diff --git a/benchmarks/src/main/scala/caliban/json/JsonEncodingBenchmark.scala b/benchmarks/src/main/scala/caliban/json/JsonEncodingBenchmark.scala
index 7c70554f6a..1baf9b0d2f 100644
--- a/benchmarks/src/main/scala/caliban/json/JsonEncodingBenchmark.scala
+++ b/benchmarks/src/main/scala/caliban/json/JsonEncodingBenchmark.scala
@@ -1,9 +1,9 @@
 package caliban.json
 
 import caliban.Value._
-import caliban.interop.circe.json.GraphQLResponseCirce.{graphQLResponseEncoder => circeEncoder}
-import caliban.interop.zio.GraphQLResponseZioJson.{graphQLResponseEncoder => zioEncoder}
-import caliban.{GraphQLResponse, ResponseValue}
+import caliban.interop.circe.json.GraphQLResponseCirce.{ graphQLResponseEncoder => circeEncoder }
+import caliban.interop.zio.GraphQLResponseZioJson.{ graphQLResponseEncoder => zioEncoder }
+import caliban.{ GraphQLResponse, ResponseValue }
 import com.github.plokhotnyuk.jsoniter_scala.core._
 import org.openjdk.jmh.annotations._
 
diff --git a/build.sbt b/build.sbt
index b2708faf2d..4861d0d57d 100644
--- a/build.sbt
+++ b/build.sbt
@@ -753,7 +753,13 @@ lazy val enableMimaSettingsJVM =
     mimaFailOnProblem     := enforceMimaCompatibility,
     mimaPreviousArtifacts := previousStableVersion.value.map(organization.value %% moduleName.value % _).toSet,
     mimaBinaryIssueFilters ++= Seq(
-      ProblemFilters.exclude[IncompatibleMethTypeProblem]("caliban.execution.Executor#ReducedStepExecutor.makeQuery")
+      ProblemFilters.exclude[IncompatibleMethTypeProblem]("caliban.execution.Executor#ReducedStepExecutor.makeQuery"),
+      ProblemFilters.exclude[DirectMissingMethodProblem]("caliban.parsing.adt.Type.$init$"),
+      ProblemFilters.exclude[DirectMissingMethodProblem]("caliban.introspection.adt.__Type.*"),
+      ProblemFilters.exclude[DirectMissingMethodProblem]("caliban.introspection.adt.__InputValue.*"),
+      ProblemFilters.exclude[FinalMethodProblem]("caliban.parsing.adt.Type*"),
+      ProblemFilters.exclude[MissingTypesProblem]("caliban.introspection.adt.__Type$"),
+      ProblemFilters.exclude[MissingTypesProblem]("caliban.introspection.adt.__InputValue$")
     )
   )
 
diff --git a/core/src/main/scala-2/caliban/schema/ArgBuilderDerivation.scala b/core/src/main/scala-2/caliban/schema/ArgBuilderDerivation.scala
index 692a4b6f60..be5d5ae816 100644
--- a/core/src/main/scala-2/caliban/schema/ArgBuilderDerivation.scala
+++ b/core/src/main/scala-2/caliban/schema/ArgBuilderDerivation.scala
@@ -3,10 +3,10 @@ package caliban.schema
 import caliban.CalibanError.ExecutionError
 import caliban.InputValue
 import caliban.Value._
-import caliban.schema.Annotations.GQLDefault
-import caliban.schema.Annotations.GQLName
+import caliban.schema.Annotations.{ GQLDefault, GQLName, GQLOneOfInput }
 import magnolia1._
 
+import scala.collection.compat._
 import scala.language.experimental.macros
 
 trait CommonArgBuilderDerivation {
@@ -25,19 +25,44 @@ trait CommonArgBuilderDerivation {
     override def map[A, B](from: EitherExecutionError[A])(fn: A => B): EitherExecutionError[B] = from.map(fn)
   }
 
-  def join[T](ctx: CaseClass[ArgBuilder, T]): ArgBuilder[T] =
-    (input: InputValue) =>
+  def join[T](ctx: CaseClass[ArgBuilder, T]): ArgBuilder[T] = new ArgBuilder[T] {
+
+    private val params = {
+      val arr = Array.ofDim[(String, EitherExecutionError[Any])](ctx.parameters.length)
+      ctx.parameters.zipWithIndex.foreach { case (p, i) =>
+        val label   = p.annotations.collectFirst { case GQLName(name) => name }.getOrElse(p.label)
+        val default = p.typeclass.buildMissing(p.annotations.collectFirst { case GQLDefault(v) => v })
+        arr(i) = (label, default)
+      }
+      arr
+    }
+
+    private val required = params.collect { case (label, default) if default.isLeft => label }
+
+    override private[schema] val partial: PartialFunction[InputValue, Either[ExecutionError, T]] = {
+      case InputValue.ObjectValue(fields) if required.forall(fields.contains) => fromFields(fields)
+    }
+
+    def build(input: InputValue): Either[ExecutionError, T] =
+      input match {
+        case InputValue.ObjectValue(fields) => fromFields(fields)
+        case _                              => Left(ExecutionError("expected an input object"))
+      }
+
+    private[this] def fromFields(fields: Map[String, InputValue]): Either[ExecutionError, T] =
       ctx.constructMonadic { p =>
-        input match {
-          case InputValue.ObjectValue(fields) =>
-            val label   = p.annotations.collectFirst { case GQLName(name) => name }.getOrElse(p.label)
-            val default = p.annotations.collectFirst { case GQLDefault(v) => v }
-            fields.get(label).fold(p.typeclass.buildMissing(default))(p.typeclass.build)
-          case value                          => p.typeclass.build(value)
-        }
+        val idx              = p.index
+        val (label, default) = params(idx)
+        val field            = fields.getOrElse(label, null)
+        if (field ne null) p.typeclass.build(field) else default
       }
+  }
 
-  def split[T](ctx: SealedTrait[ArgBuilder, T]): ArgBuilder[T] = input =>
+  def split[T](ctx: SealedTrait[ArgBuilder, T]): ArgBuilder[T] =
+    if (ctx.annotations.contains(GQLOneOfInput())) makeOneOfBuilder(ctx)
+    else makeSumBuilder(ctx)
+
+  private def makeSumBuilder[T](ctx: SealedTrait[ArgBuilder, T]): ArgBuilder[T] = input =>
     (input match {
       case EnumValue(value)   => Some(value)
       case StringValue(value) => Some(value)
@@ -53,6 +78,26 @@ trait CommonArgBuilderDerivation {
         }
       case None        => Left(ExecutionError(s"Can't build a trait from input $input"))
     }
+
+  private def makeOneOfBuilder[A](ctx: SealedTrait[ArgBuilder, A]): ArgBuilder[A] = new ArgBuilder[A] {
+
+    private def inputError(input: InputValue) =
+      ExecutionError(s"Invalid oneOf input $input for trait ${ctx.typeName.short}")
+
+    override val partial: PartialFunction[InputValue, Either[ExecutionError, A]] = {
+      val xs = ctx.subtypes.map(_.typeclass).toList.asInstanceOf[List[ArgBuilder[A]]]
+
+      val checkSize: PartialFunction[InputValue, Either[ExecutionError, A]] = {
+        case InputValue.ObjectValue(f) if f.size != 1 =>
+          Left(ExecutionError("Exactly one key must be specified for oneOf inputs"))
+      }
+      xs.foldLeft(checkSize)(_ orElse _.partial)
+    }
+
+    def build(input: InputValue): Either[ExecutionError, A] =
+      partial.applyOrElse(input, (in: InputValue) => Left(inputError(in)))
+  }
+
 }
 
 trait ArgBuilderDerivation extends CommonArgBuilderDerivation {
diff --git a/core/src/main/scala-2/caliban/schema/SchemaDerivation.scala b/core/src/main/scala-2/caliban/schema/SchemaDerivation.scala
index 6089a58717..40bbd51d48 100644
--- a/core/src/main/scala-2/caliban/schema/SchemaDerivation.scala
+++ b/core/src/main/scala-2/caliban/schema/SchemaDerivation.scala
@@ -1,8 +1,9 @@
 package caliban.schema
 
+import caliban.CalibanError.ValidationError
 import caliban.Value._
 import caliban.introspection.adt._
-import caliban.parsing.adt.Directive
+import caliban.parsing.adt.{ Directive, Directives }
 import caliban.schema.Annotations._
 import caliban.schema.Types._
 import magnolia1._
@@ -65,8 +66,8 @@ trait CommonSchemaDerivation[R] {
       if (_isValueType) {
         if (isScalarValueType(ctx)) makeScalar(getName(ctx), getDescription(ctx))
         else ctx.parameters.head.typeclass.toType_(isInput, isSubscription)
-      } else if (isInput)
-        makeInputObject(
+      } else if (isInput) {
+        lazy val tpe: __Type = makeInputObject(
           Some(ctx.annotations.collectFirst { case GQLInputName(suffix) => suffix }
             .getOrElse(customizeInputTypeName(getName(ctx)))),
           getDescription(ctx),
@@ -81,14 +82,16 @@ trait CommonSchemaDerivation[R] {
                 p.annotations.collectFirst { case GQLDefault(v) => v },
                 p.annotations.collectFirst { case GQLDeprecated(_) => () }.isDefined,
                 p.annotations.collectFirst { case GQLDeprecated(reason) => reason },
-                Some(p.annotations.collect { case GQLDirective(dir) => dir }.toList).filter(_.nonEmpty)
+                Some(p.annotations.collect { case GQLDirective(dir) => dir }.toList).filter(_.nonEmpty),
+                () => Some(tpe)
               )
             )
             .toList,
           Some(ctx.typeName.full),
           Some(getDirectives(ctx))
         )
-      else
+        tpe
+      } else
         makeObject(
           Some(getName(ctx)),
           getDescription(ctx),
@@ -172,11 +175,13 @@ trait CommonSchemaDerivation[R] {
         case _          => false
       }
 
-      if (isEnum && subtypes.nonEmpty && !isInterface && !isUnion)
+      val isOneOfInput = ctx.annotations.contains(GQLOneOfInput())
+
+      if (isEnum && subtypes.nonEmpty && !isInterface && !isUnion && !isOneOfInput) {
         makeEnum(
           Some(getName(ctx)),
           getDescription(ctx),
-          subtypes.collect { case (__Type(_, Some(name), description, _, _, _, _, _, _, _, _, _), annotations) =>
+          subtypes.collect { case (__Type(_, Some(name), description, _, _, _, _, _, _, _, _, _, _), annotations) =>
             __EnumValue(
               name,
               description,
@@ -188,7 +193,18 @@ trait CommonSchemaDerivation[R] {
           Some(ctx.typeName.full),
           Some(getDirectives(ctx.annotations))
         )
-      else if (!isInterface) {
+      } else if (isOneOfInput && isInput) {
+        makeInputObject(
+          Some(ctx.annotations.collectFirst { case GQLInputName(suffix) => suffix }
+            .getOrElse(customizeInputTypeName(getName(ctx)))),
+          getDescription(ctx),
+          ctx.subtypes.toList.flatMap { p =>
+            p.typeclass.toType_(isInput = true).allInputFields.map(_.nullable)
+          },
+          Some(ctx.typeName.full),
+          Some(List(Directive(Directives.OneOf)))
+        )
+      } else if (!isInterface) {
         containsEmptyUnionObjects = emptyUnionObjectIdxs.contains(true)
         makeUnion(
           Some(getName(ctx)),
diff --git a/core/src/main/scala-3/caliban/schema/ArgBuilderDerivation.scala b/core/src/main/scala-3/caliban/schema/ArgBuilderDerivation.scala
index d54329bb9b..114867daec 100644
--- a/core/src/main/scala-3/caliban/schema/ArgBuilderDerivation.scala
+++ b/core/src/main/scala-3/caliban/schema/ArgBuilderDerivation.scala
@@ -2,7 +2,7 @@ package caliban.schema
 
 import caliban.CalibanError.ExecutionError
 import caliban.Value.*
-import caliban.schema.Annotations.{ GQLDefault, GQLName }
+import caliban.schema.Annotations.{ GQLDefault, GQLName, GQLOneOfInput }
 import caliban.schema.macros.Macros
 import caliban.{ CalibanError, InputValue }
 import magnolia1.Macro as MagnoliaMacro
@@ -53,10 +53,16 @@ trait CommonArgBuilderDerivation {
   inline def derived[A]: ArgBuilder[A] =
     inline summonInline[Mirror.Of[A]] match {
       case m: Mirror.SumOf[A] =>
-        makeSumArgBuilder[A](
-          recurseSum[A, m.MirroredElemLabels, m.MirroredElemTypes](),
-          constValue[m.MirroredLabel]
-        )
+        inline if (Macros.hasAnnotation[A, GQLOneOfInput]) {
+          makeOneOfBuilder[A](
+            recurseSum[A, m.MirroredElemLabels, m.MirroredElemTypes](),
+            constValue[m.MirroredLabel]
+          )
+        } else
+          makeSumArgBuilder[A](
+            recurseSum[A, m.MirroredElemLabels, m.MirroredElemTypes](),
+            constValue[m.MirroredLabel]
+          )
 
       case m: Mirror.ProductOf[A] =>
         makeProductArgBuilder(
@@ -68,11 +74,11 @@ trait CommonArgBuilderDerivation {
   private def makeSumArgBuilder[A](
     _subTypes: => List[(String, List[Any], ArgBuilder[Any])],
     traitLabel: String
-  ) = new ArgBuilder[A] {
+  ): ArgBuilder[A] = new ArgBuilder[A] {
     private lazy val subTypes = _subTypes
     private val emptyInput    = InputValue.ObjectValue(Map.empty)
 
-    def build(input: InputValue): Either[ExecutionError, A] =
+    final def build(input: InputValue): Either[ExecutionError, A] =
       input.match {
         case EnumValue(value)   => Right(value)
         case StringValue(value) => Right(value)
@@ -91,28 +97,69 @@ trait CommonArgBuilderDerivation {
       }
   }
 
+  private def makeOneOfBuilder[A](
+    _subTypes: => List[(String, List[Any], ArgBuilder[Any])],
+    traitLabel: String
+  ): ArgBuilder[A] = new ArgBuilder[A] {
+
+    override val partial: PartialFunction[InputValue, Either[ExecutionError, A]] = {
+      val xs = _subTypes.map(_._3).asInstanceOf[List[ArgBuilder[A]]]
+
+      val checkSize: PartialFunction[InputValue, Either[ExecutionError, A]] = {
+        case InputValue.ObjectValue(f) if f.size != 1 =>
+          Left(ExecutionError("Exactly one key must be specified for oneOf inputs"))
+      }
+      xs.foldLeft(checkSize)(_ orElse _.partial)
+    }
+
+    def build(input: InputValue): Either[ExecutionError, A] =
+      partial.applyOrElse(input, (in: InputValue) => Left(inputError(in)))
+
+    private def inputError(input: InputValue) =
+      ExecutionError(s"Invalid oneOf input $input for trait $traitLabel")
+  }
+
   private def makeProductArgBuilder[A](
     _fields: => List[(String, ArgBuilder[Any])],
     annotations: Map[String, List[Any]]
-  )(fromProduct: Product => A) = new ArgBuilder[A] {
-    private lazy val fields = _fields
+  )(fromProduct: Product => A): ArgBuilder[A] = new ArgBuilder[A] {
+
+    private val params = Array.from(_fields.map { (label, builder) =>
+      val labelList  = annotations.get(label)
+      val default    = builder.buildMissing(labelList.flatMap(_.collectFirst { case GQLDefault(v) => v }))
+      val finalLabel = labelList.flatMap(_.collectFirst { case GQLName(name) => name }).getOrElse(label)
+      (finalLabel, default, builder)
+    })
+
+    private val required = params.collect { case (label, default, _) if default.isLeft => label }
+
+    override private[schema] val partial: PartialFunction[InputValue, Either[ExecutionError, A]] = {
+      case InputValue.ObjectValue(fields) if required.forall(fields.contains) => fromFields(fields)
+    }
 
     def build(input: InputValue): Either[ExecutionError, A] =
-      fields.view.map { (label, builder) =>
-        input match {
-          case InputValue.ObjectValue(fields) =>
-            val labelList  = annotations.get(label)
-            def default    = labelList.flatMap(_.collectFirst { case GQLDefault(v) => v })
-            val finalLabel = labelList.flatMap(_.collectFirst { case GQLName(name) => name }).getOrElse(label)
-            fields.get(finalLabel).fold(builder.buildMissing(default))(builder.build)
-          case value                          => builder.build(value)
-        }
-      }.foldLeft[Either[ExecutionError, Tuple]](Right(EmptyTuple)) { case (acc, item) =>
-        item match {
-          case Right(value) => acc.map(_ :* value)
-          case Left(e)      => Left(e)
+      input match {
+        case InputValue.ObjectValue(fields) => fromFields(fields)
+        case _                              => Left(ExecutionError("expected an input object"))
+      }
+
+    private def fromFields(fields: Map[String, InputValue]): Either[ExecutionError, A] = {
+      var i          = 0
+      val l          = params.length
+      var acc: Tuple = EmptyTuple
+      while (i < l) {
+        val (label, default, builder) = params(i)
+        val field                     = fields.getOrElse(label, null)
+        val value                     = if (field ne null) builder.build(field) else default
+        value match {
+          case Right(v)    => acc :*= v
+          case e @ Left(_) => return e.asInstanceOf[Either[ExecutionError, A]]
         }
-      }.map(fromProduct)
+        i += 1
+      }
+      Right(fromProduct(acc))
+    }
+
   }
 }
 
diff --git a/core/src/main/scala-3/caliban/schema/DerivationUtils.scala b/core/src/main/scala-3/caliban/schema/DerivationUtils.scala
index 090bc5a580..cc14690572 100644
--- a/core/src/main/scala-3/caliban/schema/DerivationUtils.scala
+++ b/core/src/main/scala-3/caliban/schema/DerivationUtils.scala
@@ -1,7 +1,7 @@
 package caliban.schema
 
 import caliban.introspection.adt.*
-import caliban.parsing.adt.Directive
+import caliban.parsing.adt.{ Directive, Directives }
 import caliban.schema.Annotations.*
 import caliban.schema.Types.*
 import magnolia1.TypeInfo
@@ -48,13 +48,13 @@ private object DerivationUtils {
     makeEnum(
       Some(getName(annotations, info)),
       getDescription(annotations),
-      subTypes.map { case (name, __Type(_, _, description, _, _, _, _, _, _, _, _, _), annotations) =>
+      subTypes.map { case (name, __Type(_, _, description, _, _, _, _, _, _, _, _, _, _), annotations) =>
         __EnumValue(
           getName(annotations, name),
           description,
           getDeprecatedReason(annotations).isDefined,
           getDeprecatedReason(annotations),
-          Some(annotations.collect { case GQLDirective(dir) => dir }.toList).filter(_.nonEmpty)
+          Some(annotations.collect { case GQLDirective(dir) => dir }).filter(_.nonEmpty)
         )
       }.sortBy(_.name),
       Some(info.full),
@@ -97,25 +97,43 @@ private object DerivationUtils {
     annotations: List[Any],
     fields: List[(String, List[Any], Schema[R, Any])],
     info: TypeInfo
-  )(isInput: Boolean, isSubscription: Boolean): __Type = makeInputObject(
-    Some(getInputName(annotations).getOrElse(customizeInputTypeName(getName(annotations, info)))),
-    getDescription(annotations),
-    fields.map { (name, fieldAnnotations, schema) =>
-      __InputValue(
-        name,
-        getDescription(fieldAnnotations),
-        () =>
-          if (schema.optional) schema.toType_(isInput, isSubscription)
-          else schema.toType_(isInput, isSubscription).nonNull,
-        getDefaultValue(fieldAnnotations),
-        getDeprecatedReason(fieldAnnotations).isDefined,
-        getDeprecatedReason(fieldAnnotations),
-        Some(getDirectives(fieldAnnotations)).filter(_.nonEmpty)
-      )
-    },
-    Some(info.full),
-    Some(getDirectives(annotations))
-  )
+  )(isInput: Boolean, isSubscription: Boolean): __Type = {
+    lazy val tpe: __Type = makeInputObject(
+      Some(getInputName(annotations).getOrElse(customizeInputTypeName(getName(annotations, info)))),
+      getDescription(annotations),
+      fields.map { (name, fieldAnnotations, schema) =>
+        val deprecationReason = getDeprecatedReason(fieldAnnotations)
+        __InputValue(
+          name,
+          description = getDescription(fieldAnnotations),
+          `type` = () =>
+            if (schema.optional) schema.toType_(isInput, isSubscription)
+            else schema.toType_(isInput, isSubscription).nonNull,
+          defaultValue = getDefaultValue(fieldAnnotations),
+          isDeprecated = deprecationReason.isDefined,
+          deprecationReason = deprecationReason,
+          directives = Some(getDirectives(fieldAnnotations)).filter(_.nonEmpty),
+          parentType = () => Some(tpe)
+        )
+      },
+      Some(info.full),
+      Some(getDirectives(annotations))
+    )
+    tpe
+  }
+
+  def mkOneOfInput[R](
+    annotations: List[Any],
+    schemas: List[Schema[R, Any]],
+    info: TypeInfo
+  ): __Type =
+    makeInputObject(
+      Some(getInputName(annotations).getOrElse(customizeInputTypeName(getName(annotations, info)))),
+      getDescription(annotations),
+      schemas.flatMap(_.toType_(isInput = true).allInputFields.map(_.nullable)),
+      Some(info.full),
+      Some(List(Directive(Directives.OneOf)))
+    )
 
   def mkObject[R](
     annotations: List[Any],
diff --git a/core/src/main/scala-3/caliban/schema/SumSchema.scala b/core/src/main/scala-3/caliban/schema/SumSchema.scala
index 7cd6dc3053..620f257a16 100644
--- a/core/src/main/scala-3/caliban/schema/SumSchema.scala
+++ b/core/src/main/scala-3/caliban/schema/SumSchema.scala
@@ -1,7 +1,7 @@
 package caliban.schema
 
 import caliban.introspection.adt.__Type
-import caliban.schema.Annotations.{ GQLInterface, GQLUnion }
+import caliban.schema.Annotations.{ GQLInterface, GQLOneOfInput, GQLUnion }
 import caliban.schema.DerivationUtils.*
 import caliban.schema.Types.makeUnion
 import magnolia1.TypeInfo
@@ -26,6 +26,7 @@ final private class SumSchema[R, A](
   }
 
   private var containsEmptyUnionObjects = false
+  private val isOneOfInput              = annotations.contains(GQLOneOfInput())
 
   def toType(isInput: Boolean, isSubscription: Boolean): __Type = {
     val _                         = schemas
@@ -33,7 +34,8 @@ final private class SumSchema[R, A](
     val isUnion                   = annotations.contains(GQLUnion())
     @threadUnsafe lazy val isEnum = subTypes.forall((_, t, _) => t.allFields.isEmpty && t.allInputFields.isEmpty)
 
-    if (!isInterface && !isUnion && subTypes.nonEmpty && isEnum) mkEnum(annotations, info, subTypes)
+    if (!isInterface && !isUnion && subTypes.nonEmpty && isEnum && !isOneOfInput) mkEnum(annotations, info, subTypes)
+    else if (isOneOfInput && isInput) mkOneOfInput(annotations, schemas.toList, info)
     else if (!isInterface) {
       containsEmptyUnionObjects = emptyUnionObjectIdxs.contains(true)
       makeUnion(
diff --git a/core/src/main/scala-3/caliban/schema/macros/Macros.scala b/core/src/main/scala-3/caliban/schema/macros/Macros.scala
index e5b5a06266..853b10099d 100644
--- a/core/src/main/scala-3/caliban/schema/macros/Macros.scala
+++ b/core/src/main/scala-3/caliban/schema/macros/Macros.scala
@@ -1,6 +1,6 @@
 package caliban.schema.macros
 
-import caliban.schema.Annotations.{ GQLExcluded, GQLField }
+import caliban.schema.Annotations.*
 import caliban.schema.Schema
 
 import scala.quoted.*
diff --git a/core/src/main/scala/caliban/introspection/Introspector.scala b/core/src/main/scala/caliban/introspection/Introspector.scala
index f6c37862da..2035ed210e 100644
--- a/core/src/main/scala/caliban/introspection/Introspector.scala
+++ b/core/src/main/scala/caliban/introspection/Introspector.scala
@@ -47,6 +47,17 @@ object Introspector extends IntrospectionDerivation {
   private val introspectionType       = introspectionSchema.toType_()
   val introspectionRootType: RootType = RootType(introspectionType, None, None)
 
+  private val oneOfDirective =
+    __Directive(
+      "oneOf",
+      Some(
+        "The `@oneOf` directive is used within the type system definition language to indicate an Input Object is a OneOf Input Object."
+      ),
+      Set(__DirectiveLocation.INPUT_OBJECT),
+      _ => Nil,
+      isRepeatable = false
+    )
+
   /**
    * Generates a schema for introspecting the given type.
    */
@@ -70,6 +81,9 @@ object Introspector extends IntrospectionDerivation {
       .values
       .toList
       .sortBy(_.name.getOrElse(""))
+
+    val hasOneOf = types.exists(_._isOneOfInput)
+
     val resolver = __Introspection(
       __Schema(
         rootType.description,
@@ -77,7 +91,7 @@ object Introspector extends IntrospectionDerivation {
         rootType.mutationType,
         rootType.subscriptionType,
         types,
-        directives ++ rootType.additionalDirectives
+        directives ++ (if (hasOneOf) List(oneOfDirective) else Nil) ++ rootType.additionalDirectives
       ),
       args => types.find(_.name.contains(args.name))
     )
diff --git a/core/src/main/scala/caliban/introspection/adt/__InputValue.scala b/core/src/main/scala/caliban/introspection/adt/__InputValue.scala
index 73c58b7bb9..3ab5e06124 100644
--- a/core/src/main/scala/caliban/introspection/adt/__InputValue.scala
+++ b/core/src/main/scala/caliban/introspection/adt/__InputValue.scala
@@ -6,6 +6,8 @@ import caliban.parsing.adt.Directive
 import caliban.parsing.Parser
 import caliban.schema.Annotations.GQLExcluded
 
+import scala.annotation.tailrec
+
 case class __InputValue(
   name: String,
   description: Option[String],
@@ -13,7 +15,8 @@ case class __InputValue(
   defaultValue: Option[String],
   isDeprecated: Boolean = false,
   deprecationReason: Option[String] = None,
-  @GQLExcluded directives: Option[List[Directive]] = None
+  @GQLExcluded directives: Option[List[Directive]] = None,
+  @GQLExcluded parentType: () => Option[__Type] = () => None
 ) {
   def toInputValueDefinition: InputValueDefinition = {
     val default       = defaultValue.flatMap(v => Parser.parseInputValue(v).toOption)
@@ -29,4 +32,20 @@ case class __InputValue(
   }
 
   private[caliban] lazy val _type: __Type = `type`()
+  private[caliban] lazy val _parentType   = parentType()
+
+  /**
+   * Makes the [[`type`]] nullable as required by the spec for OneOf Input Objects
+   */
+  private[caliban] def nullable: __InputValue = {
+    @tailrec
+    def loop(tpe: __Type): __Type =
+      (tpe.kind, tpe.ofType) match {
+        case (__TypeKind.NON_NULL, Some(inner)) => loop(inner)
+        case _                                  => tpe
+      }
+
+    val t = loop(_type)
+    copy(`type` = () => t)
+  }
 }
diff --git a/core/src/main/scala/caliban/introspection/adt/__Type.scala b/core/src/main/scala/caliban/introspection/adt/__Type.scala
index db11ec27f7..c870fd168b 100644
--- a/core/src/main/scala/caliban/introspection/adt/__Type.scala
+++ b/core/src/main/scala/caliban/introspection/adt/__Type.scala
@@ -21,7 +21,8 @@ case class __Type(
   ofType: Option[__Type] = None,
   specifiedBy: Option[String] = None,
   @GQLExcluded directives: Option[List[Directive]] = None,
-  @GQLExcluded origin: Option[String] = None
+  @GQLExcluded origin: Option[String] = None,
+  isOneOf: Option[Boolean] = None
 ) { self =>
   final override lazy val hashCode: Int = super.hashCode()
 
@@ -150,6 +151,8 @@ case class __Type(
       case _                                       => Set.empty
     }
 
+  private[caliban] val _isOneOfInput: Boolean = isOneOf.getOrElse(false)
+
   private[caliban] def mapInnerType(f: __Type => __Type): __Type = {
     def loop(t: __Type): __Type =
       t.ofType match {
diff --git a/core/src/main/scala/caliban/parsing/adt/Directive.scala b/core/src/main/scala/caliban/parsing/adt/Directive.scala
index 4d862853bd..e3cc12df1f 100644
--- a/core/src/main/scala/caliban/parsing/adt/Directive.scala
+++ b/core/src/main/scala/caliban/parsing/adt/Directive.scala
@@ -9,6 +9,7 @@ object Directives {
   final val LazyDirective       = "lazy"
   final val NewtypeDirective    = "newtype"
   final val DeprecatedDirective = "deprecated"
+  final val OneOf               = "oneOf"
 
   def isDeprecated(directives: List[Directive]): Boolean =
     directives.exists(_.name == DeprecatedDirective)
@@ -16,11 +17,15 @@ object Directives {
   def deprecationReason(directives: List[Directive]): Option[String] =
     findDirective(directives, DeprecatedDirective, "reason")
 
-  def isNewType(directives: List[Directive]): Boolean          =
+  def isNewType(directives: List[Directive]): Boolean =
     directives.exists(_.name == NewtypeDirective)
+
   def newTypeName(directives: List[Directive]): Option[String] =
     findDirective(directives, NewtypeDirective, "name")
 
+  def isOneOf(directives: List[Directive]): Boolean =
+    directives.exists(_.name == OneOf)
+
   private def findDirective(directives: List[Directive], name: String, argument: String): Option[String] =
     directives.collectFirst {
       case f if f.name == name =>
diff --git a/core/src/main/scala/caliban/parsing/adt/Type.scala b/core/src/main/scala/caliban/parsing/adt/Type.scala
index b3b0506734..0dcca87e07 100644
--- a/core/src/main/scala/caliban/parsing/adt/Type.scala
+++ b/core/src/main/scala/caliban/parsing/adt/Type.scala
@@ -4,7 +4,8 @@ import scala.annotation.tailrec
 
 sealed trait Type extends Serializable { self =>
   val nonNull: Boolean
-  lazy val nullable: Boolean = !nonNull
+
+  final def nullable: Boolean = !nonNull
 
   override def toString: String = self match {
     case Type.NamedType(name, nonNull)  => if (nonNull) s"$name!" else name
diff --git a/core/src/main/scala/caliban/schema/Annotations.scala b/core/src/main/scala/caliban/schema/Annotations.scala
index 25c7a6c0a3..39b662b9f1 100644
--- a/core/src/main/scala/caliban/schema/Annotations.scala
+++ b/core/src/main/scala/caliban/schema/Annotations.scala
@@ -76,4 +76,9 @@ object Annotations extends AnnotationsVersionSpecific {
    */
   case class GQLNonNullable() extends StaticAnnotation
 
+  /**
+   * Annotation to make a sealed trait as a GraphQL @oneOf input
+   */
+  case class GQLOneOfInput() extends StaticAnnotation
+
 }
diff --git a/core/src/main/scala/caliban/schema/ArgBuilder.scala b/core/src/main/scala/caliban/schema/ArgBuilder.scala
index 78ce9157e5..985f97d213 100644
--- a/core/src/main/scala/caliban/schema/ArgBuilder.scala
+++ b/core/src/main/scala/caliban/schema/ArgBuilder.scala
@@ -35,6 +35,20 @@ trait ArgBuilder[T] { self =>
    */
   def build(input: InputValue): Either[ExecutionError, T]
 
+  private[schema] def partial: PartialFunction[InputValue, Either[ExecutionError, T]] =
+    new PartialFunction[InputValue, Either[ExecutionError, T]] {
+      final def isDefinedAt(x: InputValue): Boolean             = build(x).isRight
+      final def apply(x: InputValue): Either[ExecutionError, T] = build(x)
+
+      final override def applyOrElse[A1 <: InputValue, B1 >: Either[ExecutionError, T]](
+        x: A1,
+        default: A1 => B1
+      ): B1 = {
+        val maybeMatch = build(x)
+        if (maybeMatch.isRight) maybeMatch else default(x)
+      }
+    }
+
   /**
    * Builds a value of type `T` from a missing input value.
    * By default, this delegates to [[build]], passing it NullValue.
diff --git a/core/src/main/scala/caliban/schema/Schema.scala b/core/src/main/scala/caliban/schema/Schema.scala
index 24657497f6..56b8e57c06 100644
--- a/core/src/main/scala/caliban/schema/Schema.scala
+++ b/core/src/main/scala/caliban/schema/Schema.scala
@@ -469,22 +469,23 @@ trait GenericSchema[R] extends SchemaDerivation[R] with TemporalSchema {
     ev2: Schema[RB, B]
   ): Schema[RB, A => B] =
     new Schema[RB, A => B] {
-      private lazy val inputType                 = ev1.toType_(true)
-      private val unwrappedArgumentName          = "value"
-      override def arguments: List[__InputValue] = {
+      private lazy val inputType        = ev1.toType_(true)
+      private val unwrappedArgumentName = "value"
+
+      private def mkValueType = List(
+        __InputValue(
+          unwrappedArgumentName,
+          None,
+          () => if (ev1.nullable || ev1.canFail) inputType else inputType.nonNull,
+          None
+        )
+      )
+
+      override lazy val arguments: List[__InputValue] = {
         val input = inputType.allInputFields
-        if (input.nonEmpty) input
-        else
-          handleInput(List.empty[__InputValue])(
-            List(
-              __InputValue(
-                unwrappedArgumentName,
-                None,
-                () => if (ev1.nullable || ev1.canFail) inputType else inputType.nonNull,
-                None
-              )
-            )
-          )
+        if (inputType._isOneOfInput) mkValueType
+        else if (input.nonEmpty) input
+        else handleInput(List.empty[__InputValue])(mkValueType)
       }
 
       override def nullable: Boolean                                         = ev2.nullable
@@ -508,6 +509,7 @@ trait GenericSchema[R] extends SchemaDerivation[R] with TemporalSchema {
           case __TypeKind.SCALAR | __TypeKind.ENUM | __TypeKind.LIST =>
             // argument was not wrapped in a case class
             onUnwrapped
+          case _ if inputType._isOneOfInput                          => onUnwrapped
           case _                                                     => onWrapped
         }
     }
diff --git a/core/src/main/scala/caliban/schema/Types.scala b/core/src/main/scala/caliban/schema/Types.scala
index 72b72da762..3ddf7eaf9c 100644
--- a/core/src/main/scala/caliban/schema/Types.scala
+++ b/core/src/main/scala/caliban/schema/Types.scala
@@ -1,7 +1,7 @@
 package caliban.schema
 
 import caliban.introspection.adt._
-import caliban.parsing.adt.Directive
+import caliban.parsing.adt.{ Directive, Directives }
 
 import scala.annotation.tailrec
 
@@ -97,7 +97,8 @@ object Types {
         if (args.includeDeprecated.getOrElse(false)) Some(fields)
         else Some(fields.filter(!_.isDeprecated)),
       origin = origin,
-      directives = directives
+      directives = directives,
+      isOneOf = Some(Directives.isOneOf(directives.getOrElse(Nil)))
     )
 
   def makeUnion(
@@ -240,4 +241,5 @@ object Types {
       case __TypeKind.NON_NULL => t.ofType.map(name)
       case _                   => t.name
     }).getOrElse("")
+
 }
diff --git a/core/src/main/scala/caliban/transformers/Transformer.scala b/core/src/main/scala/caliban/transformers/Transformer.scala
index 11a157f491..7c7ca5f677 100644
--- a/core/src/main/scala/caliban/transformers/Transformer.scala
+++ b/core/src/main/scala/caliban/transformers/Transformer.scala
@@ -246,18 +246,18 @@ object Transformer {
 
     val typeVisitor: TypeVisitor =
       TypeVisitor.fields.modify { field =>
-        def loop(parentType: Option[String])(arg: __InputValue): Option[__InputValue] =
-          parentType.flatMap(map.get) match {
+        def loop(arg: __InputValue): Option[__InputValue] =
+          arg._parentType.flatMap(_.name).flatMap(map.get) match {
             case Some(s) if arg._type.isNullable && s.contains(arg.name) =>
               None
             case _                                                       =>
               lazy val newType = arg._type.mapInnerType { t =>
-                t.copy(inputFields = t.inputFields(_).map(_.flatMap(loop(t.name))))
+                t.copy(inputFields = t.inputFields(_).map(_.flatMap(loop)))
               }
               Some(arg.copy(`type` = () => newType))
           }
 
-        field.copy(args = field.args(_).flatMap(loop(None)))
+        field.copy(args = field.args(_).flatMap(loop))
       }
 
     protected val typeNames: Set[String]                                             = Set.empty
diff --git a/core/src/main/scala/caliban/validation/FragmentValidator.scala b/core/src/main/scala/caliban/validation/FragmentValidator.scala
index e1de0b7f64..7a66bad171 100644
--- a/core/src/main/scala/caliban/validation/FragmentValidator.scala
+++ b/core/src/main/scala/caliban/validation/FragmentValidator.scala
@@ -105,7 +105,7 @@ object FragmentValidator {
 
           fields.foreach {
             case field @ SelectedField(
-                  __Type(_, Some(name), _, _, _, _, _, _, _, _, _, _),
+                  __Type(_, Some(name), _, _, _, _, _, _, _, _, _, _, _),
                   _,
                   _
                 ) if isConcrete(field.parentType) =>
diff --git a/core/src/main/scala/caliban/validation/Validator.scala b/core/src/main/scala/caliban/validation/Validator.scala
index 8b6e2e29ad..1f9a041a76 100644
--- a/core/src/main/scala/caliban/validation/Validator.scala
+++ b/core/src/main/scala/caliban/validation/Validator.scala
@@ -374,14 +374,35 @@ object Validator {
               "If any operation defines more than one variable with the same name, it is ambiguous and invalid. It is invalid even if the type of the duplicate variable is the same."
             )
           } *> validateAll(op.variableDefinitions) { v =>
-            val t = Type.innerType(v.variableType)
-            failWhen(context.rootType.types.get(t).map(_.kind).exists {
+            val t = context.rootType.types.get(Type.innerType(v.variableType))
+
+            val v1 = failWhen(t.map(_.kind).exists {
               case __TypeKind.OBJECT | __TypeKind.UNION | __TypeKind.INTERFACE => true
               case _                                                           => false
             })(
               s"Type of variable '${v.name}' is not a valid input type.",
               "Variables can only be input types. Objects, unions, and interfaces cannot be used as inputs."
             )
+
+            val v2 =
+              if (
+                t match {
+                  case Some(t0) => t0._isOneOfInput
+                  case _        => false
+                }
+              )
+                Some(
+                  failWhen(v.variableType.nullable)(
+                    s"Variable '${v.name}' cannot be nullable.",
+                    "Variables used for OneOf Input Object fields must be non-nullable."
+                  ) *> validateOneOfInputValue(
+                    context.variables.getOrElse(v.name, NullValue),
+                    s"Variable '${v.name}'"
+                  )
+                )
+              else None
+
+            v2.fold(v1)(v1 *> _)
           } *> {
             validateAll(variableUsages)(v =>
               failWhen(!op.variableDefinitions.exists(_.name == v))(
@@ -562,23 +583,40 @@ object Validator {
     val fieldArgsNonNull = fieldArgs.filter(_._type.kind == __TypeKind.NON_NULL)
     val providedArgs     = field.arguments
 
-    val v1 = validateAllNonEmpty(fieldArgsNonNull)(arg =>
-      (arg.defaultValue, field.arguments.get(arg.name)) match {
+    val v1 = validateAllNonEmpty(fieldArgsNonNull.flatMap { arg =>
+      val arg0 = field.arguments.get(arg.name)
+      val opt1 = (arg.defaultValue, arg0) match {
         case (None, None) | (None, Some(NullValue)) =>
-          failValidation(
-            s"Required argument '${arg.name}' is null or missing on field '${field.name}' of type '${currentType.name
-                .getOrElse("")}'.",
-            "Arguments can be required. An argument is required if the argument type is non‐null and does not have a default value. Otherwise, the argument is optional."
+          Some(
+            failValidation(
+              s"Required argument '${arg.name}' is null or missing on field '${field.name}' of type '${currentType.name
+                  .getOrElse("")}'.",
+              "Arguments can be required. An argument is required if the argument type is non‐null and does not have a default value. Otherwise, the argument is optional."
+            )
           )
         case (Some(_), Some(NullValue))             =>
-          failValidation(
-            s"Required argument '${arg.name}' is null on '${field.name}' of type '${currentType.name
-                .getOrElse("")}'.",
-            "Arguments can be required. An argument is required if the argument type is non‐null and does not have a default value. Otherwise, the argument is optional."
+          Some(
+            failValidation(
+              s"Required argument '${arg.name}' is null on '${field.name}' of type '${currentType.name
+                  .getOrElse("")}'.",
+              "Arguments can be required. An argument is required if the argument type is non‐null and does not have a default value. Otherwise, the argument is optional."
+            )
           )
-        case _                                      => zunit
+        case _                                      => None
       }
-    )
+      val opt2 =
+        if (arg._type.innerType._isOneOfInput)
+          Some(
+            validateOneOfInputValue(
+              arg0.getOrElse(NullValue),
+              s"Argument '${arg.name}' on field '${field.name}'"
+            )
+          )
+        else None
+
+      combineOptionalValidations(opt1, opt2)
+    })(identity)
+
     val v2 = validateAllNonEmpty(providedArgs) { case (arg, argValue) =>
       fieldArgs.find(_.name == arg) match {
         case Some(inputValue) =>
@@ -649,6 +687,23 @@ object Validator {
     }
   } *> ValueValidator.validateInputTypes(inputValue, argValue, context, errorContext)
 
+  private def validateOneOfInputValue(
+    inputValue: InputValue,
+    errorContext: => String
+  ): EReader[Any, ValidationError, Unit] =
+    ZPure
+      .whenCase(inputValue match {
+        case InputValue.ObjectValue(fields) if fields.size == 1 => fields.headOption.map(_._2)
+        case vv: InputValue.VariableValue                       => Some(vv)
+        case _                                                  => None
+      }) { case None | Some(NullValue) =>
+        failValidation(
+          s"$errorContext is not a valid OneOf Input Object",
+          "OneOf Input Object arguments must specify exactly one non-null key"
+        )
+      }
+      .unit
+
   private def checkVariableUsageAllowed(
     variableDefinition: VariableDefinition,
     inputValue: __InputValue
@@ -865,7 +920,7 @@ object Validator {
     }
 
   private[caliban] def validateInputObject(t: __Type): EReader[Any, ValidationError, Unit] = {
-    lazy val inputObjectContext = s"""InputObject '${t.name.getOrElse("")}'"""
+    lazy val inputObjectContext = s"""${if (t._isOneOfInput) "OneOf " else ""}InputObject '${t.name.getOrElse("")}'"""
 
     def noDuplicateInputValueName(
       inputValues: List[__InputValue],
@@ -877,17 +932,40 @@ object Validator {
       noDuplicateName[__InputValue](inputValues, _.name, messageBuilder, explanatory)
     }
 
+    def noDuplicatedOneOfOrigin(inputValues: List[__InputValue]): EReader[Any, ValidationError, Unit] = {
+      val resolveOrigin  = (i: __InputValue) =>
+        i._parentType.flatMap(_.origin).getOrElse("<unexpected validation error>")
+      val messageBuilder = (i: __InputValue) =>
+        s"$inputObjectContext is extended by a case class with multiple arguments: ${resolveOrigin(i)}"
+      val explanatory    = "All case classes used as arguments to OneOf Input Objects must have exactly one field"
+      noDuplicateName[__InputValue](inputValues, resolveOrigin, messageBuilder, explanatory)
+    }
+
     def validateFields(fields: List[__InputValue]): EReader[Any, ValidationError, Unit] =
       validateAll(fields)(validateInputValue(_, inputObjectContext)) *>
         noDuplicateInputValueName(fields, inputObjectContext)
 
+    def validateOneOfFields(fields: List[__InputValue]): EReader[Any, ValidationError, Unit] =
+      noDuplicatedOneOfOrigin(fields) *>
+        validateAll(fields) { f =>
+          failWhen(f.defaultValue.isDefined)(
+            s"$inputObjectContext argument has a default value",
+            "Fields of OneOf input objects cannot have default values"
+          ) *>
+            failWhen(!f._type.isNullable)(
+              s"$inputObjectContext argument is not nullable",
+              "All of OneOf input fields must be declared as nullable in the schema according to the spec"
+            )
+        }
+
     t.allInputFields match {
-      case Nil    =>
+      case Nil                       =>
         failValidation(
           s"$inputObjectContext does not have fields",
           "An Input Object type must define one or more input fields"
         )
-      case fields => validateFields(fields)
+      case fields if t._isOneOfInput => validateOneOfFields(fields) *> validateFields(fields)
+      case fields                    => validateFields(fields)
     }
   }
 
diff --git a/core/src/test/scala/caliban/RenderingSpec.scala b/core/src/test/scala/caliban/RenderingSpec.scala
index 7dcf5603e2..441ea8ae4c 100644
--- a/core/src/test/scala/caliban/RenderingSpec.scala
+++ b/core/src/test/scala/caliban/RenderingSpec.scala
@@ -4,17 +4,19 @@ import caliban.CalibanError.ParsingError
 import caliban.TestUtils._
 import caliban.introspection.adt.{ __Type, __TypeKind }
 import caliban.parsing.Parser
-import caliban.parsing.adt.Definition.{ TypeSystemDefinition, TypeSystemExtension }
 import caliban.parsing.adt.Definition.TypeSystemDefinition.TypeDefinition
 import caliban.parsing.adt.Definition.TypeSystemDefinition.TypeDefinition.{
   EnumValueDefinition,
   FieldDefinition,
   InputValueDefinition
 }
+import caliban.parsing.adt.Definition.{ TypeSystemDefinition, TypeSystemExtension }
 import caliban.parsing.adt.{ Definition, Directive }
 import caliban.rendering.DocumentRenderer
+import caliban.schema.Annotations.GQLOneOfInput
 import caliban.schema.Schema.auto._
 import caliban.schema.ArgBuilder.auto._
+import caliban.schema.{ ArgBuilder, Schema }
 import zio.{ IO, ZIO }
 import zio.test.Assertion._
 import zio.test._
@@ -179,7 +181,51 @@ object RenderingSpec extends ZIOSpecDefault {
         test("kitchen sink with query")(roundTrip("document-tests/kitchen-sink-query.graphql")),
         test("compact query")(roundTrip("document-tests/query-compact.graphql", isCompact = true)),
         test("compact kitchen sink")(roundTrip("document-tests/kitchen-sink-compact.graphql", isCompact = true))
-      )
+      ),
+      suite("OneOf input objects") {
+        def expected(label: String) =
+          s"""schema {
+             |  query: Queries
+             |}
+             |
+             |input FooInput @oneOf {
+             |  stringValue: String
+             |  otherStringField: String
+             |  intValue: FooIntInput
+             |  otherIntField: FooInt2Input
+             |}
+             |
+             |input FooInt2Input {
+             |  intValue: Int!
+             |}
+             |
+             |input FooIntInput {
+             |  intValue: Int!
+             |}
+             |
+             |type Queries {
+             |  foo($label: FooInput!): String!
+             |}""".stripMargin
+
+        List(
+          test("as value types") {
+            case class Queries(foo: Foo => String)
+
+            implicit val schema: Schema[Any, Queries] = Schema.gen
+            val resolver                              = RootResolver(Queries(_.toString))
+
+            assertTrue(graphQL(resolver).render == expected("value"))
+          },
+          test("wrapped in a case class") {
+            case class Queries(foo: Foo.Wrapped => String)
+
+            implicit val schema: Schema[Any, Queries] = Schema.gen
+            val resolver                              = RootResolver(Queries(_.toString))
+
+            assertTrue(graphQL(resolver).render == expected("fooInput"))
+          }
+        )
+      }
     )
 
   private def roundTrip(file: String, isCompact: Boolean = false) =
@@ -190,4 +236,25 @@ object RenderingSpec extends ZIOSpecDefault {
       reparsed <- Parser.parseQuery(rendered).either
     } yield assertTrue(input == rendered, reparsed.isRight))
 
+  @GQLOneOfInput
+  sealed trait Foo
+
+  object Foo {
+    case class ArgA(stringValue: String)      extends Foo
+    case class ArgB(otherStringField: String) extends Foo
+    case class ArgC(intValue: FooInt)         extends Foo
+    case class ArgD(otherIntField: FooInt2)   extends Foo
+
+    case class FooInt(intValue: Int)
+    case class FooInt2(intValue: Int)
+
+    case class Wrapped(fooInput: Foo)
+  }
+
+  implicit val fooIntAb: ArgBuilder[Foo.FooInt]        = ArgBuilder.gen
+  implicit val fooInt2Ab: ArgBuilder[Foo.FooInt2]      = ArgBuilder.gen
+  implicit val fooAb: ArgBuilder[Foo]                  = ArgBuilder.gen
+  implicit val fooIntSchema: Schema[Any, Foo.FooInt]   = Schema.gen
+  implicit val fooInt2Schema: Schema[Any, Foo.FooInt2] = Schema.gen
+  implicit val fooSchema: Schema[Any, Foo]             = Schema.gen
 }
diff --git a/core/src/test/scala/caliban/TestUtils.scala b/core/src/test/scala/caliban/TestUtils.scala
index df9a26c73f..203529fec8 100644
--- a/core/src/test/scala/caliban/TestUtils.scala
+++ b/core/src/test/scala/caliban/TestUtils.scala
@@ -103,6 +103,15 @@ object TestUtils {
   case class CharacterInArgs(@GQLDirective(Directive("lowercase")) names: List[String])
   case class CharacterObjectArgs(character: CharacterInput)
 
+  @GQLOneOfInput
+  sealed trait NameOrOrigin
+  object NameOrOrigin {
+    case class ByName(name: String)     extends NameOrOrigin
+    case class ByOrigin(origin: Origin) extends NameOrOrigin
+
+    case class Wrapper(nameOrOrigin: NameOrOrigin)
+  }
+
   @GQLDescription("Queries")
   case class Query(
     @GQLDescription("Return all characters from a given origin") characters: CharactersArgs => List[Character],
@@ -118,6 +127,12 @@ object TestUtils {
     @GQLDeprecated("Use `characters`") character: CharacterArgs => UIO[Option[Character]]
   )
 
+  @GQLDescription("Queries")
+  case class QueryWithOneOf(
+    exists: CharacterObjectArgs => Boolean,
+    character: NameOrOrigin.Wrapper => List[Character]
+  )
+
   @GQLDescription("Mutations")
   case class MutationIO(deleteCharacter: CharacterArgs => UIO[Unit])
 
@@ -126,6 +141,7 @@ object TestUtils {
   implicit val characterSchema: Schema[Any, Character]           = genAll
   implicit val querySchema: Schema[Any, Query]                   = genAll
   implicit val queryIOSchema: Schema[Any, QueryIO]               = genAll
+  implicit val queryOneOfSchema: Schema[Any, QueryWithOneOf]     = genAll
   implicit val mutationIOSchema: Schema[Any, MutationIO]         = genAll
   implicit val subscriptionIOSchema: Schema[Any, SubscriptionIO] = genAll
 
@@ -144,6 +160,15 @@ object TestUtils {
       args => ZIO.succeed(characters.find(c => c.name == args.name))
     )
   )
+  val resolverOneOf            = RootResolver(
+    QueryWithOneOf(
+      args => characters.exists(_.name == args.character.name),
+      {
+        case NameOrOrigin.Wrapper(NameOrOrigin.ByName(name))     => characters.filter(c => c.name == name)
+        case NameOrOrigin.Wrapper(NameOrOrigin.ByOrigin(origin)) => characters.filter(c => c.origin == origin)
+      }
+    )
+  )
   val resolverWithMutation     = RootResolver(
     resolverIO.queryResolver,
     MutationIO(_ => ZIO.unit)
diff --git a/core/src/test/scala/caliban/execution/DefaultValueSpec.scala b/core/src/test/scala/caliban/execution/DefaultValueSpec.scala
index 3acd508cb1..d5a6d0e2d9 100644
--- a/core/src/test/scala/caliban/execution/DefaultValueSpec.scala
+++ b/core/src/test/scala/caliban/execution/DefaultValueSpec.scala
@@ -222,10 +222,10 @@ object DefaultValueSpec extends ZIOSpecDefault {
         case class Query(testDefault: TestInput => Int)
         val interpreter = graphQL(RootResolver(Query(i => i.intValue))).interpreter
 
-        interpreter.flatMap(_.execute(introspectionQuery)).map { response =>
+        interpreter.flatMap(_.execute(introspectionQuery)).map(_.data.toString).map { response =>
           assertTrue(
-            response.data.toString ==
-              """{"__schema":{"queryType":{"name":"Query"},"mutationType":null,"subscriptionType":null,"types":[{"kind":"SCALAR","name":"Boolean","fields":null,"inputFields":null},{"kind":"SCALAR","name":"Float","fields":null,"inputFields":null},{"kind":"SCALAR","name":"Int","fields":null,"inputFields":null},{"kind":"OBJECT","name":"Query","fields":[{"name":"testDefault","description":null,"args":[{"name":"intValue","description":null,"defaultValue":"1"}]}],"inputFields":null},{"kind":"SCALAR","name":"String","fields":null,"inputFields":null},{"kind":"OBJECT","name":"__Directive","fields":[{"name":"name","description":null,"args":[]},{"name":"description","description":null,"args":[]},{"name":"locations","description":null,"args":[]},{"name":"args","description":null,"args":[{"name":"includeDeprecated","description":null,"defaultValue":null}]},{"name":"isRepeatable","description":null,"args":[]}],"inputFields":null},{"kind":"ENUM","name":"__DirectiveLocation","fields":null,"inputFields":null},{"kind":"OBJECT","name":"__EnumValue","fields":[{"name":"name","description":null,"args":[]},{"name":"description","description":null,"args":[]},{"name":"isDeprecated","description":null,"args":[]},{"name":"deprecationReason","description":null,"args":[]}],"inputFields":null},{"kind":"OBJECT","name":"__Field","fields":[{"name":"name","description":null,"args":[]},{"name":"description","description":null,"args":[]},{"name":"args","description":null,"args":[{"name":"includeDeprecated","description":null,"defaultValue":null}]},{"name":"type","description":null,"args":[]},{"name":"isDeprecated","description":null,"args":[]},{"name":"deprecationReason","description":null,"args":[]}],"inputFields":null},{"kind":"OBJECT","name":"__InputValue","fields":[{"name":"name","description":null,"args":[]},{"name":"description","description":null,"args":[]},{"name":"type","description":null,"args":[]},{"name":"defaultValue","description":null,"args":[]},{"name":"isDeprecated","description":null,"args":[]},{"name":"deprecationReason","description":null,"args":[]}],"inputFields":null},{"kind":"OBJECT","name":"__Schema","fields":[{"name":"description","description":null,"args":[]},{"name":"queryType","description":null,"args":[]},{"name":"mutationType","description":null,"args":[]},{"name":"subscriptionType","description":null,"args":[]},{"name":"types","description":null,"args":[]},{"name":"directives","description":null,"args":[]}],"inputFields":null},{"kind":"OBJECT","name":"__Type","fields":[{"name":"kind","description":null,"args":[]},{"name":"name","description":null,"args":[]},{"name":"description","description":null,"args":[]},{"name":"fields","description":null,"args":[{"name":"includeDeprecated","description":null,"defaultValue":null}]},{"name":"interfaces","description":null,"args":[]},{"name":"possibleTypes","description":null,"args":[]},{"name":"enumValues","description":null,"args":[{"name":"includeDeprecated","description":null,"defaultValue":null}]},{"name":"inputFields","description":null,"args":[{"name":"includeDeprecated","description":null,"defaultValue":null}]},{"name":"ofType","description":null,"args":[]},{"name":"specifiedBy","description":null,"args":[]}],"inputFields":null},{"kind":"ENUM","name":"__TypeKind","fields":null,"inputFields":null}]}}"""
+            response ==
+              """{"__schema":{"queryType":{"name":"Query"},"mutationType":null,"subscriptionType":null,"types":[{"kind":"SCALAR","name":"Boolean","fields":null,"inputFields":null},{"kind":"SCALAR","name":"Float","fields":null,"inputFields":null},{"kind":"SCALAR","name":"Int","fields":null,"inputFields":null},{"kind":"OBJECT","name":"Query","fields":[{"name":"testDefault","description":null,"args":[{"name":"intValue","description":null,"defaultValue":"1"}]}],"inputFields":null},{"kind":"SCALAR","name":"String","fields":null,"inputFields":null},{"kind":"OBJECT","name":"__Directive","fields":[{"name":"name","description":null,"args":[]},{"name":"description","description":null,"args":[]},{"name":"locations","description":null,"args":[]},{"name":"args","description":null,"args":[{"name":"includeDeprecated","description":null,"defaultValue":null}]},{"name":"isRepeatable","description":null,"args":[]}],"inputFields":null},{"kind":"ENUM","name":"__DirectiveLocation","fields":null,"inputFields":null},{"kind":"OBJECT","name":"__EnumValue","fields":[{"name":"name","description":null,"args":[]},{"name":"description","description":null,"args":[]},{"name":"isDeprecated","description":null,"args":[]},{"name":"deprecationReason","description":null,"args":[]}],"inputFields":null},{"kind":"OBJECT","name":"__Field","fields":[{"name":"name","description":null,"args":[]},{"name":"description","description":null,"args":[]},{"name":"args","description":null,"args":[{"name":"includeDeprecated","description":null,"defaultValue":null}]},{"name":"type","description":null,"args":[]},{"name":"isDeprecated","description":null,"args":[]},{"name":"deprecationReason","description":null,"args":[]}],"inputFields":null},{"kind":"OBJECT","name":"__InputValue","fields":[{"name":"name","description":null,"args":[]},{"name":"description","description":null,"args":[]},{"name":"type","description":null,"args":[]},{"name":"defaultValue","description":null,"args":[]},{"name":"isDeprecated","description":null,"args":[]},{"name":"deprecationReason","description":null,"args":[]}],"inputFields":null},{"kind":"OBJECT","name":"__Schema","fields":[{"name":"description","description":null,"args":[]},{"name":"queryType","description":null,"args":[]},{"name":"mutationType","description":null,"args":[]},{"name":"subscriptionType","description":null,"args":[]},{"name":"types","description":null,"args":[]},{"name":"directives","description":null,"args":[]}],"inputFields":null},{"kind":"OBJECT","name":"__Type","fields":[{"name":"kind","description":null,"args":[]},{"name":"name","description":null,"args":[]},{"name":"description","description":null,"args":[]},{"name":"fields","description":null,"args":[{"name":"includeDeprecated","description":null,"defaultValue":null}]},{"name":"interfaces","description":null,"args":[]},{"name":"possibleTypes","description":null,"args":[]},{"name":"enumValues","description":null,"args":[{"name":"includeDeprecated","description":null,"defaultValue":null}]},{"name":"inputFields","description":null,"args":[{"name":"includeDeprecated","description":null,"defaultValue":null}]},{"name":"ofType","description":null,"args":[]},{"name":"specifiedBy","description":null,"args":[]},{"name":"isOneOf","description":null,"args":[]}],"inputFields":null},{"kind":"ENUM","name":"__TypeKind","fields":null,"inputFields":null}]}}"""
           )
         }
       }
diff --git a/core/src/test/scala/caliban/execution/ExecutionSpec.scala b/core/src/test/scala/caliban/execution/ExecutionSpec.scala
index ca833b42c4..8976106c63 100644
--- a/core/src/test/scala/caliban/execution/ExecutionSpec.scala
+++ b/core/src/test/scala/caliban/execution/ExecutionSpec.scala
@@ -1,13 +1,15 @@
 package caliban.execution
 
+import java.util.UUID
 import caliban.CalibanError.ExecutionError
+import caliban.InputValue.ObjectValue
 import caliban.Macros.gqldoc
 import caliban.TestUtils._
 import caliban.Value.{ BooleanValue, IntValue, NullValue, StringValue }
 import caliban._
 import caliban.introspection.adt.__Type
 import caliban.parsing.adt.LocationInfo
-import caliban.schema.Annotations.{ GQLInterface, GQLName, GQLValueType }
+import caliban.schema.Annotations.{ GQLInterface, GQLName, GQLOneOfInput, GQLValueType }
 import caliban.schema.ArgBuilder.auto._
 import caliban.schema.Schema.auto._
 import caliban.schema._
@@ -1431,6 +1433,52 @@ object ExecutionSpec extends ZIOSpecDefault {
           r6 == List("m2", "m1-f2", "m1-f1", "m1"),
           r7 == List("m2-f2", "m2-f1", "m2", "m1-f2", "m1-f1", "m1")
         )
+      },
+      test("oneOf inputs") {
+
+        @GQLOneOfInput
+        sealed trait Foo
+        object Foo {
+          case class FooString(stringValue: String) extends Foo
+          case class FooInt(intValue: Int)          extends Foo
+
+          case class Wrapper(fooInput: Foo)
+        }
+
+        case class Queries(foo: Foo.Wrapper => String, fooUnwrapped: Foo => String)
+
+        implicit val fooStringAb: ArgBuilder[Foo.FooString] = ArgBuilder.gen
+        implicit val fooIntAb: ArgBuilder[Foo.FooInt]       = ArgBuilder.gen
+        implicit val fooAb: ArgBuilder[Foo]                 = ArgBuilder.gen
+        implicit val schema: Schema[Any, Queries]           = Schema.gen
+
+        val api: GraphQL[Any] = graphQL(
+          RootResolver(
+            Queries(
+              {
+                case Foo.Wrapper(Foo.FooString(value)) => value
+                case Foo.Wrapper(Foo.FooInt(value))    => value.toString
+              },
+              {
+                case Foo.FooString(value) => value
+                case Foo.FooInt(value)    => value.toString
+              }
+            )
+          )
+        )
+
+        val cases = List(
+          gqldoc("""{ foo(fooInput: {stringValue: "hello"}) }""")                 -> """{"foo":"hello"}""",
+          gqldoc("""{ foo(fooInput: {intValue: 42}) }""")                         -> """{"foo":"42"}""",
+          gqldoc("""{ fooUnwrapped(value: {intValue: 42}) }""")                   -> """{"fooUnwrapped":"42"}""",
+          gqldoc("""query Foo($args: FooInput!){ fooUnwrapped(value: $args) }""") -> """{"fooUnwrapped":"42"}"""
+        )
+
+        ZIO.foldLeft(cases)(assertCompletes) { case (acc, (query, expected)) =>
+          api.interpreter
+            .flatMap(_.execute(query, variables = Map("args" -> ObjectValue(Map("intValue" -> IntValue(42))))))
+            .map(response => assertTrue(response.data.toString == expected))
+        }
       }
     )
 }
diff --git a/core/src/test/scala/caliban/introspection/IntrospectionSpec.scala b/core/src/test/scala/caliban/introspection/IntrospectionSpec.scala
index 72cd48a0ba..8303139f91 100644
--- a/core/src/test/scala/caliban/introspection/IntrospectionSpec.scala
+++ b/core/src/test/scala/caliban/introspection/IntrospectionSpec.scala
@@ -36,6 +36,7 @@ object IntrospectionSpec extends ZIOSpecDefault {
           kind
           name
           description
+          isOneOf
           fields(includeDeprecated: true) {
             name
             description
@@ -122,10 +123,10 @@ object IntrospectionSpec extends ZIOSpecDefault {
       test("introspect schema") {
         val interpreter = graphQL(resolverIO).interpreter
 
-        interpreter.flatMap(_.execute(fullIntrospectionQuery)).map { response =>
+        interpreter.flatMap(_.execute(fullIntrospectionQuery)).map(_.data.toString).map { response =>
           assertTrue(
-            response.data.toString ==
-              """{"__schema":{"queryType":{"name":"QueryIO"},"mutationType":null,"subscriptionType":null,"types":[{"kind":"SCALAR","name":"Boolean","description":null,"fields":null,"inputFields":null,"interfaces":null,"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"Captain","description":null,"fields":[{"name":"shipName","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"CaptainShipName","ofType":null}},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"SCALAR","name":"CaptainShipName","description":"Description of custom scalar emphasizing proper captain ship names","fields":null,"inputFields":null,"interfaces":null,"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"Character","description":null,"fields":[{"name":"name","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"nicknames","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}}}},"isDeprecated":false,"deprecationReason":null},{"name":"origin","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"ENUM","name":"Origin","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"role","description":null,"args":[],"type":{"kind":"UNION","name":"Role","ofType":null},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"Engineer","description":null,"fields":[{"name":"shipName","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"SCALAR","name":"Float","description":null,"fields":null,"inputFields":null,"interfaces":null,"enumValues":null,"possibleTypes":null},{"kind":"SCALAR","name":"Int","description":null,"fields":null,"inputFields":null,"interfaces":null,"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"Mechanic","description":null,"fields":[{"name":"shipName","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"ENUM","name":"Origin","description":null,"fields":null,"inputFields":null,"interfaces":null,"enumValues":[{"name":"BELT","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"EARTH","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"MARS","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"MOON","description":null,"isDeprecated":true,"deprecationReason":"Use: EARTH | MARS | BELT"}],"possibleTypes":null},{"kind":"OBJECT","name":"Pilot","description":null,"fields":[{"name":"shipName","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"QueryIO","description":"Queries","fields":[{"name":"characters","description":"Return all characters from a given origin","args":[{"name":"origin","description":null,"type":{"kind":"ENUM","name":"Origin","ofType":null},"defaultValue":null}],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"Character","ofType":null}}}},"isDeprecated":false,"deprecationReason":null},{"name":"character","description":null,"args":[{"name":"name","description":null,"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"defaultValue":null}],"type":{"kind":"OBJECT","name":"Character","ofType":null},"isDeprecated":true,"deprecationReason":"Use `characters`"}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"UNION","name":"Role","description":null,"fields":null,"inputFields":null,"interfaces":null,"enumValues":null,"possibleTypes":[{"kind":"OBJECT","name":"Captain","ofType":null},{"kind":"OBJECT","name":"Engineer","ofType":null},{"kind":"OBJECT","name":"Mechanic","ofType":null},{"kind":"OBJECT","name":"Pilot","ofType":null}]},{"kind":"SCALAR","name":"String","description":null,"fields":null,"inputFields":null,"interfaces":null,"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"__Directive","description":null,"fields":[{"name":"name","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"description","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"locations","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"ENUM","name":"__DirectiveLocation","ofType":null}}}},"isDeprecated":false,"deprecationReason":null},{"name":"args","description":null,"args":[{"name":"includeDeprecated","description":null,"type":{"kind":"SCALAR","name":"Boolean","ofType":null},"defaultValue":null}],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__InputValue","ofType":null}}}},"isDeprecated":false,"deprecationReason":null},{"name":"isRepeatable","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"Boolean","ofType":null}},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"ENUM","name":"__DirectiveLocation","description":null,"fields":null,"inputFields":null,"interfaces":null,"enumValues":[{"name":"ARGUMENT_DEFINITION","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"ENUM","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"ENUM_VALUE","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"FIELD","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"FIELD_DEFINITION","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"FRAGMENT_DEFINITION","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"FRAGMENT_SPREAD","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"INLINE_FRAGMENT","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"INPUT_FIELD_DEFINITION","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"INPUT_OBJECT","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"INTERFACE","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"MUTATION","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"OBJECT","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"QUERY","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"SCALAR","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"SCHEMA","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"SUBSCRIPTION","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"UNION","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"VARIABLE_DEFINITION","description":null,"isDeprecated":false,"deprecationReason":null}],"possibleTypes":null},{"kind":"OBJECT","name":"__EnumValue","description":null,"fields":[{"name":"name","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"description","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"isDeprecated","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"Boolean","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"deprecationReason","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"__Field","description":null,"fields":[{"name":"name","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"description","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"args","description":null,"args":[{"name":"includeDeprecated","description":null,"type":{"kind":"SCALAR","name":"Boolean","ofType":null},"defaultValue":null}],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__InputValue","ofType":null}}}},"isDeprecated":false,"deprecationReason":null},{"name":"type","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Type","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"isDeprecated","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"Boolean","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"deprecationReason","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"__InputValue","description":null,"fields":[{"name":"name","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"description","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"type","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Type","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"defaultValue","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"isDeprecated","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"Boolean","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"deprecationReason","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"__Schema","description":null,"fields":[{"name":"description","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"queryType","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Type","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"mutationType","description":null,"args":[],"type":{"kind":"OBJECT","name":"__Type","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"subscriptionType","description":null,"args":[],"type":{"kind":"OBJECT","name":"__Type","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"types","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Type","ofType":null}}}},"isDeprecated":false,"deprecationReason":null},{"name":"directives","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Directive","ofType":null}}}},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"__Type","description":null,"fields":[{"name":"kind","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"ENUM","name":"__TypeKind","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"name","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"description","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"fields","description":null,"args":[{"name":"includeDeprecated","description":null,"type":{"kind":"SCALAR","name":"Boolean","ofType":null},"defaultValue":null}],"type":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Field","ofType":null}}},"isDeprecated":false,"deprecationReason":null},{"name":"interfaces","description":null,"args":[],"type":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Type","ofType":null}}},"isDeprecated":false,"deprecationReason":null},{"name":"possibleTypes","description":null,"args":[],"type":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Type","ofType":null}}},"isDeprecated":false,"deprecationReason":null},{"name":"enumValues","description":null,"args":[{"name":"includeDeprecated","description":null,"type":{"kind":"SCALAR","name":"Boolean","ofType":null},"defaultValue":null}],"type":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__EnumValue","ofType":null}}},"isDeprecated":false,"deprecationReason":null},{"name":"inputFields","description":null,"args":[{"name":"includeDeprecated","description":null,"type":{"kind":"SCALAR","name":"Boolean","ofType":null},"defaultValue":null}],"type":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__InputValue","ofType":null}}},"isDeprecated":false,"deprecationReason":null},{"name":"ofType","description":null,"args":[],"type":{"kind":"OBJECT","name":"__Type","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"specifiedBy","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"ENUM","name":"__TypeKind","description":null,"fields":null,"inputFields":null,"interfaces":null,"enumValues":[{"name":"ENUM","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"INPUT_OBJECT","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"INTERFACE","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"LIST","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"NON_NULL","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"OBJECT","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"SCALAR","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"UNION","description":null,"isDeprecated":false,"deprecationReason":null}],"possibleTypes":null}],"directives":[{"name":"skip","description":"The 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types. 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@@ -150,10 +151,20 @@ object IntrospectionSpec extends ZIOSpecDefault {
 
         val interpreter = (graphQL(resolverIO) @@ hideWrapper).interpreter
 
-        interpreter.flatMap(_.execute(fullIntrospectionQuery)).map { response =>
+        interpreter.flatMap(_.execute(fullIntrospectionQuery)).map(_.data.toString).map { response =>
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-            response.data.toString ==
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+          )
+        }
+      },
+      test("introspect schema with OneOf input objects") {
+        val interpreter = graphQL(resolverOneOf).interpreter
+
+        interpreter.flatMap(_.execute(fullIntrospectionQuery)).map(_.data.toString).map { response =>
+          assertTrue(
+            response ==
+              """{"__schema":{"queryType":{"name":"QueryWithOneOf"},"mutationType":null,"subscriptionType":null,"types":[{"kind":"SCALAR","name":"Boolean","description":null,"isOneOf":null,"fields":null,"inputFields":null,"interfaces":null,"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"Captain","description":null,"isOneOf":null,"fields":[{"name":"shipName","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"CaptainShipName","ofType":null}},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"SCALAR","name":"CaptainShipName","description":"Description of custom scalar emphasizing proper captain ship 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BELT"}],"possibleTypes":null},{"kind":"OBJECT","name":"Pilot","description":null,"isOneOf":null,"fields":[{"name":"shipName","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"QueryWithOneOf","description":"Queries","isOneOf":null,"fields":[{"name":"exists","description":null,"args":[{"name":"character","description":null,"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"INPUT_OBJECT","name":"CharacterInput","ofType":null}},"defaultValue":null}],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"Boolean","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"character","description":null,"args":[{"name":"nameOrOrigin","description":null,"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"INPUT_OBJECT","name":"NameOrOriginInput","ofType":null}},"defaultValue":null}],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"Character","ofType":null}}}},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"UNION","name":"Role","description":null,"isOneOf":null,"fields":null,"inputFields":null,"interfaces":null,"enumValues":null,"possibleTypes":[{"kind":"OBJECT","name":"Captain","ofType":null},{"kind":"OBJECT","name":"Engineer","ofType":null},{"kind":"OBJECT","name":"Mechanic","ofType":null},{"kind":"OBJECT","name":"Pilot","ofType":null}]},{"kind":"SCALAR","name":"String","description":null,"isOneOf":null,"fields":null,"inputFields":null,"interfaces":null,"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"__Directive","description":null,"isOneOf":null,"fields":[{"name":"name","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"description","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"locations","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"ENUM","name":"__DirectiveLocation","ofType":null}}}},"isDeprecated":false,"deprecationReason":null},{"name":"args","description":null,"args":[{"name":"includeDeprecated","description":null,"type":{"kind":"SCALAR","name":"Boolean","ofType":null},"defaultValue":null}],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__InputValue","ofType":null}}}},"isDeprecated":false,"deprecationReason":null},{"name":"isRepeatable","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"Boolean","ofType":null}},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"ENUM","name":"__DirectiveLocation","description":null,"isOneOf":null,"fields":null,"inputFields":null,"interfaces":null,"enumValues":[{"name":"ARGUMENT_DEFINITION","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"ENUM","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"ENUM_VALUE","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"FIELD","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"FIELD_DEFINITION","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"FRAGMENT_DEFINITION","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"FRAGMENT_SPREAD","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"INLINE_FRAGMENT","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"INPUT_FIELD_DEFINITION","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"INPUT_OBJECT","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"INTERFACE","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"MUTATION","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"OBJECT","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"QUERY","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"SCALAR","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"SCHEMA","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"SUBSCRIPTION","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"UNION","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"VARIABLE_DEFINITION","description":null,"isDeprecated":false,"deprecationReason":null}],"possibleTypes":null},{"kind":"OBJECT","name":"__EnumValue","description":null,"isOneOf":null,"fields":[{"name":"name","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"description","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"isDeprecated","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"Boolean","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"deprecationReason","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"__Field","description":null,"isOneOf":null,"fields":[{"name":"name","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"description","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"args","description":null,"args":[{"name":"includeDeprecated","description":null,"type":{"kind":"SCALAR","name":"Boolean","ofType":null},"defaultValue":null}],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__InputValue","ofType":null}}}},"isDeprecated":false,"deprecationReason":null},{"name":"type","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Type","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"isDeprecated","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"Boolean","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"deprecationReason","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"__InputValue","description":null,"isOneOf":null,"fields":[{"name":"name","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"description","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"type","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Type","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"defaultValue","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"isDeprecated","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"Boolean","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"deprecationReason","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"__Schema","description":null,"isOneOf":null,"fields":[{"name":"description","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"queryType","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Type","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"mutationType","description":null,"args":[],"type":{"kind":"OBJECT","name":"__Type","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"subscriptionType","description":null,"args":[],"type":{"kind":"OBJECT","name":"__Type","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"types","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Type","ofType":null}}}},"isDeprecated":false,"deprecationReason":null},{"name":"directives","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Directive","ofType":null}}}},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"OBJECT","name":"__Type","description":null,"isOneOf":null,"fields":[{"name":"kind","description":null,"args":[],"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"ENUM","name":"__TypeKind","ofType":null}},"isDeprecated":false,"deprecationReason":null},{"name":"name","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"description","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"fields","description":null,"args":[{"name":"includeDeprecated","description":null,"type":{"kind":"SCALAR","name":"Boolean","ofType":null},"defaultValue":null}],"type":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Field","ofType":null}}},"isDeprecated":false,"deprecationReason":null},{"name":"interfaces","description":null,"args":[],"type":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Type","ofType":null}}},"isDeprecated":false,"deprecationReason":null},{"name":"possibleTypes","description":null,"args":[],"type":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__Type","ofType":null}}},"isDeprecated":false,"deprecationReason":null},{"name":"enumValues","description":null,"args":[{"name":"includeDeprecated","description":null,"type":{"kind":"SCALAR","name":"Boolean","ofType":null},"defaultValue":null}],"type":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__EnumValue","ofType":null}}},"isDeprecated":false,"deprecationReason":null},{"name":"inputFields","description":null,"args":[{"name":"includeDeprecated","description":null,"type":{"kind":"SCALAR","name":"Boolean","ofType":null},"defaultValue":null}],"type":{"kind":"LIST","name":null,"ofType":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"__InputValue","ofType":null}}},"isDeprecated":false,"deprecationReason":null},{"name":"ofType","description":null,"args":[],"type":{"kind":"OBJECT","name":"__Type","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"specifiedBy","description":null,"args":[],"type":{"kind":"SCALAR","name":"String","ofType":null},"isDeprecated":false,"deprecationReason":null},{"name":"isOneOf","description":null,"args":[],"type":{"kind":"SCALAR","name":"Boolean","ofType":null},"isDeprecated":false,"deprecationReason":null}],"inputFields":null,"interfaces":[],"enumValues":null,"possibleTypes":null},{"kind":"ENUM","name":"__TypeKind","description":null,"isOneOf":null,"fields":null,"inputFields":null,"interfaces":null,"enumValues":[{"name":"ENUM","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"INPUT_OBJECT","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"INTERFACE","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"LIST","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"NON_NULL","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"OBJECT","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"SCALAR","description":null,"isDeprecated":false,"deprecationReason":null},{"name":"UNION","description":null,"isDeprecated":false,"deprecationReason":null}],"possibleTypes":null}],"directives":[{"name":"skip","description":"The @skip directive may be provided for fields, fragment spreads, and inline fragments, and allows for conditional exclusion during execution as described by the if argument.","locations":["FIELD","FRAGMENT_SPREAD","INLINE_FRAGMENT"],"args":[{"name":"if","description":null,"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"Boolean","ofType":null}},"defaultValue":null}]},{"name":"include","description":"The @include directive may be provided for fields, fragment spreads, and inline fragments, and allows for conditional inclusion during execution as described by the if argument.","locations":["FIELD","FRAGMENT_SPREAD","INLINE_FRAGMENT"],"args":[{"name":"if","description":null,"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"Boolean","ofType":null}},"defaultValue":null}]},{"name":"specifiedBy","description":"The @specifiedBy directive is used within the type system definition language to provide a URL for specifying the behavior of custom scalar types. The URL should point to a human-readable specification of the data format, serialization, and coercion rules. It must not appear on built-in scalar types.","locations":["SCALAR"],"args":[{"name":"url","description":null,"type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","ofType":null}},"defaultValue":null}]},{"name":"oneOf","description":"The `@oneOf` directive is used within the type system definition language to indicate an Input Object is a OneOf Input Object.","locations":["INPUT_OBJECT"],"args":[]}]}}"""
           )
         }
       },
@@ -167,8 +178,8 @@ object IntrospectionSpec extends ZIOSpecDefault {
               }
             """)
 
-        interpreter.flatMap(_.execute(query)).map { response =>
-          assertTrue(response.data.toString == """{"__type":{"name":"Captain"}}""")
+        interpreter.flatMap(_.execute(query)).map(_.data.toString).map { response =>
+          assertTrue(response == """{"__type":{"name":"Captain"}}""")
         }
       },
       test("introspect non-existent type") {
@@ -181,8 +192,8 @@ object IntrospectionSpec extends ZIOSpecDefault {
               }
             """)
 
-        interpreter.flatMap(_.execute(query)).map { response =>
-          assertTrue(response.data.toString == """{"__type":null}""")
+        interpreter.flatMap(_.execute(query)).map(_.data.toString).map { response =>
+          assertTrue(response == """{"__type":null}""")
         }
       },
       test("introspect the introspection itself") {
@@ -203,11 +214,33 @@ object IntrospectionSpec extends ZIOSpecDefault {
               }
             """)
 
-        interpreter.flatMap(_.execute(query)).map { response =>
+        interpreter.flatMap(_.execute(query)).map(_.data.toString).map { response =>
           assertTrue(
-            response.data.toString == """{"__type":{"name":"__Schema","fields":[{"name":"description"},{"name":"queryType"},{"name":"mutationType"},{"name":"subscriptionType"},{"name":"types"},{"name":"directives"}]},"__schema":{"types":[{"name":"Boolean"},{"name":"Captain"},{"name":"CaptainShipName"},{"name":"Character"},{"name":"Engineer"},{"name":"Float"},{"name":"Int"},{"name":"Mechanic"},{"name":"Origin"},{"name":"Pilot"},{"name":"QueryIO"},{"name":"Role"},{"name":"String"},{"name":"__Directive"},{"name":"__DirectiveLocation"},{"name":"__EnumValue"},{"name":"__Field"},{"name":"__InputValue"},{"name":"__Schema"},{"name":"__Type"},{"name":"__TypeKind"}]}}"""
+            response == """{"__type":{"name":"__Schema","fields":[{"name":"description"},{"name":"queryType"},{"name":"mutationType"},{"name":"subscriptionType"},{"name":"types"},{"name":"directives"}]},"__schema":{"types":[{"name":"Boolean"},{"name":"Captain"},{"name":"CaptainShipName"},{"name":"Character"},{"name":"Engineer"},{"name":"Float"},{"name":"Int"},{"name":"Mechanic"},{"name":"Origin"},{"name":"Pilot"},{"name":"QueryIO"},{"name":"Role"},{"name":"String"},{"name":"__Directive"},{"name":"__DirectiveLocation"},{"name":"__EnumValue"},{"name":"__Field"},{"name":"__InputValue"},{"name":"__Schema"},{"name":"__Type"},{"name":"__TypeKind"}]}}"""
           )
         }
+      },
+      test("introspect OneOf input object types") {
+        val interpreter = graphQL(resolverOneOf).interpreter
+
+        def query(name: String) =
+          s"""
+              query {
+                __type(name: "$name") {
+                  name
+                  isOneOf
+                }
+              }
+            """
+
+        for {
+          i  <- interpreter
+          r1 <- i.execute(query("NameOrOriginInput"))
+          r2 <- i.execute(query("CharacterInput"))
+        } yield assertTrue(
+          r1.data.toString == """{"__type":{"name":"NameOrOriginInput","isOneOf":true}}""",
+          r2.data.toString == """{"__type":{"name":"CharacterInput","isOneOf":false}}"""
+        )
       }
     )
 }
diff --git a/core/src/test/scala/caliban/schema/ArgBuilderSpec.scala b/core/src/test/scala/caliban/schema/ArgBuilderSpec.scala
index 6c92e9a0e9..d50a759383 100644
--- a/core/src/test/scala/caliban/schema/ArgBuilderSpec.scala
+++ b/core/src/test/scala/caliban/schema/ArgBuilderSpec.scala
@@ -5,6 +5,7 @@ import caliban.InputValue
 import caliban.InputValue.ObjectValue
 import caliban.schema.ArgBuilder.auto._
 import caliban.Value.{ IntValue, NullValue, StringValue }
+import caliban.schema.Annotations.GQLOneOfInput
 import zio.test.Assertion._
 import zio.test._
 
@@ -96,6 +97,56 @@ object ArgBuilderSpec extends ZIOSpecDefault {
         assertTrue(derivedAB.build(ObjectValue(Map("a" -> NullValue))) == Right(Wrapper(NullNullable))) &&
         assertTrue(derivedAB.build(ObjectValue(Map("a" -> StringValue("x")))) == Right(Wrapper(SomeNullable("x"))))
       }
-    )
+    ),
+    suite("oneOf") {
+      @GQLOneOfInput
+      sealed trait Foo
+
+      object Foo {
+        case class Arg1(stringValue: String) extends Foo
+        case class Arg2(fooString2: String)  extends Foo
+        case class Arg3(intValue: Foo1)      extends Foo
+        case class Arg4(fooInt2: Foo2)       extends Foo
+
+      }
+
+      case class Foo1(foo1: Int)
+      case class Foo2(foo2: Int)
+
+      implicit val foo1Ab: ArgBuilder[Foo1]   = ArgBuilder.gen
+      implicit val foo2Ab: ArgBuilder[Foo2]   = ArgBuilder.gen
+      implicit val f1Ab: ArgBuilder[Foo.Arg1] = ArgBuilder.gen
+      implicit val f2Ab: ArgBuilder[Foo.Arg2] = ArgBuilder.gen
+      implicit val f3Ab: ArgBuilder[Foo.Arg3] = ArgBuilder.gen
+      implicit val f4Ab: ArgBuilder[Foo.Arg4] = ArgBuilder.gen
+      val fooAb: ArgBuilder[Foo]              = ArgBuilder.gen
+
+      List(
+        test("valid input") {
+          val inputs = List(
+            Map("stringValue" -> StringValue("foo"))                    -> Foo.Arg1("foo"),
+            Map("fooString2" -> StringValue("foo2"))                    -> Foo.Arg2("foo2"),
+            Map("intValue" -> ObjectValue(Map("foo1" -> IntValue(42)))) -> Foo.Arg3(Foo1(42)),
+            Map("fooInt2" -> ObjectValue(Map("foo2" -> IntValue(24))))  -> Foo.Arg4(Foo2(24))
+          )
+
+          inputs.foldLeft(assertCompletes) { case (acc, (input, expected)) =>
+            acc && assertTrue(fooAb.build(ObjectValue(input)) == Right(expected))
+          }
+        },
+        test("invalid input") {
+          List(
+            Map("invalid"     -> StringValue("foo")),
+            Map("stringValue" -> StringValue("foo"), "fooString2" -> StringValue("foo2")),
+            Map("stringValue" -> ObjectValue(Map("value" -> StringValue("foo")))),
+            Map("stringValue" -> NullValue),
+            Map("stringValue" -> StringValue("foo"), "invalid"    -> NullValue)
+          )
+            .map(input => assertTrue(fooAb.build(ObjectValue(input)).isLeft))
+            .foldLeft(assertCompletes)(_ && _)
+
+        }
+      )
+    }
   )
 }
diff --git a/core/src/test/scala/caliban/validation/ValidationSchemaSpec.scala b/core/src/test/scala/caliban/validation/ValidationSchemaSpec.scala
index f365c1f7c0..cabbe2c5d3 100644
--- a/core/src/test/scala/caliban/validation/ValidationSchemaSpec.scala
+++ b/core/src/test/scala/caliban/validation/ValidationSchemaSpec.scala
@@ -5,7 +5,8 @@ import caliban._
 import caliban.TestUtils._
 import caliban.TestUtils.InvalidSchemas._
 import caliban.introspection.adt._
-import caliban.schema.Types
+import caliban.schema.Annotations.{ GQLDefault, GQLOneOfInput }
+import caliban.schema.{ ArgBuilder, Schema, Types }
 import caliban.schema.Schema.auto._
 import caliban.schema.ArgBuilder.auto._
 import caliban.{ GraphQL, RootResolver }
@@ -334,6 +335,67 @@ object ValidationSchemaSpec extends ZIOSpecDefault {
             )
           }
         )
-      }
+      },
+      suite("OneOf input objects")(
+        test("must have at least one input field") {
+          @GQLOneOfInput
+          sealed trait Foo
+          case object ArgA extends Foo
+          case class Queries(foo: Foo => String)
+
+          implicit val ab: ArgBuilder[Foo]          = ArgBuilder.gen
+          implicit val schema: Schema[Any, Queries] = Schema.gen
+
+          check(
+            graphQL(RootResolver(Queries(_.toString))),
+            "OneOf InputObject 'FooInput' does not have fields"
+          )
+        },
+        test("case classes extending OneOf sealed traits must have a single argument") {
+          @GQLOneOfInput
+          sealed trait Foo
+          case class ArgA(foo: String, bar: String) extends Foo
+          case class ArgB(baz: String)              extends Foo
+          case class Queries(foo: Foo => String)
+
+          implicit val ab: ArgBuilder[Foo]          = ArgBuilder.gen
+          implicit val schema: Schema[Any, Queries] = Schema.gen
+
+          check(
+            graphQL(RootResolver(Queries(_.toString))),
+            "OneOf InputObject 'FooInput' is extended by a case class with multiple arguments: caliban.validation.ValidationSchemaSpec.spec.ArgA"
+          )
+        },
+        test("cannot have default values") {
+          @GQLOneOfInput
+          sealed trait Foo
+          case class ArgA(@GQLDefault("foo") foo: String) extends Foo
+          case class ArgB(bar: String)                    extends Foo
+          case class Queries(foo: Foo => String)
+
+          implicit val ab: ArgBuilder[Foo]          = ArgBuilder.gen
+          implicit val schema: Schema[Any, Queries] = Schema.gen
+
+          check(
+            graphQL(RootResolver(Queries(_.toString))),
+            "OneOf InputObject 'FooInput' argument has a default value"
+          )
+        },
+        test("must have unique field names") {
+          @GQLOneOfInput
+          sealed trait Foo
+          case class ArgA(foo: Int)    extends Foo
+          case class ArgB(foo: String) extends Foo
+          case class Queries(foo: Foo => String)
+
+          implicit val ab: ArgBuilder[Foo]          = ArgBuilder.gen
+          implicit val schema: Schema[Any, Queries] = Schema.gen
+
+          check(
+            graphQL(RootResolver(Queries(_.toString))),
+            "OneOf InputObject 'FooInput' has repeated fields: foo"
+          )
+        }
+      )
     )
 }
diff --git a/core/src/test/scala/caliban/validation/ValidationSpec.scala b/core/src/test/scala/caliban/validation/ValidationSpec.scala
index 81e2f4eb0d..c23e54b34f 100644
--- a/core/src/test/scala/caliban/validation/ValidationSpec.scala
+++ b/core/src/test/scala/caliban/validation/ValidationSpec.scala
@@ -2,10 +2,12 @@ package caliban.validation
 
 import caliban._
 import caliban.CalibanError.ValidationError
+import caliban.InputValue.ObjectValue
 import caliban.Macros.gqldoc
 import caliban.TestUtils._
-import caliban.Value.{ BooleanValue, IntValue, StringValue }
-import caliban.schema.Annotations.GQLDefault
+import caliban.Value.{ BooleanValue, IntValue, NullValue, StringValue }
+import caliban.schema.Annotations.{ GQLDefault, GQLOneOfInput }
+import caliban.schema.{ ArgBuilder, Schema }
 import zio.{ IO, UIO, ZIO }
 import zio.test.Assertion._
 import zio.test._
@@ -383,6 +385,155 @@ object ValidationSpec extends ZIOSpecDefault {
         api.interpreter.flatMap(_.execute(query)).map { response =>
           assertTrue(response.errors.isEmpty)
         }
+      },
+      suite("OneOf input objects") {
+        import caliban.schema.Schema.auto._
+        import caliban.schema.ArgBuilder.auto._
+
+        @GQLOneOfInput
+        sealed trait Foo
+        object Foo {
+          case class FooString(fooString: String) extends Foo
+          case class FooInt(fooInt: IntObj)       extends Foo
+          case class Wrapper(fooInput: Foo)
+        }
+
+        case class IntObj(intValue: Int)
+        case class Bar(foo2: Foo => String)
+        case class Queries(foo: Foo.Wrapper => String, fooUnwrapped: Foo => String, bar: Bar)
+
+        implicit val intObjAb: ArgBuilder[IntObj]           = ArgBuilder.gen
+        implicit val fooStringAb: ArgBuilder[Foo.FooString] = ArgBuilder.gen
+        implicit val fooIntAb: ArgBuilder[Foo.FooInt]       = ArgBuilder.gen
+        implicit val fooAb: ArgBuilder[Foo]                 = ArgBuilder.gen
+        implicit val barSchema: Schema[Any, Bar]            = Schema.gen
+        implicit val schema: Schema[Any, Queries]           = Schema.gen
+
+        val api: GraphQL[Any] = graphQL(RootResolver(Queries(_.fooInput.toString, _.toString, Bar(_.toString))))
+
+        def argumentsQuery(arg1: ObjectValue, arg2: ObjectValue, arg3: ObjectValue) =
+          s"""
+             |{
+             |  foo(fooInput: ${arg1.toInputString})
+             |
+             |  fooUnwrapped(value: ${arg2.toInputString})
+             |
+             |  bar {
+             |    foo2(value: ${arg3.toInputString} )
+             |  }
+             |}
+             |""".stripMargin
+
+        val variablesQuery =
+          """
+            |query Foo($args1: FooInput!, $args2: FooInput!, $args3: FooInput!){
+            |  foo(fooInput: $args1)
+            |
+            |  fooUnwrapped(value: $args2)
+            |
+            |  bar {
+            |    foo2(value: $args3 )
+            |  }
+            |}
+            |""".stripMargin
+
+        val validInputs = List(
+          Map("fooString" -> StringValue("hello")),
+          Map("fooInt"    -> ObjectValue(Map("intValue" -> IntValue(42))))
+        ).map(ObjectValue(_))
+
+        val invalidInputs = List(
+          Map.empty[String, InputValue],
+          Map("fooString" -> NullValue),
+          Map("fooString" -> StringValue("foo"), "fooInt" -> NullValue),
+          Map("fooString" -> StringValue("foo"), "fooInt" -> ObjectValue(Map("intValue" -> IntValue(42))))
+        ).map(ObjectValue(_))
+
+        val validVariablesCases = validInputs.map(arg => Map("args1" -> arg, "args2" -> arg, "args3" -> arg))
+
+        List(
+          test("valid field arguments") {
+            val cases = validInputs.map(v => argumentsQuery(v, v, v))
+            ZIO.foldLeft(cases)(assertCompletes) { case (acc, query) =>
+              api.interpreter
+                .flatMap(_.execute(query))
+                .map(resp => acc && assertTrue(resp.errors.isEmpty))
+            }
+          },
+          test("valid variables") {
+            ZIO.foldLeft(validVariablesCases)(assertCompletes) { case (acc, variables) =>
+              api.interpreter
+                .flatMap(_.execute(variablesQuery, variables = variables))
+                .map(resp => acc && assertTrue(resp.errors.isEmpty))
+            }
+          },
+          test("invalid field arguments") {
+            val cases = for {
+              invalid <- invalidInputs
+              valid    = validInputs.head
+              _case   <- List(
+                           argumentsQuery(invalid, valid, valid),
+                           argumentsQuery(valid, invalid, valid),
+                           argumentsQuery(valid, valid, invalid)
+                         )
+            } yield _case
+
+            ZIO.foldLeft(cases)(assertCompletes) { case (acc, query) =>
+              api.interpreter
+                .flatMap(_.execute(query))
+                .map(resp =>
+                  acc && assertTrue(
+                    resp.errors.nonEmpty && resp.errors.forall {
+                      case ValidationError(msg, _, _, _) => msg.contains("is not a valid OneOf Input Object")
+                      case _                             => false
+                    }
+                  )
+                )
+            }
+          },
+          test("invalid variables") {
+            val cases = for {
+              argName <- validVariablesCases.head.keys
+              invalid <- invalidInputs
+            } yield validVariablesCases.head.updated(argName, invalid)
+
+            ZIO.foldLeft(cases)(assertCompletes) { case (acc, variables) =>
+              api.interpreter
+                .flatMap(_.execute(variablesQuery, variables = variables))
+                .map(resp =>
+                  acc && assertTrue(
+                    resp.errors.nonEmpty && resp.errors.forall {
+                      case ValidationError(msg, _, _, _) => msg.contains("is not a valid OneOf Input Object")
+                      case _                             => false
+                    }
+                  )
+                )
+            }
+          },
+          test("OneOf variables cannot be nullable") {
+            val variablesQuery =
+              """
+                |query Foo($args1: FooInput){
+                |  foo(fooInput: $args1)
+                |}
+                |""".stripMargin
+
+            api.interpreter
+              .flatMap(_.execute(variablesQuery, variables = Map("args1" -> validInputs.head)))
+              .map(resp =>
+                assertTrue(
+                  resp.errors.nonEmpty,
+                  resp.errors.forall {
+                    case ValidationError("Variable 'args1' cannot be nullable.", _, _, _) => true
+                    case _                                                                => false
+                  }
+                )
+              )
+          },
+          test("schema is valid") {
+            api.validateRootSchema.as(assertCompletes)
+          }
+        )
       }
     )
 }