How setup spray-json to set null when json element is not present?

The only way I can think of is to implement your own Protocol via read/write, which might be cumbersome. Below is a simplified example. Note that I changed the age to be an Integer instead of an Int since Int is an AnyVal, which is not nullable by default. Furthermore, I only consider the age field to be nullable, so you might need to adopt as necessary. Hope it helps.

 case class Foo (name:String, age: Integer) object MyJsonProtocol extends DefaultJsonProtocol {    implicit object FooJsonFormat extends RootJsonFormat[Foo] {      def write(foo: Foo) =        JsObject("name" -> JsString(foo.name),                 "age"  -> Option(foo.age).map(JsNumber(_)).getOrElse(JsNull))      def read(value: JsValue) = value match {        case JsObject(fields) =>          val ageOpt: Option[Integer] = fields.get("age").map(_.toString().toInt) // implicit conversion from Int to Integer          val age: Integer = ageOpt.orNull[Integer]          Foo(fields.get("name").get.toString(), age)        case _ => deserializationError("Foo expected")      }    }  }  import MyJsonProtocol._  import spray.json._  val json = """{ "name": "Meh" }""".parseJson  println(json.convertTo[Foo]) // prints Foo("Meh",null)

It seems you're out of luck

From the doc you linked:

spray-json will always read missing optional members as well as null optional members as None

You can customize the json writing, but not the reading.