Sneak Attack Spells

[Phawksin]

Howdy Playground! Long time lurker, first time poster.

First, background: I'm running a a game with a lot (like, a stupid amount) of homebrew content. Homebrew equipment, sub-classes, swapped abilities and a reflavored warlock/artificer. Every bit of homebrewed content has been discussed out-of-character with all the players and we are all onboard with all of it. With that in mind I feel pretty confident in balancing homebrew content for this game. I recognize that 90% of my stuff wouldn't fly at any other table and I'm totally okay with that. Background complete.

One of the PCs is a Rogue/Wild Mage Sorcerer who is having an absolute blast sneak attacking distracted targets, disengaging past baddies to evil spellcasters to twist fate and get advantage against spells before using advantage to sneak attack someone silly and diving back out to protect our ranged caster. No, its not the most efficient build. Yes, its a lot of fun.

My problem is she wants to develop the character to blend the magic/martial thing she has going on by being able to apply sneak attack to spells. I'm thinking about adding that feature either as a Rogue subclass or a feat. Either way it would have the following restrictions:

1) Must meet SA requirements (advantage or attacking a target within 5 feet of an ally without disad)
2) Can only deal SA damage on a spell with an attack roll (not saving throw spells)
3) the SA damage can only apply to one target effected by the spell, NOT every target effected by the spell.

While I'm worried that cantrip sneak attacks will outperform weapon sneak attacks, I don't think Its a huge deal. No more than a hand full of damage depending on ability score modifiers. However, I can't find much information on allowing this so I'm worried that it might have further reaching changes than I'm aware of. So, Playground, How broken/OP/stupid would allowing SA damage on spells actually be?

[Gritmonger]

My gut feeling would say that the must be attack spell not save, must abide by SA restrictions, only one target -
I might go further and say "cantrip only" - and figure that the advancement in rogue and sorcerer levels might taper at close to the same level of either individually. She likely won't have the total sneak attack of a full rogue, she likely won't get the benefit of twenty caster levels of cantrip improvement... but if that's not true, if she goes to 20 in cantrip casting without having to take commensurate caster levels, you might cap the sneak-attack damage unless she takes "spell sniper" as a feat.

[eastmabl]

Howdy Playground! Long time lurker, first time poster.

First, background: I'm running a a game with a lot (like, a stupid amount) of homebrew content. Homebrew equipment, sub-classes, swapped abilities and a reflavored warlock/artificer. Every bit of homebrewed content has been discussed out-of-character with all the players and we are all onboard with all of it. With that in mind I feel pretty confident in balancing homebrew content for this game. I recognize that 90% of my stuff wouldn't fly at any other table and I'm totally okay with that. Background complete.

One of the PCs is a Rogue/Wild Mage Sorcerer who is having an absolute blast sneak attacking distracted targets, disengaging past baddies to evil spellcasters to twist fate and get advantage against spells before using advantage to sneak attack someone silly and diving back out to protect our ranged caster. No, its not the most efficient build. Yes, its a lot of fun.

My problem is she wants to develop the character to blend the magic/martial thing she has going on by being able to apply sneak attack to spells. I'm thinking about adding that feature either as a Rogue subclass or a feat. Either way it would have the following restrictions:

1) Must meet SA requirements (advantage or attacking a target within 5 feet of an ally without disad)
2) Can only deal SA damage on a spell with an attack roll (not saving throw spells)
3) the SA damage can only apply to one target effected by the spell, NOT every target effected by the spell.

While I'm worried that cantrip sneak attacks will outperform weapon sneak attacks, I don't think Its a huge deal. No more than a hand full of damage depending on ability score modifiers. However, I can't find much information on allowing this so I'm worried that it might have further reaching changes than I'm aware of. So, Playground, How broken/OP/stupid would allowing SA damage on spells actually be?

Sneak attack doesn't apply to spell attacks? *checks rules* I guess sneak attacks doesn't apply to spells RAW.

Not that I've had this come into play, but I wouldn't see such a big issue with it that if a player wanted to do this, that I would say "no."

I like all three of your requirements - I might have the player choose the character she is sneak attacking with the spell. (EG spell attacks three monsters (A, B and C); I would have the player tell me which monster the sneakiest attack is against).

[Jlooney]

It seems ok just watch out for scorching Rays with SA dice. To much damage.

[Marcelinari]

It seems ok just watch out for scorching Rays with SA dice. To much damage.

Remember that SA only applies once each turn, so even scorching rays shouldn't be too bad. OP, you should work out how cantrip damage scales vis. multiclassing - doing a full 4d10+10d6 with only a one-level investment into sorcerer seems like it would be silly. That'd be... 22+35... 57 average damage? Honestly that's not even that ridiculous at level 20, but I'm convinced I haven't thought that whole thing through.

Hmm. Sorcerer 3/Rogue 17, Quicken metamagic? 4d10+4d10+9d6... 22+22+31.5... 75.5?

Let's try (Fire) Dragon Sorcerer 6/Rogue14, Quicken. 4d10+5, 4d10+5, 7d6... 8d10+7d6+10... 78.5? But with Dragon Sorcerer, of course, you can't Tide of Chaos for free advantage, so this point is moot.

So, at level 20 this shouldn't work out to be too bad in the long run, basing damage on cantrip progressions. I haven't considered outright damaging spells, but consider that 1) AOE spells don't roll to hit and therefore can't SA, 2) You can't apply SA to multiple separate attacks per round, and 3) The tides of chaos thing only works 1/long rest, unless you as DM explicitly force a roll on the Wild Magic table.

EDIT: The most dangerous iteration may be Warlock 3/ Wild Sorcerer 3/ Rogue 14. 2 Agonizing Eldritch Blasts are already pretty nasty, and an additional 7d6 SA may be overdoing it. That's what... 8d10+7d6+40? 108.5? Assuming all hit, and none crit. I'd call that overkill.

[SharkForce]

it will be a fairly substantial boost to DPR.

especially with a 2 warlock splash, it will grant massive reliability to the sneak attack damage while already dealing very near what a weapon-using fighter will just from the eldritch blast + hex alone.

now, the question of whether this is broken or not is up for debate. i will say that it is massively superior to any DPR build currently in the game. considering that, for some classes, 95% or so of their contribution to the party is reliable DPR (and this build will give extremely easy access to superior reliable DPR plus a whole bunch of other things), allowing sneak attack with spells will probably make those classes feel extremely impotent. (note that the rogue on its own deals very good DPR while doing far more than those pure DPR focused classes, but is not typically superior, which is a key difference).

now, in your specific case, it probably won't be nearly as bad. but it still has the potential to cause problems... a sorcerer can, for example, twin a firebolt and quicken a second firebolt all in one round. giving 3 attack rolls, each of which deals respectable damage, allowing you to reroll the 1s and 2s with an appropriate feat, and making your sneak attack damage very easy to land.

i would allow it situationally in parties where there is no pure DPR (and thus nobody to be made to feel useless), probably on condition of a gentleman's agreement to not abuse it with a 2 warlock splash (sorcerer is fine, they'll either need lots of sorcerer levels to provide sorcery points and won't have huge sneak attack damage, or won't be able to do the full combo terribly often, and the basic cantrip version will add a relatively small amount of DPR since they won't be able to use two weapons or crossbow master to get a second chance of dealing sneak attack damage, plus in your case it requires a third attribute).

if you *do* have pure DPR classes in the game, that makes it a bit tougher. at levels 1-10 or so, i wouldn't expect it to make a huge difference and would probably still allow it. at higher levels, it will make a much larger difference. but then again, i'd probably see about adding something to those pure DPR builds to make them able to do more at those levels anyways, so YMMV.

[tarlison]

If I have any say about it , it specifically say finesse and range weapons only and spells are not consider weapon and dont do weapon attacks and are called magic or spell attack so likely can't be use for sneak attack unless specofied

[xyianth]

As long as you stick to the 1/turn aspect of sneak attack, it shouldn't be a problem in most cases. I'd require the spell to be attack roll based, any save based spell can't crit and therefore can't benefit from sneak attack. The spells to watch out for are the spells that allow multiple attack rolls in the same turn: eldritch blast and scorching ray. These provide a massive boost in reliability of the sneak attack, since you are practically guaranteed to hit at least once.

You might want to rule that the sneak attack damage is divided up among each attack granted by the spell, then apply the extra damage on each hit. Example: warlock 2/rogue 15 uses eldritch blast. As a 17th level character, eldritch blast allows 4 attacks. As a 15th level rogue, the character has 8d6 sneak attack. If the character hits 3 of the 4 blasts, roll the full 8d6 sneak attack and add 3/4(round down) of the result. If the same character targets multiple creatures, each blast deals 1/4(round down) sneak attack damage on a hit. If any creature is not a valid sneak attack target, any sneak attack damage on that blast gets wasted. This might be a bit math heavy though, so only use it if it won't slow down the game.

[Giant2005]

The reason they took away the Rogue's second attack is because scaling sneak damage + scaling attack damage is OP. By letting the Rogue cast on a scaling cantrip that adds scaling sneak damage, his attack will scale twice and fall back into the realm of OP.
If this happens at all you will need to limit that - Cantrips can only have sneak damage if they restrict themselves to their unscaled level 1 damage, or maybe you could have sneak attacks only apply to spells with attack rolls that are cast from a level 1 spell slot.

[xyianth]

The reason they took away the Rogue's second attack is because scaling sneak damage + scaling attack damage is OP. By letting the Rogue cast on a scaling cantrip that adds scaling sneak damage, his attack will scale twice and fall back into the realm of OP.
If this happens at all you will need to limit that - Cantrips can only have sneak damage if they restrict themselves to their unscaled level 1 damage, or maybe you could have sneak attacks only apply to spells with attack rolls that are cast from a level 1 spell slot.

That makes no sense, sneak attack is 1/turn anyway. The only thing an extra attack does for a sneak attacker is increase the reliability of getting a successful sneak attack each turn.

[Giant2005]

That makes no sense, sneak attack is 1/turn anyway. The only thing an extra attack does for a sneak attacker is increase the reliability of getting a successful sneak attack each turn.

It increases your damage per turn.
1 sneak + 2 attacks > 1 sneak + 1 attack.
1 sneak + 1 fully scaling cantrip > 1 sneak + 1 attack.

Basically cantrips are designed to scale in a similar manner to martial character's attack action economy scaling. In the playtest Rogues used to get a second attack but it was removed because both scaling sneak damage and scaling attack action economy proved to be unbalanced. By allowing scaling cantrips to trigger a sneak attack, then those cantrips which are supposed to be keeping pace with attack action economy scaling would bring that issue back into play.
Having the sneak attacks apply only on non-scaling, attack roll based spells would allow the player to use his spells to sneak attack without bringing back that imbalance.

[xyianth]

It increases your damage per turn.
1 sneak + 2 attacks > 1 sneak + 1 attack.
1 sneak + 1 fully scaling cantrip > 1 sneak + 1 attack.

Basically cantrips are designed to scale in a similar manner to martial character's attack action economy scaling. In the playtest Rogues used to get a second attack but it was removed because both scaling sneak damage and scaling attack action economy proved to be unbalanced. By allowing scaling cantrips to trigger a sneak attack, then those cantrips which are supposed to be keeping pace with attack action economy scaling would bring that issue back into play.
Having the sneak attacks apply only on non-scaling, attack roll based spells would allow the player to use his spells to sneak attack without bringing back that imbalance.

First, cantrips do a horrid job of keeping up with attack scaling. (other than eldritch blast) 4dX damage is roughly equal to 2 weapon hits, yet martial characters get this at 5th level while casters must wait til 17th. Now, I am not suggesting cantrips should be strengthened, spell casters are plenty strong enough as is after all. But to suggest that scaling cantrip use with sneak attack is OP is stretching the term a bit. A rogue could dip 1 level in fighter to pick up the two weapon fighting style and get 2dX+2*dex+sneak attack damage. The same rogue could instead dip 1 level as a caster and (assuming we allow cantrips to sneak attack) do 4dX+sneak attack damage. Is 4dX really that much more OP than 2dX+2*dex? (keep in mind that unless X>8, max dex results in more damage)

Second, nothing prevents a character from gaining extra attacks via multiclassing while remaining primarily rogue. A fighter 5/rogue 15 gets 2 attacks a round with +8d6 sneak attack 1/turn. This is only 2d6 less sneak attack than a rogue 20.

I will grant you that allowing spells to sneak attack is slightly more powerful than what a single classed rogue is capable of. I'll also admit that I am not unbiased, I've always loved the concept of a spell sniper. But I seriously doubt that allowing sneak attack on spells that use attack rolls would be game breaking as long as you watch out for spells that grant multiple attack rolls.

[Giant2005]

Is 4dX really that much more OP than 2dX+2*dex? (keep in mind that unless X>8, max dex results in more damage)
It is.
4D10 averages out to 22 damage per round whereas 2D6+10 averages out to 17 damage per round and comes at the expense of the bonus action which most would agree is a massive drawback considering how much everyone seems to favor Cunning Action. Plus the crit potential (Which is very important for Assassins) is significantly higher for the Cantrip which would increase its average damage to 44, with the dual-wielder staying at a humble 24. Basically, the spell sneaker inflicts more damage, conserves his bonus action and can do the above safely from range. When all of the advantages are on way side of the fence, it is impossible to consider it anything but unbalanced.

Second, nothing prevents a character from gaining extra attacks via multiclassing while remaining primarily rogue. A fighter 5/rogue 15 gets 2 attacks a round with +8d6 sneak attack 1/turn. This is only 2d6 less sneak attack than a rogue 20.
That is true and there is nothing at all wrong with that. Multiclassing is pretty well balanced in this game - that Rogue is giving up a substantial amount of useful abilities for that extra attack. He is giving up almost nothing (A feat, a single class level or having to be an Elf) by using a Cantrip to have the same effect. If sacrificing 5 levels of Rogue abilities to gain 5 levels of a Martial class is balanced, then sacrificing next to nothing for the same effect cannot be balanced.


I will grant you that allowing spells to sneak attack is slightly more powerful than what a single classed rogue is capable of. I'll also admit that I am not unbiased, I've always loved the concept of a spell sniper. But I seriously doubt that allowing sneak attack on spells that use attack rolls would be game breaking as long as you watch out for spells that grant multiple attack rolls.
With one sneak per turn, having multiple attack rolls isn't really that much of an issue. The issue is the extra damage they bring to the table and in some cases, the potential to attack on turns other than your own and getting more sneak attacks per round because of it.
Still, even if you can't sneak attack with spells, there is nothing stopping you from being a Spell Sniper. All you need to do is be hidden and hit them from range to be a Spell Sniper. You could hit them from 1200' away as a Warlock/Sorc and snipe them to death long before they ever had a reasonable chance of figuring out where you are; or at least finding a way to get to you before they were gunned down.

[xyianth]

It is.
4D10 averages out to 22 damage per round whereas 2D6+10 averages out to 17 damage per round and comes at the expense of the bonus action which most would agree is a massive drawback considering how much everyone seems to favor Cunning Action. Plus the crit potential (Which is very important for Assassins) is significantly higher for the Cantrip which would increase its average damage to 44, with the dual-wielder staying at a humble 24. Basically, the spell sneaker inflicts more damage, conserves his bonus action and can do the above safely from range. When all of the advantages are on way side of the fence, it is impossible to consider it anything but unbalanced.

That is true and there is nothing at all wrong with that. Multiclassing is pretty well balanced in this game - that Rogue is giving up a substantial amount of useful abilities for that extra attack. He is giving up almost nothing (A feat, a single class level or having to be an Elf) by using a Cantrip to have the same effect. If sacrificing 5 levels of Rogue abilities to gain 5 levels of a Martial class is balanced, then sacrificing next to nothing for the same effect cannot be balanced.

Very well, I concede. Although, given the strength of full spellcasters, I still don't think it would be a problem including a rogue that performed sneak attacks with cantrips in a game. I also think it is a bit ironic that ~7 average damage at the cost of a feat is OP in this instance, but feats like crossbow sniper and polearm master are somehow not OP... (which add ~7 average damage as a mere part of their benefit) Not really a comment directed at you, just a random observation.


With one sneak per turn, having multiple attack rolls isn't really that much of an issue. The issue is the extra damage they bring to the table and in some cases, the potential to attack on turns other than your own and getting more sneak attacks per round because of it.
Still, even if you can't sneak attack with spells, there is nothing stopping you from being a Spell Sniper. All you need to do is be hidden and hit them from range to be a Spell Sniper. You could hit them from 1200' away as a Warlock/Sorc and snipe them to death long before they ever had a reasonable chance of figuring out where you are; or at least finding a way to get to you before they were gunned down.

That isn't a spell sniper in the same sense though. That is a long range caster sure, but the spell sniper I was referring to was the 3.5 prestige class, that specifically performed sneak attacks with spells. It wasn't about long range, it was about raw destructive power by targeting weak points from short to mid range. It used the word sniper to mean 1-shot KO, not extreme range. Sadly, it appears that this concept has been done away with in favor of simplicity/balance. :smallsigh:

[MrStabby]

I would say that Sneak Attack is open to abuse with cantrips and certainly should not be allowed as is. That said it is flavourful for some classes and it would be a shame to be without it entireley.

I would say that the abuse comes from just adding a dip of caster (as others noted) but this can be fixed. If I were in your position I would allow any caster to exchange their 6th level class ability to allow cantrips to be used instead of melee/ranged weapon attacks in other classes abilities.

If you take the ability as a Paladin you can smite on cantrips, as a Warlock/rogue you can sneak attack with Eldritch blast and as a Sorcerer/Champion your cantrips also crit on a 19 or 20. I think that the commitment to spellcasting justifies this.

To me it even seems fair for the 1/3rd spellcasting classes. If an Eldritch Knight or an Arcane Trickster want to skip their 6th level class ability to be more versative with magic in combat then it does not seem out of character for either of them.

[calebrus]

Rogues originally had Extra Attack at level 8 (if I remember correctly) in the play test.
It got the axe because it made their damage output MUCH more reliable than anyone else's was.
Allowing sneak attack on any spell with multiple attack rolls would basically turn the clock back and make them hands down and by far the most reliable DPR class in the game.
In addition, a scaling cantrip is already far better than a single weapon attack after mid-levels or so.

If you want to allow spells to sneak attack, I would limit it to cantrips from the wizard/sorc list that have an attack roll. That's what Arcane Tricksters have access to. Limit it to the cantrips that an AT could get.
Anything beyond that and we're in the realm of OP. That's already skirting it, but more than wiz cantrips would definitively cross that line.

[DanyBallon]

In order to prevent abuse, I would create a feat that allow rogue to sneak attack using cantrip only. Also cantrip must be from the following list (write down any cantrip that you would allow, i.e. cantrip with a single attack roll).
Prerequisite would be; ability to sneak attack + spellcasting class.
The prerequisite may seem excessive, but this will prevent a rogue with magic initiate to SA with cantrips without having to multiclass.

[Giant2005]

That isn't a spell sniper in the same sense though. That is a long range caster sure, but the spell sniper I was referring to was the 3.5 prestige class, that specifically performed sneak attacks with spells. It wasn't about long range, it was about raw destructive power by targeting weak points from short to mid range. It used the word sniper to mean 1-shot KO, not extreme range. Sadly, it appears that this concept has been done away with in favor of simplicity/balance. :smallsigh:
Fair enough. I wasn't aware that that was actually what a Spell Sniper was - I still kind of believe that the name supports my interpretation and Wizards screwed up when naming their sneak attacking spellslinger :D.


I would say that Sneak Attack is open to abuse with cantrips and certainly should not be allowed as is. That said it is flavourful for some classes and it would be a shame to be without it entireley.

I would say that the abuse comes from just adding a dip of caster (as others noted) but this can be fixed. If I were in your position I would allow any caster to exchange their 6th level class ability to allow cantrips to be used instead of melee/ranged weapon attacks in other classes abilities.

If you take the ability as a Paladin you can smite on cantrips, as a Warlock/rogue you can sneak attack with Eldritch blast and as a Sorcerer/Champion your cantrips also crit on a 19 or 20. I think that the commitment to spellcasting justifies this.

To me it even seems fair for the 1/3rd spellcasting classes. If an Eldritch Knight or an Arcane Trickster want to skip their 6th level class ability to be more versative with magic in combat then it does not seem out of character for either of them.



This seems like a good idea to me - the 5 level (Or in your idea's case, 6) caster dip should be balanced with a 5 level martial dip. I'd even allow it to work off any attack roll spell, regardless of level or spell slot (Which would be balanced with a martial class using their extra abilities in tandem with sneak attack, other than extra attack).
It seems a little clunky though. I'd just create a feat that enables sneak attack with spells and has the requirement of 3rd level spell slots so if a multiclassing-addict Rogue felt the need, they could meet the requirements with a bunch of different classes. It would also be available for a level 16+ pure Arcane Trickster which imo isn't too bad - around that level is when the Assassin gets his double damage ability and the Thief gets his second attack, both of which are fairly balanced against the feat suggested.

[Person_Man]

RAW Sneak Attack doesn't work with spells. Spells are not Finesse or Ranged weapons.

For homebrew, I suggest a few minor additions to the Arcane Trickster (which is perfectly playable RAW, but like every non-full caster in the game could use a few minor buffs at mid-high levels:

At level 3, Mage Hand can be cast without a Verbal Component (allowing them to cast it as an Action right before combat without revealing themselves).
At level 9 the Arcane Trickster can use Sneak Attack with any 1st level or higher spell (not cantrips) that requires one attack roll modified by Dexterity, in addition to gaining the Magical Ambush ability. Does not work with spells that allow more then one attack roll or Saving Throws in place of an attack roll. I understand that this increases the Arcane Trickster's damage potential, but by limiting to spells, you strongly limit the number of uses per day. And at this point in their progression, they really need some additional "burst" options in order to keep up with full casters.
At level 13 the Arcane Trickster can cast Mage Hand as a Bonus Action (instead of an Action), in addition to gaining the Versatile Trickster ability.
At level 14, the range of Blindsense is improved to permanent True Seeing with a range of 120 feet.
At level 15 they gain Use Magic Device (Thief ability) in addition to Slipper Mind.
At level 17 Spell Thief ability works once per Short or Long Rest (not just Long Rest) on any spell that targets only the Arcane Trickster that they pass a Saving Throw against (not just low level spells). The RAW ability is garbage.
At level 20, Stroke of Luck converts any one d20 roll into a natural 20 (critical or auto-success) once per Short or Long Rest. (Not just a hit for attack rolls or 20 for ability checks).

[Mara]

So in 3.5/PF sneak attacking with spells was really important to me since it would allow me to land sneak attacks as an arcane trickster.

In 5e, I don't need spells to sneak attack to land sneak attacks. This has lead to me asking myself, "did I ever REALLY want to sneak attack with spells or did I want to play a magical rogue that could actually hit things and do decent damage?"

I do like the idea of just blowing someone away with a cantrip, but on the other-hand I am glad that I can focus on other kinds of spells without basically giving up my "rogueness". None of my slots or cantrips have to be a blasting spell for my sneak attack to still be useful. I like that.

[xyianth]

So in 3.5/PF sneak attacking with spells was really important to me since it would allow me to land sneak attacks as an arcane trickster.

In 5e, I don't need spells to sneak attack to land sneak attacks. This has lead to me asking myself, "did I ever REALLY want to sneak attack with spells or did I want to play a magical rogue that could actually hit things and do decent damage?"

I do like the idea of just blowing someone away with a cantrip, but on the other-hand I am glad that I can focus on other kinds of spells without basically giving up my "rogueness". None of my slots or cantrips have to be a blasting spell for my sneak attack to still be useful. I like that.

I agree, but I wish the choice was still there. I liked the concept of focusing on targeting critical spots with weaker spells instead of just using stronger spells. Maybe I should make a different homebrew rogue archetype that is even less magic focused than the arcane trickster and instead fills the role of cantrip assassin. The base ability would be a custom, non-scaling cantrip that you can sneak attack with, and the more advanced abilities would allow you to trade some sneak attack dice for conditions/debuffs to impose. For the custom cantrip, I'd probably allow the rogue to change the energy type, but limit it to 1d6 damage and 60-120 foot range. Would it be too much to allow it to be dex based?

Edit: just realized that this is getting dangerously close to the mechanics of the 3.5 warlock.

[Mjolnirbear]

RAW it requires, as others have said, a light or finesse weapon. What on earth could be lighter than a spell? You don't even swing it, you point it. However, I'd limit it to cantrips, the 'light' version of a spell, that requires almost no mental effort at all, and further to single-target ones (no Acid Splash!) because "precision strike against vulnerable parts" is the opposite of "blanket the area with acid".

I'd say go for it. Keep in mind that:

* To sneak attack with a spell on an attack of opportunity, you'd need the Warcaster feat. (You can sneak attack on an attack of opportunity, because a sneak attack is only once per turn, not 'once on your turn').
* that cantrips improve with character level, not with caster level. You may wish to houserule this to caster level, but because spellcasting and opportunity attacks and sneak attacks all have inherent limits, I see no easily abusable problems, not even with haste which only gives you a regular attack and no extra spells.
* If you do find it too powerful, simply change it. No rule says that once you say yes it's a permanent decision.
* It should take the place of a feat or a class feature, unless you want to offer this to every player.

At level 20, this would be at worst: attack + attack (offhand weapon) + bonus action cantrip (quickened) + sneak attack. Plus cantrip+sneak attack on an opportunity attack.

Or Cantrip + bonus action cantrip (quickened) + sneak attack, + opportunity attack stuff.

If he goes cantrip, he loses ability bonuses to attack unless he's multiclassed into Evoker or Agonizing warlock. If he's multiclassing this much, he loses on a lot of higher-end powers IMO. And sorcery points aren't free. At level 6, that means 3 quickened cantrips or 6 twinned cantrips. A quickened Fireball would be FAR more effective.

[Giddonihah]

I once did the math for whether a feat that gave sneak attack on spells would be worth it for my Arcane Trickster Rogue.
While I don't have the exact calculations on me, I remember finding that Cantrips would be to powerful (pretty much strictly better than other Rogue options).
But for noncantrip spells it was more interesting, there are really not many spells that use an attack roll and do damage, with almost none above lvl3 spells. If you consider the opportunity cost of using a spell attack instead of a higher damaging aoe or utility spell, it gets interesting. You are essentially trading higher single target burst for less utility. In my case the additional opportunity cost of it being a feat, made me decide against getting it.

A Fire sorcerer/Rogue multiclass using scorching ray gets most of its power from the Fire Sorcerer, and that seems to be the highest case scenario, but thats mostly a scorching ray thing than a problem with sneak attacking with spells.

[xyianth]

I decided to go down the new rogue archetype path to address this concept. I posted it here (http://www.giantitp.com/forums/showthread.php?405631-Deadeye-Spellslinger-a-rogue-archetype-to-sneak-attack-with-cantrips). I'd welcome any feedback you have.

[Person_Man]

A Fire sorcerer/Rogue multiclass using scorching ray gets most of its power from the Fire Sorcerer, and that seems to be the highest case scenario, but thats mostly a scorching ray thing than a problem with sneak attacking with spells.

Just keep in mind that a Sorcerer has metamagic, and thus probably doesn't need Sneak Attack added onto his non-cantrip spells. When he wants to go nova, he can cast spells out of a higher slot and/or use Metamagic. Multiclassing would reduce both options, so it would probably be a wash.