While using the Twin Eidolon class feature, the summoner can assume the shape of his eidolon, copying all of its evolutions, form, and abilities. Are feats also copied?
Also, can a summoner use his own feats while transformed?
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The wording indicates to me that the summoner is meant to keep his own feats, but not "copy" the eidolon's feats. Specifically, that the summoner keeps his feats is supported by the idea that the rules emphasize the keeping of racial bonus feats - why keep racial feats if you don't keep other feats? Additionally, there is simply nothing indicating that you also gain the eidolon's feats. They list what you do get, and feats are not on that list.
Note: feats having to do with natural attacks, or feats that have a Strength, Dexterity, or Consitution score requirement do not get carried over in the case that the eidolon's ability scores do not meet the requirements. In fact, any feats for which the eidolon does not meet the requirements reasonably cannot be used.
Description copied from d20pfsrd.com, emphasis mine:
Twin Eidolon (Su)
At 20th level, a summoner and his eidolon share a true connection. As a standard action, the summoner can assume the shape of his eidolon, copying all of its evolutions, form, and abilities. His Strength, Dexterity, and Constitution scores change to match the base scores of his eidolon. He can choose to have any gear that he carries become absorbed by his new form, as with spells from the polymorph subschool. Items with continuous effects continue to function while absorbed in this way. The summoner loses his natural attacks and all racial traits (except bonus feats, skills, and languages) in favor of the abilities granted by his eidolon’s evolutions. The summoner retains all of his class features. The summoner can keep this form for a number of minutes per day equal to his summoner level. This duration does not need to be consecutive, but it must be spent in 1-minute increments. The summoner can end this effect as a free action.