I wrote a skill that allows players to enter vanish, but I found that they can constantly attack others in stealth without canceling magic, so is it possible to add a cancel on attack parameter? Thank you

You're looking for cancel_on_damage- the builtin cloak spell uses it for this very reason.

https://github.com/elBukkit/MagicPlugin/blob/master/Magic/src/main/resources/examples/survival/spells/cloak.yml#L69

i mean when im in cloak attacking others wont actually cancel the magic isnt it? only if i got damaged

Oh hm my bad, you are right- I got that option confused.

Unfortunately I don't think there is an option for that, but it should be pretty easy to add.

(I'll keep this open and let you know when I put that in)