Calling io.circe.Decoder#decodeList

Decoder.scala

import io.circe._
import io.circe.parser._
import io.circe.syntax._

scala> val ints = "[1,2,3]".asJson
ints: io.circe.Json = "[1,2,3]"

scala> Decoder.decodeList[Int].decodeJson(ints)
res1: io.circe.Decoder.Result[List[Int]] = Left(DecodingFailure([A]List[A], List()))

What's the right way to call this function?

I presume you mean:

@ val ints = parse("[1,2,3]").right.get 
ints: Json = [
  1,
  2,
  3
]
@ ints.as[List[Int]].right.get 
res13: List[Int] = List(1, 2, 3)