Thorn Whip says "Make a melee spell attack roll". The range on this attack is 30ft. Warcaster says "When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack." Is the caster's threat range now 30ft if they have both of these? In other words, can they get an AOO when a creature moves from 30 to 35 feet away from them if they use Thorn Whip?
To expand on the answers above, the opportunity attack is triggered first, then you cast the spell. So even if the spell changed your reach (which it doesn't), the spell isn't cast until after the opportunity attack has been provoked.
The spell is cast instead of the opportunity attack, but you must still trigger the opportunity to attack by normal Opportunity Attack rules first. So your reach is still whatever your weapon's reach is.
Thorn Whip is not ideal to use in this circumstance for most people, generally there are better options. Since it would pull them closer, except, they're still probably adjacent to you at the time of the opportunity attack, so that portion of the spell effect is wasted. That just leaves you with the spell's damage. Which is, well... not the greatest.
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I'm probably laughing.
It is apparently so hard to program Aberrant Mind and Clockwork Soul spell-swapping into dndbeyond they had to remake the game without it rather than implement it.
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Thorn Whip says "Make a melee spell attack roll". The range on this attack is 30ft. Warcaster says "When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack." Is the caster's threat range now 30ft if they have both of these? In other words, can they get an AOO when a creature moves from 30 to 35 feet away from them if they use Thorn Whip?
No. Nothing about either the spell or the fear changes your reach.
Spell range does not effect reach.
To expand on the answers above, the opportunity attack is triggered first, then you cast the spell. So even if the spell changed your reach (which it doesn't), the spell isn't cast until after the opportunity attack has been provoked.
The spell is cast instead of the opportunity attack, but you must still trigger the opportunity to attack by normal Opportunity Attack rules first. So your reach is still whatever your weapon's reach is.
Thorn Whip is not ideal to use in this circumstance for most people, generally there are better options. Since it would pull them closer, except, they're still probably adjacent to you at the time of the opportunity attack, so that portion of the spell effect is wasted. That just leaves you with the spell's damage. Which is, well... not the greatest.
I'm probably laughing.
It is apparently so hard to program Aberrant Mind and Clockwork Soul spell-swapping into dndbeyond they had to remake the game without it rather than implement it.