example.scala

package things

import java.time.Instant

import com.sksamuel.avro4s.AvroSchema
import org.apache.avro.Schema

object World extends App {

  sealed trait User {
    val name: String
  }

  case class SpecialUser(name: String, age: Int, birthday: Instant) extends User
  case class PlainUser(name: String) extends User
  case class BetterButNotAsGoodAsSpecial(name: String, age: Int) extends User

  val schema: Schema = AvroSchema[User]
  val output: String = schema.toString

  println(output)
}

output.json

[
  {
    "type": "record",
    "name": "BetterButNotAsGoodAsSpecial",
    "namespace": "things.World",
    "fields": [
      {
        "name": "name",
        "type": "string"
      },
      {
        "name": "age",
        "type": "int"
      }
    ]
  },
  {
    "type": "record",
    "name": "PlainUser",
    "namespace": "things.World",
    "fields": [
      {
        "name": "name",
        "type": "string"
      }
    ]
  },
  {
    "type": "record",
    "name": "SpecialUser",
    "namespace": "things.World",
    "fields": [
      {
        "name": "name",
        "type": "string"
      },
      {
        "name": "age",
        "type": "int"
      },
      {
        "name": "birthday",
        "type": {
          "type": "long",
          "logicalType": "timestamp-millis"
        }
      }
    ]
  }
]