I'm without a book at the moment, but I have a quick question for you.
An item in an Aegis, what happens? Let's say you have an item (overall it's a PeCo effect level 35, no Penetration). The Aegis is level 40. Does the item work? Do you have to roll for it? When you roll is it stress or simple?
So would you have to roll a 15 (35-20) and beat 35 to have it work?
Assuming the item is foreign, its effects would be resisted. The aegis will lower the penetration of the effect by half the aegis level (20). Since originally the penetration was 0, it is now -20. Hence, the effect will work on anyone that has no magic resistance, but will fail to affect anyone that has even Magic Resistance 0.
[The aegis spell itself only says the effects are resisted, not being perfectly clear on how. Since the aegis lowers the penetration totals of magical creatures by the same amount, and reduces a magus' casting total by it, this is my interpretation. But it isn't strictly spelled out in the RAW.]
There is no roll involved.
OK, got my book now.
The Aegis redues the casting total by half its level, not the Penetration. The item would be resisted, I'm just not sure if you would roll or not, that's still unclear to me.
Indeed, the spell design is not clear on this. The other option would be for the item to penetrate the level of the Aegis to take effect, as with a parma magica. I think I would prefer that last option, since magical items don't have a casting total. But it's harsh...i'm not very satisfied with that rule..
Of what roll are you speaking about ? Maybe you would get more help if you could explain =)
What roll? Activating an item involves no roll, so there is no roll to make. The existence of the aegis doesn't change that.
The aegis does reduce the "penetration totals for magical creatures" by half the aegis level. Not just casting totals. Note that in that case too, there is (generally) no roll involved.
It is ambiguous. I prefer items to not fizzle when entering an aegis, however, and like the possibilities of people using magical items within a foreign aegis. Since magical creatures don't have a casting total too but get their penetration reduced, and since the item is already within the aegis so effectively "casts" the spell from within it, I'm comfortable with reading the spell like that.
Yes, and yours is a very reasonnable approach to this. This is probably a case that may vary from Saga to Saga.
Anyway, here is some thoughts about it if anyone care 
We had this problem in our campaign. My idea makes it very difficult for magical items to work in an Aegis. Too much for my taste.
But your idea also means that the Aegis only adds to the effectiveness of already existing Magic Resistance. The same magical object will be have to penetrate if used from outside the boundary of the Aegis unto something withing - but not if used from within ? It means any magical object will be effective against something/one who have no magic resistance. It give protection only to those who already have some...and I don't like that either, I like the Aegis to protect all that is within.
To solve this, I would suggest making a house rule so that it need to penetrate half the level of the Aegis. YMMV !
This problem has not yet showed up in our saga, but being the SG, I considered the problem for quite some time.
Here are the house-rules I came up with for consideration:
-Items wielded by anyone who participated in the Aegis ritual are not affected by any of the following rules, as are all effects imbued into items inside the Aegis while it was working or being inside the Aegis when it was cast. (Thus at most made three seasons ago.)
-Effects turned on before entering the Aegis are dispelled upon entering the Aegis.
-Effects turned on inside the aegis suffer a penetration’s reduction by half of the Aegis’ level. (But a bonus for the Aura inside the Aegis as usual)
-I would house-rule that any effect with a final penetration below 10 fizzels. This is similar to casting totals and prevents intruding magi to use all items without restriction that do not need penetration. (like moving boulders, destroying walls….)
What is your opinion on the part about effect fizzle when penetration drops below 10?
I think it's a very good idea, to have even the non-penetration items suffer from being in a foreign Aegis. Otherwise it's too easy and powerful to use items, say during a Wizard's War when sneaking into the enemy's Sanctum.
I'd perhaps say, that following the mechanics of a penalty of half the Aegis' level being suitable, the Penetration of items suffer. If penetraion is below 0, it does not work! It's harsh, but actually gives the item-munchkins something to think about, when making Penetration a low-(or no-)priority for items not meant to directly affect a Magus or someone with Magic Resistance. So invisible intruders, magic keys to all locks, things killing or rendering guards irelevant aren't as easy. And forget about teleporting in and out!
Also, I'd have items used by those present during the casting of Aegis, or anyone with tokens, used normally.
Any item not used as above, will cut it's effects when entering/crossing the Aegis boundary.
Upon further contemplation, this seems the best solution. Looks like another house rule.
Vetrenius: I prefer the limit of 0. Zero is simpler, allows more leeway, and makes sense. I understand that you want to mirror the result that magi can cast spells with a casting total up to 10 below the spell's level - did you mean the effect fails if its penetration drops to below (-10)? I don't think this is a good idea; spontaneous spells are cast differently, and invested effects set off differently - I see no need to establish this analogy, and am perfectly happy with letting items fail. The limit of 10 seems very arbitrary and mechanical. That said, I don't think it's a very bad house rule, it's perfectly fine.
The real problem is that any solution might have the effect of causing items to be too powerful or too weak.
This might be the way to go. By doing it this way you have the Aegis causing items to fizzle even against beings without resistance, but also give item inventors the opportunity to make an item that can work in an Aegis.
That was the problem I was referring to. There is no roll for an item, but an item like this is closer to a spell rather than a creature's power.
My idea would be that (barring that house rule above) would be that you would have to roll, basically taking the level of the effect, subtract half the level of the Aegis, and then make a stress roll to see if it could work or not.
I like the house rule, but would like to hear a final say from the editors on this for clarity.
This has never happened to us but I think the easiest solution would be to make the aegis grant everybody inside it a 1/4 to 1/2 its level in MR.
A wand odf insta-kill with 0 penetration would be quite ineffective inside an aegis, then, since everything (servants, animals, doors and buildings...) would haver a MR of half the level of the aegis.
If you make that stack with parma magica and form bonus, makes a magus' home base quite a fortress, that is the original idea of the aegis, after all.
Cheers,
Xavi
Great idea!!!
Slight modification: Give them a bonus to the MR of Aegis/2 
That way, the Aegis has exactly the same effects when casting spells on maguses (ie - level/2 and must bypass MR), and everything else is protected as well 
This solution solves almost all problems. Just what about a mage using a spell with range personal on hinself? I would rule it reduces his casting total by Level/2 if the Aegis penetrates his parma. If the Aegis' penetration cannot breach the parma, the casting total is not changed.
A bonus to MR won't fizzle the item's magic. I don't like the image of someone engulfing the whole area with fire only to have it burn nothing, not even the dry twigs. I prefer to lower the penetration and have the effect fizzle if relevant, makes for a better image IMO.