Magic mobs which are disguised will cause an arm swing animation for all spell casts. Mentioned by Remo on Discord as well, and here's a video showcasing the issue. This is jarring for certain magic mobs who may have passive spells or interval spells which don't necessarily have any visual component, so it just looks like they are swinging their arms repeatedly. It is also a bit of an issue for mobs such as dragons, as they appear to have a janky-looking wing punch whenever any interval or triggered spells occur.

Can you try putting swing_arm: false in the mob trigger?

If that's the problem I could also add a way to turn that off by default.

Unfortunately this doesn't seem to work, at least where I tried to place it:

Neither of these positions in the config stop the behaviour, unless it should be somewhere else?

That looks like the right place- is it only disguised mobs?

I don't have a setup right now that can work with LibsDisguises (it hasn't been updated) so it's kind of hard for me to test.

But testing with an undisguised mob, it seems to work:

noswing:
  type: zombie
  health: 50
  interval: 1000
  triggers:
    interval:
      swing_arm: false
      cast:
        missile: 100
        none: 100

I don't do anything special to trigger animations for disguised mobs, and I can't see how LibsDisguises would "know" a spell cast is going on- but you could try adding animations: false to the disguise config. I honestly don't know what that does, it refers to setAddEntityAnimations in the Disguise API.

Complete oversight on my part, sorry! 😅 I misread the initial message, and was trying with arm_swing instead of swing_arm like you suggested. swing_arm: false works perfectly and prevents the animation, thank you!