Under Exact XP on p.399 of the Core Rulebook, it states that the XP for the encounter should be tallied and then divided by the number of party members. Each party member is then awarded this amount of XP.
My question is:
Do I count the summoner's eidolon when doing this calculation? Or is the eidolon lumped in with the summoner?
I'm assuming that the eidolon's XP is tracked separately from the summoner's rather than his level being slaved-- I could be wrong about that!
The Eidolon is a class feature of the summoner and the abilities of the Eidolon are derived from the Summoner's level and class features, not independently:
An eidolon’s abilities are determined by the summoner’s level and by the choices made using its evolution pool.
The Eidolon improves when the Summoner levels up, and just as summoned creatures as from the Summon Monster etc. spells don't count as members of the party when it comes to tallying up experience, the summoned Eidolon isn't considered separate to the Summoner who brings it to the table when accounting for experience or encounter difficulty.
Just like a Druid's animal companion, Summoned Monsters or Natures Allies, or Familiars, Eidolons are part of a class feature, and are therefore considered part of the owning character from the point of view of encounter or party composition. In a similar fashion, an enemy NPC Summoner is not given a challenge rating separate to his Eidolon - the Eidolon is part of the summoner's basic capabilities.
Sadly, I can't think of anywhere that this is explicitly spelled out in the rules.
