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~1st question~
Do I count the HP of the summoner before or after summoning the eidolon?
1st level Summoner w/ Con 12 (+1) HP is 9 (1d8+1)
his eidolons con is 14 (+2) giving temp HP 7 (1d10+2)
Before Summoning example
His HP stays at 9 since he does not alter his standard HP score
After summoning example
His HP bumps up to 10 while with the eidolon due to a higher Con score. And drops by 1 when it is dismissed.
~2nd question~
This question is moot if the answer to question 1 is "After" but if the answer is before then does wearing a belt of mighty constitution effect both Con scores (and thus both HP pools)
Same Example
His HP is now 10 and his Temp HP is now 8 due to the increased constitution of both himself and his eidolon.
~3rd question~
Would toughness improve based on total HD or just the summoners?
Same example lets make him level 6 though (6d8 HP, and 5d10 Temp) would it give +6 HP or +6 HP and +5 Temp HP since the eidolon uses my feats?

Arbalester |

When you summon your eidolon, you get his hp as temporary hp added to yours. Like all temp hp, it is lost first when damage is taken, and when it runs out, your eidolon is unsummoned. Unlike normal temp hp, it can be healed through the eidolon healing spells. (I forget the names, check the APG).
As for whose hp gets upped with a belt of constitution, it depends on who's wearing it: You or your eidolon. I haven't checked Synthesist much; can a Synthesist's eidolon even wear gear? A regular summoner's eidolon can. If it can wear gear, you and your eidolon share slots. So if you put on a magic belt, your eidolon can't wear a magic belt, and vice versa. So only one of you can wear a magic belt. If you put on a belt of +Con, your hp will go up; that's your normal hp. If your eidolon wears the belt (if it even can), it's hp will go up, which is the temp hp you gain when you summon it.
With Toughness, once again, who's taking the feat? You or your eidolon? Does a Synthesist eidolon get feats? I think it does. Anyway, once again: If you take the feat, your hp goes up. If the eidolon takes Toughness, it's hp goes up, so you'll get more temp hp from summoning it.
I hope that clears a few things up. Synthesist is a surprisingly complicated archetype.

Malfus |

When you summon your eidolon, you get his hp as temporary hp added to yours. Like all temp hp, it is lost first when damage is taken, and when it runs out, your eidolon is unsummoned. Unlike normal temp hp, it can be healed through the eidolon healing spells. (I forget the names, check the APG).
As for whose hp gets upped with a belt of constitution, it depends on who's wearing it: You or your eidolon. I haven't checked Synthesist much; can a Synthesist's eidolon even wear gear? A regular summoner's eidolon can. If it can wear gear, you and your eidolon share slots. So if you put on a magic belt, your eidolon can't wear a magic belt, and vice versa. So only one of you can wear a magic belt. If you put on a belt of +Con, your hp will go up; that's your normal hp. If your eidolon wears the belt (if it even can), it's hp will go up, which is the temp hp you gain when you summon it.
With Toughness, once again, who's taking the feat? You or your eidolon? Does a Synthesist eidolon get feats? I think it does. Anyway, once again: If you take the feat, your hp goes up. If the eidolon takes Toughness, it's hp goes up, so you'll get more temp hp from summoning it.
I hope that clears a few things up. Synthesist is a surprisingly complicated archetype.
When the Eidolon is out, just think of it having the same class levels and gear as you do (except for armor). That's about how it works.

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When you summon your eidolon, you get his hp as temporary hp added to yours. Like all temp hp, it is lost first when damage is taken, and when it runs out, your eidolon is unsummoned. Unlike normal temp hp, it can be healed through the eidolon healing spells. (I forget the names, check the APG).
I understand its temp HP but using the Eidolons "Fused Link" ability I can sacrifice my own HP in order to keep my eidolon up. Thus my Summoners HP amount is fairly important.
As for whose hp gets upped with a belt of constitution, it depends on who's wearing it: You or your eidolon. I haven't checked Synthesist much; can a Synthesist's eidolon even wear gear? A regular summoner's eidolon can. If it can wear gear, you and your eidolon share slots. So if you put on a magic belt, your eidolon can't wear a magic belt, and vice versa. So only one of you can wear a magic belt. If you put on a belt of +Con, your hp will go up; that's your normal hp. If your eidolon wears the belt (if it even can), it's hp will go up, which is the temp hp you gain when you summon it.
A synthesist and his eidolon are considered a single creature and when fused they use all of the synthesists gear. This works pretty obviously with most gear (Boots of Springing and Striding would improve my eidolon speed for example) but since we use both the summoner and the eidolons HP it becomes a problem when we refer to HP increases.
With Toughness, once again, who's taking the feat? You or your eidolon? Does a Synthesist eidolon get feats? I think it does. Anyway, once again: If you take the feat, your hp goes up. If the eidolon takes Toughness, it's hp goes up, so you'll get more temp hp from summoning it.
Synthesist Eidolons do not gain Feats or Skills. The question is do they benefit from my feats? I'm going to answer myself no on this one since there is no text that says it would.

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You can take feats for your synthesist whose prerequisites you only meet while in synthesized form, at least that has been the consensus on the boards and I think some developers said so too, like SKR. So you could take power attack or even resilient eidolon, and etc etc.
Oh that is nice. I hadn't even thought of that as a problem.

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You figure the summoner's HP using it's own CON. If you are wearing a belt it increases both scores. Your example would have 14 w/o Eidolon 16 w/ if he was wearing a belt.
So your hp would be 10/8....if you get hit for 17 damage, you lose 7 temp HP, and 10 normal HP. Because you need to keep 1 HP in your Temp. pool to keep the eidolon around. I'd rule that you'd be staggered at that point, but still have the eidolon.

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You figure the summoner's HP using it's own CON. If you are wearing a belt it increases both scores. Your example would have 14 w/o Eidolon 16 w/ if he was wearing a belt.
So your hp would be 10/8....if you get hit for 17 damage, you lose 7 temp HP, and 10 normal HP. Because you need to keep 1 HP in your Temp. pool to keep the eidolon around. I'd rule that you'd be staggered at that point, but still have the eidolon.
And that finished my questioning. Thanks for the info all.
So far everything I read is right save one thing ... You can't "heal" your edi , Its Temp. Hp , Temp. Hp cant be healed... What you can do is Just burn your own hp with your link and restore its hp then say Drink a potion or heal your self/get healed... But even thats up to your dm.
Actually you can Heal your edi. However you may only use the eidolon specific heals in order to do this, The Cure list or any other heals wont work. Heres a link to the FAQ

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Summoner HP and Eidolon Temp HP are calculated separately and independently of each other. You do not gain extra hit points from your Eidolon.
Not quite accurate.
Your hit points are adjusted by your constitution, which does change. In the OP's example, 1 regulare hit point is gained when fused, and lost when the eidolon is dismissed.
At higher levels, this can lead to an instant death scenario.
As for whose hp gets upped with a belt of constitution, it depends on who's wearing it: You or your eidolon. I haven't checked Synthesist much; can a Synthesist's eidolon even wear gear?
In the case of an item increasing a physical stat, both the summoner and the eidolon gain the benefit. It is a bonus, not a base stat.
In the above example with a +2 Con belt, the synthesist would have a 15 Con unfused and a 16 Con fused. Yes, this means the synthesist double dips on Con, once for normal hit points, once for temp hit points.

joeyfixit |

~1st question~
Do I count the HP of the summoner before or after summoning the eidolon?
1st level Summoner w/ Con 12 (+1) HP is 9 (1d8+1)
his eidolons con is 14 (+2) giving temp HP 7 (1d10+2)Before Summoning example
His HP stays at 9 since he does not alter his standard HP score
After summoning example
His HP bumps up to 10 while with the eidolon due to a higher Con score. And drops by 1 when it is dismissed.
~2nd question~
This question is moot if the answer to question 1 is "After" but if the answer is before then does wearing a belt of mighty constitution effect both Con scores (and thus both HP pools)Same Example
His HP is now 10 and his Temp HP is now 8 due to the increased constitution of both himself and his eidolon.~3rd question~
Would toughness improve based on total HD or just the summoners?
Same example lets make him level 6 though (6d8 HP, and 5d10 Temp) would it give +6 HP or +6 HP and +5 Temp HP since the eidolon uses my feats?
As far as I know, both Eidelon and Summoner will benefit from an item like a belt. So both the Summoner and the Eidelon will get the bonus HP.
Also, while an Synthesist's Eidelon cannot take feats or skills, the Summoner can, and the Eidelon should benefit from them. Feats like Rapid Shot, for example, or Stealthy, or Power Attack are things that the Summoner can take and then benefit from while in Fused (if the Fused form meets prerequisites).
Whether or not a Fused Eidelon benefits from Toughness is another matter, and worth its own thread, I would say.