then a player announces they want to use silvery barbs, what happens?

1. Does the spell make the creature roll again, despite having succeeded (and if they fail the roll again and have a use of Legendary Resistance left, they may choose to succeed again, thus using two uses)?
2. Does the spell happen but effectively do nothing, because Legendary Resistance already declared that "it chooses to succeed instead"?
3. Is it not possible to cast the spell because they technically failed the roll, even if the resolution in the end was a succeeded saving throw?

Dale M's answer to this question touches on this, but I was wanting to understand this interaction in more depth.

Given that silvery barbs has the casting time that says:

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

So what happens if a monster with Legendary Resistance:

If [monster] fails a saving throw, it can choose to succeed instead.

succeeds due to said Legendary Resistance.

If I, as the DM, announce to the players that the monster succeeds a saving throw, but not how the monster succeeded (i.e. due to using a Legendary Resistance), and then a player announces they want to use silvery barbs, what happens?

1. Does the spell make the creature roll again, despite having succeeded only due to Legendary Resistance (and if they fail the roll again and have a use of Legendary Resistance left, they may choose to succeed again, thus using two uses)?
2. Does the spell happen but effectively do nothing, because Legendary Resistance already declared that "it chooses to succeed instead"?
3. Is it not possible to cast the spell because they technically failed the roll, even if the resolution in the end was a succeeded saving throw?

Dale M's answer to this question touches on this, but I was wanting to understand this interaction in more depth.