One of my players in my new game is requisting a spell that would allow his street shaman to "See through walls", not only is this possible with the rules but it would be dirt cheap.
My quistion is this: How can I incorperate such a spell and still maintain a simblance of balance, primarly because this spell would open up a lot of opertunitys for spellscasting against someone that can't fight back. (Line of sight) So far the only way to keep him from blasting foes in the next room over is to limit it to mana spells, (Lightning bolts and such can't fly through walls) but this wouldn't stop alot of his spells and it could also be used to become a UBER sniper. I don't wana deny him a obviously possible spell, but I also don't want to make the game unbalanced.

Just because he can see through the wall doesn't mean he can "see" them in order to satisfy spellcasting requirements.

In fact, I think there are already a couple of spells that are functionally the same as his request. They don't however provide LOS for purposes of spellcasting. Check Street Magic.

I remember that there was a similar thread a while back about the invisibility spell. If you cast invisibility on a door, could you then see people on the other side? Could the people on the other side see through it as well? And could you cast direct combat spells through the door? I think the consensus at the time was "Yes" to all three questions.

IIRC natural light (not magically bent light) must be able to travel in a straight line between the subject and the the caster to maintain LOS. This means if a physical object like a wall or door is in between the caster and the target (regardless if it is visible or not), then you don't have LOS and anyone on the other side can't be targeted. This is also why mage sight goggles work, as they naturally bend light so you can maintain LOS.

That being said, the ability to look through walls is still powerful as it will give you vital information (Oh look, the guards have set up an ambush for us behind this door. Lets go the long way around.).

Two things:

The only way this spell would be feasable is if it works both ways he's not seeing through walls, shadowrun magic can't do that outside of the ridiculous sense enemies spell. So the only way for him to accomplish this feat is to atcually chang ethe properties of the wall itself, a taxing undertaking.

Also flat out wouldn't allow it to satisfy the line of site requirement.

In the end anything a player can come up with could have likely been come up with by someone else and should have. Therefore I would presume the reason such a thing doesn't exist is because it's just not possible. Easy peasy.

"Detect Life" I think you mean, and yes that is quite nuts.

Simply don't allow it. Magic is powerful as it is, he just needs to use Mage Sight Goggles and find a simple way of poking holes into things. Then cast away.

I would caution allowing this, and this comes from a mostly-magician player.

Clairvoiyance does this, right? No spellcasting though.

Might as well use clairvoyance and a HMG with a tripod and APDS ammo. Then you can really start poking holes in people through walls.

I'm thinking I'm going to have it as a lower drain version of clairvoyance, why real light is needed to convay the metal image and theirfor magical link I don't know but as long as he can't go blasting people with Shatter while in the room over everything should be peachy.

I don't see what is wrong with being able to see through a wall but not being able to cast through the wall. You can do that already with a short astral projection anyway. I think to keep game mechanics in balance you just need to reinforce the point that while you can see the people on the other side of the wall, you don't have LOS with them and thus can't cast spells against them (unless your willing to have someone act as an astral spotter and do ritual sorcery (which is just plain silly).