I'm not a maths person, so I'm asking if anyone has any opinion on this: would substituting a sidekick for the steel defender be too overpowered, underpowered, or would it not make a difference?
First and foremost: that is a conversation to have with your DM. That sounds like a fun side quest to me, though.
Second: I would argue that having both is better than one over the other.
Third: it would absolutely be overpowered. Sidekicks, as I understand them, are separate characters with progression, personality, and their own action economy. Personality is easy to get around, and the action economy can follow the normal rules, but the progression with the stats and particularly the class features would put it far beyond the capabilities of the Steel Defender, in my opinion. You also technically lose the Mending heal too, but, again, that is an easy fix.
Forth: That said, nothing is stopping you from saying it is shaped like a person or whatever you had in mind and equipping it with arms and armor (if I remember correctly).
Substituting a sidekick for the Steel Defender would drastically change the dynamics of the Battlesmith subclass, as many/most of the Battlesmith's later-level features hinge on specific actions and abilities of the Steel Defender. Arcane Jolt works with the Defender, and the Improved Defender feature is an upgrade package specifically for the original stat block. You'd likely have to homebrew quite a bit for this solution to work, and to your question of balancing?
The generic answer is "most definitively", as a Tasha's sidekick is drastically more capable than a Steel Defender. That said, you can rein it back some by still requiring the artificer to command their defender via a bonus action and by eliminating the Extra Attack feature of the Warrior's stat block. Assuming, of course, that you're using the Warrior sidekick rules. The Warrior can gain the same skill proficiencies as the Defender and the Defender actually has better saves (your proficiency bonus to both its Dex and Con), and the 'Defender' martial role is basically the Defensive Pounce ability.
Eliminating the Extra Attack option means the critter is mostly in line with the original defender, save that it gains additional bonuses to its stats as the critter levels that the original Defender does not. That shouldn't be too enormous a deal, but it's something to be aware of if you're simply adding sidekick levels to the Defender's stat block. The Defender does scale with the artificer's level, just...poorly. It can't really do anything else. 5e is such that the math is actually very loose and wobbly, and slanted heavily in favor of the players. 'Balance' is not nearly so important as people think, with one very crucial exception. And that is action economy.
One of the only ways to legitimately challenge a party of competent PCs is to outnumber them significantly. It's why the CR calculations are so heavily biased in favor of single large enemies - an adventuring party will almost always have an enormous advantage over a single large 'dangerous' critter. Once the PCs are outnumbered the balance starts to actually shift, but that means a DM who wants the option to challenge their PCs with numbers needs to limit the number of PCs on the field. You are effectively asking your DM to make your Steel Defender the equivalent of a second entire PC under your artificer's direct control. That numbers advantage is powerful and should be taken into account when making this decision.
Thanks, Yurie, very good points and well thought out. Toraf, the idea was that I'd use steel defender to a point.
The concept is a warforged interested in reproducing, so she's working on creating more warforged. To begin with she'd not be able to (defender) but at a later point she might achieve something sapient. It's really not a big deal what I use, I was interested in the thoughts people might have really.
I'm not a maths person, so I'm asking if anyone has any opinion on this: would substituting a sidekick for the steel defender be too overpowered, underpowered, or would it not make a difference?
First and foremost: that is a conversation to have with your DM. That sounds like a fun side quest to me, though.
Second: I would argue that having both is better than one over the other.
Third: it would absolutely be overpowered. Sidekicks, as I understand them, are separate characters with progression, personality, and their own action economy. Personality is easy to get around, and the action economy can follow the normal rules, but the progression with the stats and particularly the class features would put it far beyond the capabilities of the Steel Defender, in my opinion. You also technically lose the Mending heal too, but, again, that is an easy fix.
Forth: That said, nothing is stopping you from saying it is shaped like a person or whatever you had in mind and equipping it with arms and armor (if I remember correctly).
Yeah, fourth point is the one I'm probably going to go with. I was just wondering. You're spot on about progression.
I miss the old UA artificer, when the creation leveled up with the character.
Substituting a sidekick for the Steel Defender would drastically change the dynamics of the Battlesmith subclass, as many/most of the Battlesmith's later-level features hinge on specific actions and abilities of the Steel Defender. Arcane Jolt works with the Defender, and the Improved Defender feature is an upgrade package specifically for the original stat block. You'd likely have to homebrew quite a bit for this solution to work, and to your question of balancing?
The generic answer is "most definitively", as a Tasha's sidekick is drastically more capable than a Steel Defender. That said, you can rein it back some by still requiring the artificer to command their defender via a bonus action and by eliminating the Extra Attack feature of the Warrior's stat block. Assuming, of course, that you're using the Warrior sidekick rules. The Warrior can gain the same skill proficiencies as the Defender and the Defender actually has better saves (your proficiency bonus to both its Dex and Con), and the 'Defender' martial role is basically the Defensive Pounce ability.
Eliminating the Extra Attack option means the critter is mostly in line with the original defender, save that it gains additional bonuses to its stats as the critter levels that the original Defender does not. That shouldn't be too enormous a deal, but it's something to be aware of if you're simply adding sidekick levels to the Defender's stat block. The Defender does scale with the artificer's level, just...poorly. It can't really do anything else. 5e is such that the math is actually very loose and wobbly, and slanted heavily in favor of the players. 'Balance' is not nearly so important as people think, with one very crucial exception. And that is action economy.
One of the only ways to legitimately challenge a party of competent PCs is to outnumber them significantly. It's why the CR calculations are so heavily biased in favor of single large enemies - an adventuring party will almost always have an enormous advantage over a single large 'dangerous' critter. Once the PCs are outnumbered the balance starts to actually shift, but that means a DM who wants the option to challenge their PCs with numbers needs to limit the number of PCs on the field. You are effectively asking your DM to make your Steel Defender the equivalent of a second entire PC under your artificer's direct control. That numbers advantage is powerful and should be taken into account when making this decision.
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I’m curious as to why you would want to do this? Not knocking the idea or anything just genuinely curious.
Thanks, Yurie, very good points and well thought out. Toraf, the idea was that I'd use steel defender to a point.
The concept is a warforged interested in reproducing, so she's working on creating more warforged. To begin with she'd not be able to (defender) but at a later point she might achieve something sapient. It's really not a big deal what I use, I was interested in the thoughts people might have really.