circe-validation.scala

import cats.syntax.apply._
import io.circe._
import io.circe.literal._

object JsonValidation{
  def validate[A: Decoder]( f: HCursor => ACursor, pred: A => Boolean, err: A => String ): Decoder[A] =
    Decoder.instance(
      f andThen{ ac =>
        ac.as[A].flatMap( a =>
          if ( pred( a ) )
            Right( a )
          else
            Left( DecodingFailure( err( a ), ac.history ) ) )
      }
    )
}

implicit val someRequestDecoder: Decoder[SomeRequest] ={
  def stringOfLength( min: Int, max: Int )( f: HCursor => ACursor ): Decoder[String] =
    JsonValidation.validate[String]( f, s => s.length >= min && s.length <= max,
                         s => s"Length must be between $min and $max; was ${s.length}" )

  val name = stringOfLength( 1, 10 )( _.downField( "name" ) )
  val desc = stringOfLength( 0, 20 )( _.downField( "description" ) )

  ( name, desc ).mapN( SomeRequest )
}

case class SomeRequest( name: String, description: String )

val name = "A name too long"
val desc = "this description is longer than 20 characters"

val json = json"""{ "name": $name, "description": $desc }"""

Decoder[SomeRequest].accumulating.apply( json.hcursor )

Ok, I think I understand what was the confusion (at least from my perspective): my code is attaching the validation of the individual fields to the containing JSON element. The accumulation is not working as apparently 1 element can only have 1 error.