Hi, I'm curious as to what is supposed to increase from might of the master for the Steel Defender.

My understanding is that as the proficiency bonus increases so do the various options. So this means when it starts it calculated on the base +2 proficiency but then when my proficiency goes up to +3 it should now have

+4 dex and +5 con saving throws

+5 Atheltics and +5 perception

15 Passive Perception

+5 to hit and 1d8 + 3 force damage on force-empowered rend

I just want to confirm this is correct as the beyond Steel Defender isn't calculating properly.

Thank you!

The following numbers increase by 1 when your proficiency bonus increases by 1: the defender’s skill and saving throw bonuses (above), the bonuses to hit and damage of its rend attack, and the number of hit points restored by its Repair action (below).

Yes, at level 5, the bonuses should increase as you wrote.

Might of the Master. The following numbers increase by 1 when your proficiency bonus increases by 1: the defender’s skill and saving throw bonuses (above), the bonuses to hit and damage of its rend attack, and the number of hit points restored by its Repair action (below).

Read literally, yes, you get an SD at the base stats you see in its entry and then only add +1 when your bonus increases in the future. So, base stats, possibly up to +4 depending on character level.

But... that leaves the door open about what happens if you're multiclassed, and unlock the SD when your proficiency bonus is already +3 or higher? Does the SD only benefit from future PB increases, and miss out on any that you've already unlocked? Or, is the intent that any bonus above +2 is intended to be retroactively applied to the SD? Or, is this an indication that actually all PB bonus is intended to be added, including the base +2 that you have at character levels 1-4? Is it something weird where you're intended to look at the PB value in your Artificer class table, and only give the SD the benefit of whatever value is printed there based on your class level instead of character level? Something else? 

For a class feature that has the benefit of knowing what worked and didn't work about the  Beast Master, the SD's Might of the Master feature is a giant and bewildering step backwards. 

Add your proficiency bonus to the beast’s AC, attack rolls, and damage rolls, as well as to any saving throws and skills it is proficient in. 

5E multiclassing by design doesn't care about what order classes are taken in, and yet, that rule is now broken for the first time with Artificer? Here we have one of their newest releases getting wrong somethign that was gotten right years ago. WTF. 

Also, theoretically, if you were to get an Ioun stone of Mastery, which increases your proficiency bonus by 1 when you attune to it, you could attune to it, and unattune to it, over and over, and because Might of the Master never says that its bonus is lowered if your proficiency bonus is lowered, you can theoretically gain an infinite bonus to the Steel Defender's Skill and Saving Throw Bonuses, Hit and Damage rolls, and Hit Points restored by the Repair Action.

I know, no DM (including me), would ever allow it, but by RAW, it is technically allowed.

Well I'm not sure that one-way ratcheting would be required by the current wording... but yes, if the intent is really that you always add your current proficiency bonus (or current proficiency bonus minus 2) to the stat block instead of just future PB increases, that could certainly have been worded more clearly to avoid any possible confusion. 

Yes, the RAW is different from the RAI, and it should be written better to avoid confusion, but my exploitation of this is technically legal.

What a surprise, a rule that needs to be written better. Let me get a ladder so I can add it to the top of the pile...