I see no exception for measuring to a Swarms base in regards to whether or not anyone else would gain flank from it, so my tentative answer is, if I've modeled your example "picture" correctly, that both Friendly models would gain Flank from the Swarm and vice versa.
The Swarm trait does not prevent them from flanking, but their limited types of attacks might.
To flank you need:
1. Positioning & ability to act: Easy
2. A melee weapon in hand: nope
or ability to make an unarmed attack: ???
3. Opponent within reach: Messy, but if on top of the opponent, I suppose this works, but that's odd to be using square X to be on opposite sides and square Y for them to be within reach. So I'll focus on part 2.
Looking at the Spider Swarm for instance, they have neither a weapon in hand nor an unarmed attack they can make. Their Swarming Bites action does not have the attack trait nor does it have an attack roll.
A quick perusal of the other Swarms shows the same result, though someday there might be a Swarm that can flank. Maybe a swarm of tiny fey w/ spears for instance, since even if they don't attack w/ them, they'd have a melee weapon(s) in hand). Or a swarm that can lash out, maybe with a mental energy whip.
So no current swarms can flank, even though technically a swarm could if they had an attack action (and hopefully reach too for clarity!).
The Swarm trait does not prevent them from flanking, but their limited types of attacks might.
To flank you need:
1. Positioning & ability to act: Easy
2. A melee weapon in hand: nope
or ability to make an unarmed attack: ???
3. Opponent within reach: Messy, but if on top of the opponent, I suppose this works, but that's odd to be using square X to be on opposite sides and square Y for them to be within reach. So I'll focus on part 2.Looking at the Spider Swarm for instance, they have neither a weapon in hand nor an unarmed attack they can make. Their Swarming Bites action does not have the attack trait nor does it have an attack roll.
A quick perusal of the other Swarms shows the same result, though someday there might be a Swarm that can flank. Maybe a swarm of tiny fey w/ spears for instance, since even if they don't attack w/ them, they'd have a melee weapon(s) in hand). Or a swarm that can lash out, maybe with a mental energy whip.
So no current swarms can flank, even though technically a swarm could if they had an attack action (and hopefully reach too for clarity!).
To be fair, none of the bestiary entries list the specific traits for any of a monsters attacks, so saying that Swarming Bites doesn't have the attack trait feels reaching. Granted, since the damage is automatic, MAP doesn't really matter.
Regarding the potential swarm of tiny fey with spears, their attack would probably be handled the same, as Swarming Stabs or something similar.
I feel like the intention here is that there are so many bites happening at once that it is basically impossible to avoid every bite, so no attack roll necessary. Each individual rat has a bite attack after all, so why shouldn't they count as being able to perform an "unarmed attack" for Flank?
This does appear to need an official ruling, but my house rule will be swarm flanking is a thing. From a realism stand point, I feel like a Swarm would be even more effective than an Orc with an axe at splitting your attention between targets, especially if you have a ton of rats/spiders/etc... under your feet!
The Swarm trait does not prevent them from flanking, but their limited types of attacks might.
To flank you need:
1. Positioning & ability to act: Easy
2. A melee weapon in hand: nope
or ability to make an unarmed attack: ???
3. Opponent within reach: Messy, but if on top of the opponent, I suppose this works, but that's odd to be using square X to be on opposite sides and square Y for them to be within reach. So I'll focus on part 2.Looking at the Spider Swarm for instance, they have neither a weapon in hand nor an unarmed attack they can make. Their Swarming Bites action does not have the attack trait nor does it have an attack roll.
A quick perusal of the other Swarms shows the same result, though someday there might be a Swarm that can flank. Maybe a swarm of tiny fey w/ spears for instance, since even if they don't attack w/ them, they'd have a melee weapon(s) in hand). Or a swarm that can lash out, maybe with a mental energy whip.
So no current swarms can flank, even though technically a swarm could if they had an attack action (and hopefully reach too for clarity!).
To be fair, none of the bestiary entries list the specific traits for any of a monsters attacks, so saying that Swarming Bites doesn't have the attack trait feels reaching. Granted, since the damage is automatic, MAP doesn't really matter.
Regarding the potential swarm of tiny fey with spears, their attack would probably be handled the same, as Swarming Stabs or something similar.
I feel like the intention here is that there are so many bites happening at once that it is basically impossible to avoid every bite, so no attack roll necessary. Each individual rat has a bite attack after all, so why shouldn't they count as being able to perform an "unarmed attack" for Flank?
This does appear to need an official ruling, but my house rule will be swarm flanking is a thing. From a realism stand point, I feel like a Swarm would be even more effective than an Orc with an axe at...
I accept the overstep on looking for the attack trait before checking to see if other monsters list that one. Whoopsies. (And they do list most traits.)
The point with spears (unintended pun) for the swarm is that they have weapons, so qualify to flank. Whether they have an attack action w/ a weapon isn't checked for, that's only for unarmed.
It's an odd loophole I don't enjoy, but I was also imagining the spears might give them reach 5', which seems pretty important. Unlike the rats, who at normal size don't threaten even w/ their bite. Heck, they're not even statted any more.
Yet I think too that a deadly swarm behind you would be very distracting, which is the point of flanking. But unfortunately that argument would apply to a raging Barbarian 10' away as well. Or arrows flying at you from a rank of archers.
Is it immediately distracting you NOW? If they don't have poky bits and smashy parts intruding on your space, maybe not enough.
Being ambivalent, I think I'd lean toward no flanking because most of my players do PFS.