class User(val name: String, val isAdmin: Boolean)

it is serialialized in object with "admin" property, but not "is_admin", is this a bug?

Not related to Kotlin, this is how Jackson names properties. It strips the "getter prefix", which is

get

normally and

is

for boolean properties. So with

isEnabled

as a getter you will get

enabled

as property.

but getter is generated by Kotlin, so it is Kotlin related

Yes, but Kotlin just follows Java conventions here.