diff --git a/codegen/src/main/scala/caliban/codegen/ClientWriter.scala b/codegen/src/main/scala/caliban/codegen/ClientWriter.scala
index e639639bc..978ba0ea8 100644
--- a/codegen/src/main/scala/caliban/codegen/ClientWriter.scala
+++ b/codegen/src/main/scala/caliban/codegen/ClientWriter.scala
@@ -249,9 +249,8 @@ object ClientWriter {
         )
       }
     val args = field.args match {
-      case Nil => ""
-      case list =>
-        s"(${list.map(arg => s"${safeName(arg.name)}: ${writeType(arg.ofType)}").mkString(", ")})"
+      case Nil  => ""
+      case list => s"(${writeArgumentFields(list)})"
     }
     val argBuilder = field.args match {
       case Nil => ""
@@ -263,7 +262,7 @@ object ClientWriter {
   }
 
   def writeArgumentFields(args: List[InputValueDefinition]): String =
-    s"${args.map(arg => s"${safeName(arg.name)}: ${writeType(arg.ofType)}").mkString(", ")}"
+    s"${args.map(arg => s"${safeName(arg.name)}: ${writeType(arg.ofType)}${if (arg.ofType.nullable) " = None" else ""}").mkString(", ")}"
 
   def writeArguments(field: FieldDefinition): String =
     if (field.args.nonEmpty) {
diff --git a/codegen/src/test/scala/caliban/codegen/ClientWriterSpec.scala b/codegen/src/test/scala/caliban/codegen/ClientWriterSpec.scala
index ccb70c56e..302045f43 100644
--- a/codegen/src/test/scala/caliban/codegen/ClientWriterSpec.scala
+++ b/codegen/src/test/scala/caliban/codegen/ClientWriterSpec.scala
@@ -356,6 +356,37 @@ object Client {
     def nicknames: SelectionBuilder[Character, List[String]] = Field("nicknames", ListOf(Scalar()))
   }
 
+}
+"""
+              )
+            )
+          },
+          testM("default to None for optional arguments") {
+            val schema =
+              """
+              type Query {
+                characters(
+                  first: Int!
+                  last: Int
+                ): String
+              }""".stripMargin
+
+            assertM(
+              gen(schema),
+              equalTo(
+                """import caliban.client.FieldBuilder._
+import caliban.client.SelectionBuilder._
+import caliban.client._
+import caliban.client.Operations._
+
+object Client {
+
+  type Query = RootQuery
+  object Query {
+    def characters(first: Int, last: Option[Int] = None): SelectionBuilder[RootQuery, Option[String]] =
+      Field("characters", OptionOf(Scalar()), arguments = List(Argument("first", first), Argument("last", last)))
+  }
+
 }
 """
               )
diff --git a/core/src/main/scala/caliban/parsing/adt/Type.scala b/core/src/main/scala/caliban/parsing/adt/Type.scala
index 369ea7a76..f30f663fb 100644
--- a/core/src/main/scala/caliban/parsing/adt/Type.scala
+++ b/core/src/main/scala/caliban/parsing/adt/Type.scala
@@ -2,7 +2,10 @@ package caliban.parsing.adt
 
 import scala.annotation.tailrec
 
-sealed trait Type
+sealed trait Type {
+  val nonNull: Boolean
+  var nullable: Boolean = !nonNull
+}
 
 object Type {