PFS: Summoner-Eidelon with a reach weapon?

[Elder Basilisk]

I'm thinking of making a summoner for PFS and I was thinking of giving my (biped) Eidelon a reach weapon (Lucerne Hammer or Dwarven Longhammer). What are the potential issues with this?

It seems like this will make it difficult to mix natural attacks with weapon attacks once the Eidelon gets the size increase to large. Should I just go with a greatsword instead?

Also, are there any issues with eventually getting a large sized weapon for my Eidelon when it increases in size? It doesn't seem like the iconic barbarian has any trouble with her large bastard sword, but you never know.

[Rosc]

Pretty sure you can buy a large weapon. The real problem is going to be the in investment of either gold or time, since a magic item can't be resized. So you'll either buy an early magic weapon that you have to throw out (unless you plan on having the summoner use it) or get by with nothing better than masterwork weapons until the Large evolution kicks in. Last option is the silliest: get a large weapon early, have the Summoner act as the Eidolon's golf caddie and only pass it to them after casting Enlarge Person.

As for claws and weapons, I'm fairly certain you can 5foot step in the middle of a full attack.

Last but not least, remember a big sized AoO machine won't always work with PFS groups. It might not fit in a narrow passage, and some well meaning melee-happy thickies of the group will leap bravely on to enemy spears in blatant disregard of your tactical advantage. And of course, it runs the risk of your combat pet overshadowing the PC martials if you go overboard.

[Elder Basilisk]

5 foot step solves the reach/natural attacks issue for a medium eidolon. Attacks at 10 feet with mfg reach weapon, 5 foot step adjacent and use natural attacks. I guess that still works at large size but only if you attack with the reach weapon at 15 feet, and 5 foot step to ten feet to make natural attacks. I suppose that Flying and careful attention to diagonals will make that easier to pull off-though I was hoping to avoid airburst fireball Levels of trigonometry.

As far as overshadowing other PCs, I think that's a possibility with any high level eidolon that isn't deliberately nerfed. But, I guess it would be a good idea to have reduce person as a known spell just in case the eidolon runs into small spaces. It's probably going to come up more often than wanting to use enlarge person on an already large eidolon for a huge eidolon.

[Dafydd]

My suggestion, drop the weapon and take the tail, sting, reach (sting) as your first 3 evo points spent.

You can then have a reach weapon, and 2 naturals (biped starts with 2 claws) without any penalties to any of the attacks. Also, without limbs (arms) you lose the use of your first 2 natural attacks, and need to spend evos on getting others to replace em. Oh, and the reach evolution does not leave a hole in the middle like a reach weapon.

You can also later add Push and Pull to the Sting. Push to shove the foe back as it tries to move around the demon spawn. Pull to bring someone who is trying to escape Back into claw shredders.

It also solves the magic weapon issue of what happens when you go big. Sting and claws will both scale up with Large size.

As for how to get the Eidolon into tight corners, there is dismiss and recall it (not good mid combat), or just have it be medium and use Evo Surge (3rd level spell) to go large when in combat (best done as first action in combat, but second is not terrible. Either way Lesser rod of Quickening exists to eventually make your first turn Quicken Haste and Evolution Surge.