D&D 5E Arcane Trickster/Bladesinger - When to dip?

We're starting a new campaign at 3rd level, which will be going to roughly 15th level.

I'm playing a Half Elf spellthief character, and am planning to go with a mix of Arcane Trickster and Bladesinger. I'd like a bit more casting utility than a straight AT, so I'm planning to dip at least two levels into Bladesinger, and possibly as many as 5 levels.

The rest of the party is a Barbarian tank, Life Cleric healer, and Sorlock blaster. I will be the Face, Trapfinder, Striker, Scout, and Utility Caster. In combat, I'll sprinkle in control spells as needed, but mostly Sneak Attack with Booming Blade/Rapier/Disengage when melee is an option, and Hand Crossbow/Hide for ranged. (Bladesong ends if you use a two handed weapon, so no normal crossbows or bows.)

Stat breakdown is as follows:
S10
D18 (17+1 HE)
C16 (15+1 HE)
I16
W12
Ch15 (13+2 HE)

My dillema is deciding how to distribute my levels, specifically when during my progression to mix in the Bladesinger levels.

I don't want to dilute my levels too much and overly delay ASIs, so it looks like the best option would be to stick to groups of 4 or so levels.

One possibility is to start with straight AT for 4 and then add in the 2 to 5 levels of BS. That would mean starting as AT3 with Cunning Action, 2d6 Sneak Attack, and a few limited spells, but waiting until 5th level overall for access to rituals, and 6th overall for Bladesong. Plus I'd be stuck with only 1st level spells until 7th level overall, which is similar spell progression to a straight AT, albeight with a wider variety of cantrips and 1st level spells than a straight AT.

I could approach it from the other side, with 1 level of AT and then 5 levels of BS. That would mean starting at AT1/BS2, and having rituals and Bladesong from the start and access to 2nd/3rd level spells at approximately normal character level, but minimal Sneak Attack damage and having to wait for 7th level overall for Cunning Action, which I think would out too much if a cramp on my melee/ranged usefulness.

I could also split the difference, starting at something like AT2/BS1, followed by 2 more levels of AT, then 1-4 more levels of BS. That gets me rituals right off the bat at 3rd level overall, and only delays my ASIs by 1 level, but still delays Bladesong to 6th level and Level 2 spells until 7th level.

What are your thoughts? When would you suggest taking the Bladesinger levels?

And should I stick to just 2 levels of Bladesinger, or is it worth pushing Bladesinger up to 5 for 3rd level spells, even if they might not come online until 9th level overall or higher?

Thanks!

First, I applaud that you're closely watching you ASI progression - not doing so is a good way to accidentally shoot yourself in the foot. That said, the ability scores you have there are quite a bit above point buy so that it's not nearly as big a deal.

Another point to think about is your level 5 power bump - of which rogue doesn't have as big as some other characters, but wizard does in terms of 3rd level spells.

BB/GFB is a great way to do more with a single attack. But with sneak attack if you miss you lose it all, and BB/GFB aren't even the attack action to enable two weapon fighting bonus action. So once you have a sufficient amount of sneak attack, going for Bladesinger's extra attack at Wizard 6 is worthwhile.

Another thing worthwhile is using your concentration slot, so it's not an unused (and therefore wasted) resource. I'd make sure you have enough slots that you can pull out concentration most combats. Don't forget Shadow Blade, a 2nd level spell from Xanthar's which looks like it might be just right for you since it will also enable sneak attack by granting advantage in dim light.

As a side note, if you do have advantage a lot, the Elven Accuracy feat from Xanthar's is also very nice, and as a half-elf you qualify.

Go far enough for slots to do concentration spells, as well as Shield & Absorb Elements to offset your squishiness. Plus whatever else Utility Caster covers. So I'd recommend more Bladesinger levels.

So I'd start Rogue 1 but then would head for Wizard levels. 2 for baldesinging, 4 for ASI/feat, 5 for 3rd level spells. Then back to Rogue. And once your SA becomes enough that a second attack for a second chance to apply it is worth more than BB/GWB, then take Wizard 6 for. How much rogue and/or wizard you want at that point is up to you. Wiz 8 is another ASI as well as advancing casting. Rogue is full of goodies.

Thanks for the comments.

Like you said, ASIs aren't as crucial to this character, since I rolled well on stats. So I think I can afford to delay the first one by a few levels.

Regarding TWF and Extra Attack, I'm taking Find Familiar with an Owl familiar, whose Help action will grant me Advantage in a lot of situations. So that will help ensure my lone BB attack hits, without eating up my bonus action like TWF would. And leaving my bonus action free will allow me to consistently Disengage, to help force the extra BB damage from enemy movement and help ensure my survivability in melee combat.

Elven Accuracy is nice, but it loses a bit of its luster since I don't have an odd DEX or INT score to round out. I'm most likely going to go straight +2s to DEX and INT with my ASIs, to get both up to 20 ASAP. (I had originally planned on going High Elf or DMG Variant Eladrin for 19 DEX and 16 INT, and taking Elven Accuracy with my first ASI since it'd make more sense in that case, but the party needs a Face and the Half-Elf's extra +2 CHA and skill proficiencies pushed me over the edge. Basically trading a bit of extra combat efficiency for extra social utility and a more well-rounded team.)

I think I've settled on starting Rogue2/Wiz1, since Cunning Action comes at Rogue2, and Disengage will be one of my go-to combat tricks, especially with the added movement from Bladesong. My next level will then be Rogue2/Wiz2 for Bladesong.

I believe I'm going to aim for Rogue4/Wiz5 by level 9, which has 2d6 Sneak Attack, two ASIs, and 3rd level spells, and then straight Rogue from there for the remainder of the campaign. The question is which route to take getting there, with Rogue2/Wiz5 first for 2nd/3rd level spells earlier on, or Rogue4/Wiz2 first for extra Sneak Attack and more lower-level spells early on. Basically for levels 4 to 9, do I want to be a Bladesinger with a bit more thievery, or an Arcane Trickster with a bit more magic?

(If I head towards Wiz5 first, at least Shadow Blade will help offset the lost Sneak Attack dice during those levels.)

I'd like a mix of battlefield control spells, out of combat utility spells, and self-preservation spells. Between the two classes, the spells I'm shooting for include:
0: Booming Blade, Minor Illusion, Prestidigitation, Mage Hand, Message, Toll the Dead, Mending
1: Find Familiar, Detect Magic, Absorb Elements, Feather Fall, Shield, Fog Cloud, Grease, Silent Image, Tasha'a Hideous Laughter, Comprehend Languages, Alarm
2: Shadow Blade, Mirror Image, Misty Step, Invisibility, Web, Suggestion
3: Fly, Blink, Leomund's Tiny Hut

I've got Concentration covered with some battlefield control spells like Fog Cloud and Web, as well as Shadow Blade and Fly.

Apologies for the slight necropost here, but I'm guessing you can't have leveled up too much since starting this thread.

I'm playing a very similar character now, in a fairly similar albeit larger party (we have a barbarian, cleric, and a sorlock too, but also a paladin and a lore bard). We used point buy, so my stats aren't nearly as good as yours, and because others have CHA covered I dumped it.

Going 2/1 then 2/2 sounds great. Expertise, cunning action, and a big library of utility spells with ritual casting is awesome. Then bladesong vs the 2nd sneak attack die comes down to whether you want to boost offense or defense; I'd probably feel it out through level 3 and see what you think you need more. The spells learned are similar and the slots are identical at that point.

I went 3/2 at level 5. You get almost the same spell slots as going 2/3 (arcane recovery being the only difference), and with only 2-3 2nd level slots, if you're planning to cast shadow blade to keep pace with damage you may as well just get the sneak attack die that never runs out and doesn't use concentration, unless you're fighting in dim light a lot and/or have few combats per day, then maybe it's a different story. Plus you get your fancy mage hand, a couple more cantrips, and three more 1st level spells. Compare that to one other 2nd level spell (since shadow blade is almost a spell tax at that point) that competes with shadow blade for slots.

At level 6, assuming you went 3/2, you're deciding between an ASI (boosting DEX presumably) and 2nd level spells. The ASI has the advantage of boosting your AC, but with bladesong, cunning action and shield, you're likely more worried about non-AC-targeting ways of getting killed. Offensively, between your familiar and shadow blade's extra damage and advantage in dim light, I'd argue that shadow blade with up to 4 uses a day is superior to a DEX bump, especially if you're aiming for 4/5 at L9 (otherwise you want to think about whether you want to be 5/2 at L7 for uncanny dodge and more sneak attack).

Level 7 is an ASI (finally!), which you may as well get on the wizard side since it also comes with spells known as well as 3rd level slots (3d8 shadow blade for those big fights!).

Level 8 is either third level spells or another ASI. In my case I really wanted Resilient (CON) at this point, but I had 13 CON and felt strapped for HP. In your case you might rather have 3rd level spells, but I'd also consider Mobile. One of the drawbacks of the multiclass is that you're usually using up your bonus action for the first two rounds starting up bladesong and casting shadow blade; getting that free quasi-disengage helps you more than it would a straight AT.

I'm currently at Level 8, 4/4, and it's been great. Between my owl and shadow blade I rarely don't have advantage, and with elven accuracy (I'm full elf, so started with 17 DEX) I'm constantly rolling three times. It's been ages since I've missed an attack. The biggest drawbacks for me at the moment are (1) so much demand on my bonus action from bladesong and shadow blade means I end up stuck in melee more than I'd like, (2) I wish I weren't so dependent on concentration to keep my damage up, since it makes fun spells like web, levitate, and Tasha's harder to fit in, and (3) although I'm usually able to avoid damage entirely, if something does get through it's rough, having delayed uncanny dodge and evasion, and not having a lot of HP. I was nearly taken out by a devourer the other night after I charged in with mirror image, haste and shield spell in hand for a potential AC of 26, but the DM somehow rolled a 4 against mirror image followed by a crit on the attack (she seems to roll crits a lot against me, including twice despite disadvantage... hmm...).

I'm certainly happiest when we can get a round to prepare, since then I can cast mirror image and start blade song, letting me jump into melee with shadow blade and some serious defenses up in the first round. The sorlock often will twin haste on me and our barbarian, which is a highly recommended tactic (though I've been hesitant to use it to do the full on readied second sneak attack, since I want to keep my reaction free for shield, or later, uncanny dodge; but a second straight shadow blade attack is still nice).

If I were starting at level 8 I think I'd probably be better off at 7/1, with uncanny dodge, evasion, and almost as much at will damage as a third level shadow blade (plus the potential to add a 2nd level shadow blade on top a couple times a day), leaving tanking to the barbarian and the paladin and sticking to booming blade/disengage with some Tasha's or maybe hold person mixed in. But I got to play with 2nd level spells for two levels when I wouldn't have had them otherwise, and I get a broader magic toolbox.

I'm currently pretty torn on where to go from here. My original plan was to stick to rogue levels until I get magical ambush, then bring the hypnotic patterns from hiding at level 14 (one level later than a pure AT, but with lots more uses a day). I think I will at least go rogue 5 next for the damage mitigation. But I might grab that 5th wizard level and fear after that to mix things up a bit. I'm pretty sure that I want to be 9/6 at level 15: imposed disadvantage on level 3 debuffs, easily getting in and out of touch range to cast bestow curse with no concentration if using the 5th level slot, maybe even from invisible to be hidden when it's cast, or taking two attacks with a 5th level shadow blade and 5d6 sneak attack. If the campaign continues from there there are a lot of directions to go in with fun possibilities.

Thanks for the detailed write-up, Esker.

I'm currently AT3/Wiz1, with:

STR 10
DEX 18
CON 16
INT 16
WIS 12
CHA 15

Skills: Arcana, Deception, Insight, Investigation, Perception (Expertise), Persuasion, Sleight of Hand, Stealth (Expertise)

AT Cantrips: Mage Hand, Message, Toll the Dead
Wiz Cantrips: Booming Blade, Minor Illusion, Prestidigitation
AT 1st Level: Absorb Elements, Silent Image, Tasha's Hideous Laughter
Wiz 1st Level: Alarm (scribed), Detect Magic, Feather Fall, Find Familiar, Fog Cloud, Grease, Illusory Script (scribed), Shield

Out of combat, I'm the party scout, trapfinder, face, and utility caster.

In combat, my tactics currently tend towards Hiding and Sneak Attacking with a Light Crossbow, and sometimes using my Rapier to BB+Disengage when a melee enemy closes on me. I drop the occasional spell, like Tasha's, Fog Cloud, or Grease, and use Shield/Absorb Elements to mitigate damage as needed.

At 5th level, I'm picking up Wiz2, to get Bladesong. This AC/Concentration/Movement increase coincides with BB's damage increase, and my combat tactics will switch to focusing more on being a melee striker with BB + Disengage. The Barbarian and Cleric are usually engaged with at least one melee enemy apiece, so between that and having my Owl familiar Help, I can pretty reliably guarantee Sneak Attacks in melee. And I'll still have a Light Crossbow or Hand Crossbow (when Bladesong is active) for ranged Sneak Attacks as needed, plus Toll the Dead for other ranged attacks when I can't get Sneak Attack, the enemy is behind cover, or the enemy has a high AC.

Almost a carbon copy of my path, apart from my being an eladrin (DMG version) and not having any charisma or social interaction skills (the bard has persuasion and deception expertised and the cleric has high insight). I also wound up going wizard first to balance out my saving throws at the cost of a skill and some weapon proficiency. I get some weapons back from elf and of course picked rapier of course as my bladesinger weapon, so the only thing I'm really missing is hand crossbow, but usually if I have to attack from range it's early in combat, so I just wait to activate bladesong. But just like you, I'm the resident scout, trapfinder and utility caster, and I played as ranged with a light crossbow until L5, then switched to melee to take advantage of booming blade's extra damage.

STR: 8
DEX 17 (to 18 at L7)
CON 13 (to 14 at L8)
INT 16
WIS 12
CHA 8

Skills: Acrobatics, Arcana, Investigation, Perception (Expertise), Sleight of Hand, Stealth (Expertise), +Performance (L5)

AT Cantrips: Mage Hand, Message, Booming Blade
Wiz Cantrips: Green Flame Blade, Minor Illusion, Prestidigitation
AT 1st Level: Absorb Elements, Silent Image, Tasha's Hideous Laughter, (+Disguise Self at L8)
Wiz 1st Level: Alarm, Comprehend Languages, Detect Magic, Find Familiar, Fog Cloud, Shield, (+Identify, +False Life at L5 (nice use of a 2nd level slot before you have actual 2nd level spells, but will be first to swap out once I need the preparation slot))
Wiz 2nd Level: (+Shadow Blade, Levitate at L6), (+Mirror Image, Web at L7), Skywrite (scribed)

May I ask the obvious question that hasn't been asked? If you have a sorlock in your party with Charisma as their main stat, why would you need to be the party face?

Because they didn't take any Face skills.

(That player is fairly introverted anyway, and doesn't really like to take much part in the Social side of the game.)

Ah, gotcha. That sucks, but it makes sense.

EDIT: As others have pointed out, I misunderstood the spell level progression of multiclass spellcasters. I don't want to delete this post as it may serve to clear up other's confusion as well. Basically when you level up in a multiclass spellcasting class, you cannot learn a spell outside your individual class' spell progression. Despite the extra spell slots and the wording of some of the classes, you cant skip ahead with multiclass spell slots, which was my misunderstanding.

I'm currently playing this build and want to weigh in on my level progression. The progression takes into consideration the multiclass spell-level progression table (AT counts as 1/3 caster, while BS is full caster). I am trying to maximize the free spells, or swapped spell, you get at each level. Reducing the need to scrounge for spell scrolls later and picking up crucial spells ASAP.

1-5
R1, W1, R2, R3, W2

This is basically the same progression as RogueJP and Esker mentioned above. When you hit Wizard 2, you want to be able to pick up 2nd level spells. Getting up to R3 for Arcane Trickster is required before you hit W2. During this time, the build plays as party scout, utility, and ranged attacker. You CAN melee, but there's little upside over ranged.

6-10
R4, W3, W4, R5, W5

You can either pick up Shadowblade at 6 and keep it as one of your AT spell slots from here on out, or you can snag it at level 5 as W2. Since we get a buff to cantrips at level 5, I postponed Shadowblade to level 6 so I could use it as an AT spell slot. Not a huge deal, but technically AT's still have to use material components and cannot benefit from the Wizard's spell focus. Consider what material component you will have to be using. R4 is ASI #1 pumping Dex. W4 is ASI #2 picking up War Caster for advantage on concentration checks. Swap these two around if you are in a smaller group or are finding yourself failing a lot of concentration checks. You want to pick up Blink ASAP, as Mirror Image and Blink are the two best lower level non-concentration defensive spells available to you. Pick up counterspell, dispel magic, see invisibility, and hypnotic pattern during these levels.

11-15
W6, R6, R7, R8, R9

Basically just bee-lining for Magical Ambush, getting it at level 15. It's okay, if you wanted to be some super wizard, you would have gone full caster, instead of this gishy thing. AT/BS excels at disrupting opponents, not necessarily disabling (although still possible, just costly). Save your spell slots for the defensive spells like Mirror Image, Blink, Counterspell, Dispel Magic, See Invisibility, Shield, Absorb Elements (reconsider due to Uncanny dodge and/or Evasion - maybe it's better to save the spell slot). Picking up a few AT spells in this bracket, I like Dimension Door (AT8 any school), Charm Monster, and Geas. These are spells worth keeping for the rest of your career. The 1 ASI in this bracket can be used to pickup Resilient (WIS) to help us keep control of our agency when we need it most.

16-20
W7, W8,W9, W10, R10

Grabbing some nice spells such as Steel Wind Strike, Teleportation Circle, Wall of Force, Globe of Invulnerability, Disintegrate, Mass Suggestion, True Seeing, Plane Shift, Symbol, and Project Image. Notice I didn't put many damage spells in there. That's because we aren't very good as an offensive caster. We excel at disrupting and disabling. With Magical Ambush now, we can drop Hypnotic Pattern or Slow from stealth, or polymorph their shaman, or charming their monsters. The last 2 ASIs in this bracket both go towards boosting Dex or Int. If you happen to have a DM who will allow you to roll for your stats, this build could benefit from an 18 DEX and 16 INT out the gate.