How to get 1d6 sneak attack per 3 caster levels for an arcane caster.

[begooler]

The following will grant you a good amount of sneak attack for a caster who doesn't have to have levels in rogue. There's a considerable amount of feats and levels involved, so obviously you would apply this to a build where those levels are already useful. This chain also gives you the ability to persist and extend many other buffs, and also lots of spell slots.

Start out with some levels of wizard (suggested school focus Divination,) and beguiler (or other spontaneous casting class of your choice.) Choose Extend Spell as one of your low level feats.

After level 6:
Take a level in Unseen Seer from Complete Mage (Which gives 1d6 sneak attack itself)
Take enough levels in Ultimate Magus (also Complete Mage) so that you can cast 2nd level beguiler/sorcerer spells.
Take the second level of Unseen Seer. Use advanced learning to learn the Ranger Spell Hunter's Eye from PHB2 as a beguiler/sorcerer. (or you can learn it as a wizard and use the UM's class feature to learn as a spontaneous caster later.)

Take Persistent Spell (perhaps as 5th level UM bonus feat.) You also want Metamagic School Focus: Divination, and probably Practiced Spellcaster for both of your classes. It's not necessary, but would also make a whole lot of sense to take Versatile Spellcaster from Races of the Dragon to use your beguiler/sorcerer slots to cast wizard spells.

At the point at which you can cast 5th level wizard spells, you can use UM's augmented casting to spend a wizard 5th level slot to apply Persistent Spell to a divination spell you know as a beguiler/sorcerer. Persistent Hunter's Eye gives you 1d6 sneak attack per three caster levels. Now you have lots of sneak attack all the time and you're not even a rogue. You can also persist other divination buffs like Critical Strike, so long as you know them on your spontaneous caster's list.

At the point where you can cast 6th level wizard spells, you can Persist any other non-divination buff your spontaneous caster class has as well. (Wraithstrike would be sweet if you're a melee-er, Displacement is also nice...)

Application: for a melee/caster willing to give up the BAB, or an arcane caster who uses rays or melee touches.

[dextercorvia]

Or, you can use all of those feats and shenanigans to raise your caster level a little and blast.

Wizard5/WildMage1/UltimateMagus4

Human with Two Flaws and Spellgifted Trait
Feats: Alacritous Cogitation, Practiced Spellcaster, Searing Spell, Bloodline of Fire, Spell Thematics, Firey Burst, Energy Substitution

is a 10th level caster with a CL of up to 17+1d6.

[Thespianus]

After level 6:
Take a level in Unseen Seer from Complete Mage (Which gives 1d6 sneak attack itself)
Take the second level of Unseen Seer. Use advanced learning to learn the Ranger Spell Hunter's Eye from PHB2 as a beguiler/sorcerer. (or you can learn it as a wizard and use the UM's class feature to learn as a spontaneous caster later.)

Unseen Seer doesn't give you Sneak Attack, it just adds to the Sneak Attack you already have.

However, it can be argued that when casting Hunter's Eye at 2nd level, you "get" Sneak Attack, and at that level, you'll get the extra sneak attack from Unseen Seer.

But, no, Unseen Seer doesn't grant Sneak Attack as a class feature.

[Akal Saris]

Alternativel:

Wiz 6: take Martial Study (Shadow Jaunt)
Wiz 12: take Martial Stance (Assassin's Stance)

Now you have 2d6 sneak attack on all the time, and can take Craven at 15, unseen seer levels grant sneak attack, etc.

[dextercorvia]

Martial Stance can be taken as a Fighter Bonus feat, right? Then the Martial Wizard variant can get it at 10.

[Ravens_cry]

How about Arcane Trickster?

[MeeposFire]

Or, you can use all of those feats and shenanigans to raise your caster level a little and blast.

Wizard5/WildMage1/UltimateMagus4

Human with Two Flaws and Spellgifted Trait
Feats: Alacritous Cogitation, Practiced Spellcaster, Searing Spell, Bloodline of Fire, Spell Thematics, Firey Burst, Energy Substitution

is a 10th level caster with a CL of up to 17+1d6.

What are you using to get into ultimate magus? Spontaneous divination or some other trick?

[Kurald Galain]

Take one level in Spellthief, that should let you qualify for both Arcane Trickster and Unseen Seer, and still give you some nice abilities.

[dextercorvia]

What are you using to get into ultimate magus? Spontaneous divination or some other trick?

Alacritous Cogitation is what was listed. (Please note I'm not using UM to advance Wizard twice, just using the +1CL bumps at 1 and 4.

[TheSteelRat]

Martial Stance can be taken as a Fighter Bonus feat, right? Then the Martial Wizard variant can get it at 10.

Why waste the feats, when you can use Heroics (2nd lvl Sor/Wiz spell, 10 min/lvl) to get it when convenient? Add an Extend Spell (to allow earlier casting) or Quickened Spell (if in combat) to flavour as necessary. Though I like getting Combat Expertise, especially if you're mostily using save-or-lose spells, and have a good enough feel that the extra BAB isn't necessary for touch attacks.

Couldn't the Generic Spellcaster from UA get Sneak Attack as well?

I think this perhaps would work better with an Artificer / archivist / Incantrix in the party. They'd have access to the spell (item/prayerbook/borrowed, respectively) and could improve it (Metamagic Item for Artificer or Incantrix's metamagic boosting of existing spell effects). Then again, what problem isn't solved with adding an Artificer or Incantrix to the mix?

[sreservoir]

Couldn't the Generic Spellcaster from UA get Sneak Attack as well?

UA generic classes can get basically anything in core other than bardic music and prepared casting.

[begooler]

Unseen Seer doesn't give you Sneak Attack, it just adds to the Sneak Attack you already have.

However, it can be argued that when casting Hunter's Eye at 2nd level, you "get" Sneak Attack, and at that level, you'll get the extra sneak attack from Unseen Seer.

But, no, Unseen Seer doesn't grant Sneak Attack as a class feature.

Good point. And I would not rule that Hunter's Eye let's you qualify for having sneak attack. So to take advantage of the Unseen Seer's 1d6, you'd have to already have one rogue/spelltheif etc level. That would make sense anyway as resources are already being invested in getting sneak attack.

Arcane trickster is a great PRC to follow up with, however, you do start to talk about investment in a lot of different skills.