diff --git a/api-gen-runtime/api/api-gen-runtime.api b/api-gen-runtime/api/api-gen-runtime.api
index 99dcdd8..06c2125 100644
--- a/api-gen-runtime/api/api-gen-runtime.api
+++ b/api-gen-runtime/api/api-gen-runtime.api
@@ -453,6 +453,7 @@ public final class sh/christian/ozone/api/runtime/BlobSerializer : kotlinx/seria
 
 public final class sh/christian/ozone/api/runtime/BuildXrpcJsonConfigurationKt {
 	public static final fun buildXrpcJsonConfiguration (Lkotlinx/serialization/modules/SerializersModule;)Lkotlinx/serialization/json/Json;
+	public static synthetic fun buildXrpcJsonConfiguration$default (Lkotlinx/serialization/modules/SerializersModule;ILjava/lang/Object;)Lkotlinx/serialization/json/Json;
 }
 
 public final class sh/christian/ozone/api/runtime/ImmutableListSerializer : kotlinx/serialization/KSerializer {
diff --git a/api-gen-runtime/src/commonMain/kotlin/sh/christian/ozone/api/runtime/buildXrpcJsonConfiguration.kt b/api-gen-runtime/src/commonMain/kotlin/sh/christian/ozone/api/runtime/buildXrpcJsonConfiguration.kt
index 7bfd9c0..c06cbc6 100644
--- a/api-gen-runtime/src/commonMain/kotlin/sh/christian/ozone/api/runtime/buildXrpcJsonConfiguration.kt
+++ b/api-gen-runtime/src/commonMain/kotlin/sh/christian/ozone/api/runtime/buildXrpcJsonConfiguration.kt
@@ -6,7 +6,7 @@ import kotlinx.serialization.modules.SerializersModule
 /**
  * JSON configuration for serializing and deserializing lexicon objects with the given module.
  */
-fun buildXrpcJsonConfiguration(module: SerializersModule): Json = Json {
+fun buildXrpcJsonConfiguration(module: SerializersModule = Json.serializersModule): Json = Json {
   ignoreUnknownKeys = true
   classDiscriminator = "${'$'}type"
   serializersModule = module
diff --git a/bluesky/build.gradle.kts b/bluesky/build.gradle.kts
index e0baed5..6f6a569 100644
--- a/bluesky/build.gradle.kts
+++ b/bluesky/build.gradle.kts
@@ -35,6 +35,12 @@ dependencies {
 }
 
 lexicons {
+  namespace.set("sh.christian.ozone.api.xrpc")
+
+  defaults {
+    generateUnknownsForSealedTypes.set(false)
+  }
+
   generateApi("BlueskyApi") {
     packageName.set("sh.christian.ozone")
     withKtorImplementation("XrpcBlueskyApi")
diff --git a/bluesky/src/commonMain/kotlin/sh/christian/ozone/BlueskyJson.kt b/bluesky/src/commonMain/kotlin/sh/christian/ozone/BlueskyJson.kt
index eaccebb..62247ba 100644
--- a/bluesky/src/commonMain/kotlin/sh/christian/ozone/BlueskyJson.kt
+++ b/bluesky/src/commonMain/kotlin/sh/christian/ozone/BlueskyJson.kt
@@ -6,4 +6,4 @@ import sh.christian.ozone.api.runtime.buildXrpcJsonConfiguration
 /**
  * JSON configuration for serializing and deserializing Bluesky API objects.
  */
-val BlueskyJson: Json = buildXrpcJsonConfiguration(Json.serializersModule)
+val BlueskyJson: Json = buildXrpcJsonConfiguration()
diff --git a/generator/src/main/kotlin/sh/christian/ozone/api/generator/ApiConfiguration.kt b/generator/src/main/kotlin/sh/christian/ozone/api/generator/ApiConfiguration.kt
index 215a5b6..071495e 100644
--- a/generator/src/main/kotlin/sh/christian/ozone/api/generator/ApiConfiguration.kt
+++ b/generator/src/main/kotlin/sh/christian/ozone/api/generator/ApiConfiguration.kt
@@ -3,6 +3,7 @@ package sh.christian.ozone.api.generator
 import java.io.Serializable
 
 data class ApiConfiguration(
+  val namespace: String,
   val packageName: String,
   val interfaceName: String,
   val implementationName: String?,
diff --git a/generator/src/main/kotlin/sh/christian/ozone/api/generator/DefaultsConfiguration.kt b/generator/src/main/kotlin/sh/christian/ozone/api/generator/DefaultsConfiguration.kt
new file mode 100644
index 0000000..52b0f61
--- /dev/null
+++ b/generator/src/main/kotlin/sh/christian/ozone/api/generator/DefaultsConfiguration.kt
@@ -0,0 +1,7 @@
+package sh.christian.ozone.api.generator
+
+import java.io.Serializable
+
+data class DefaultsConfiguration(
+  val generateUnknownsForSealedTypes: Boolean,
+) : Serializable
diff --git a/generator/src/main/kotlin/sh/christian/ozone/api/generator/LexiconApiGenerator.kt b/generator/src/main/kotlin/sh/christian/ozone/api/generator/LexiconApiGenerator.kt
index 5acd44a..b237cff 100644
--- a/generator/src/main/kotlin/sh/christian/ozone/api/generator/LexiconApiGenerator.kt
+++ b/generator/src/main/kotlin/sh/christian/ozone/api/generator/LexiconApiGenerator.kt
@@ -6,6 +6,7 @@ import com.squareup.kotlinpoet.CodeBlock
 import com.squareup.kotlinpoet.FileSpec
 import com.squareup.kotlinpoet.FunSpec
 import com.squareup.kotlinpoet.KModifier
+import com.squareup.kotlinpoet.MemberName
 import com.squareup.kotlinpoet.ParameterSpec
 import com.squareup.kotlinpoet.ParameterizedTypeName.Companion.parameterizedBy
 import com.squareup.kotlinpoet.PropertySpec
@@ -219,7 +220,16 @@ class LexiconApiGenerator(
         PropertySpec
           .builder("client", TypeNames.HttpClient)
           .addModifiers(KModifier.PRIVATE)
-          .initializer(CodeBlock.of("httpClient.%M()", withXrpcConfiguration))
+          .initializer(
+            buildCodeBlock {
+              if (environment.defaults.generateUnknownsForSealedTypes) {
+                val moduleMemberName = MemberName(configuration.namespace, "XrpcSerializersModule")
+                add("httpClient.%M(%M)", withXrpcConfiguration, moduleMemberName)
+              } else {
+                add("httpClient.%M()", withXrpcConfiguration)
+              }
+            }
+          )
           .build()
       )
       .apply {
diff --git a/generator/src/main/kotlin/sh/christian/ozone/api/generator/LexiconClassFileCreator.kt b/generator/src/main/kotlin/sh/christian/ozone/api/generator/LexiconClassFileCreator.kt
index a464618..e8bec90 100644
--- a/generator/src/main/kotlin/sh/christian/ozone/api/generator/LexiconClassFileCreator.kt
+++ b/generator/src/main/kotlin/sh/christian/ozone/api/generator/LexiconClassFileCreator.kt
@@ -1,12 +1,17 @@
 package sh.christian.ozone.api.generator
 
 import com.squareup.kotlinpoet.AnnotationSpec
+import com.squareup.kotlinpoet.ClassName
 import com.squareup.kotlinpoet.CodeBlock
 import com.squareup.kotlinpoet.FileSpec
 import com.squareup.kotlinpoet.FunSpec
 import com.squareup.kotlinpoet.KModifier
+import com.squareup.kotlinpoet.MemberName
 import com.squareup.kotlinpoet.ParameterizedTypeName.Companion.parameterizedBy
+import com.squareup.kotlinpoet.PropertySpec
+import com.squareup.kotlinpoet.TypeSpec.Kind.INTERFACE
 import com.squareup.kotlinpoet.TypeVariableName
+import com.squareup.kotlinpoet.buildCodeBlock
 import com.squareup.kotlinpoet.withIndent
 import sh.christian.ozone.api.generator.builder.EnumClass
 import sh.christian.ozone.api.generator.builder.EnumEntry
@@ -31,6 +36,8 @@ class LexiconClassFileCreator(
 
   private val sealedRelationships = mutableListOf<SealedRelationship>()
 
+  private val unionTypes = mutableListOf<ClassName>()
+
   fun createClassForLexicon(document: LexiconDocument) {
     val enums = mutableMapOf<EnumClass, MutableSet<EnumEntry>>()
 
@@ -49,6 +56,14 @@ class LexiconClassFileCreator(
           context.typeAliases().forEach { addTypeAlias(it) }
 
           sealedRelationships += context.sealedRelationships()
+
+          if (environment.defaults.generateUnknownsForSealedTypes) {
+            unionTypes += context.types().filter { type ->
+              type.kind == INTERFACE &&
+                  type.name!!.endsWith("Union") &&
+                  type.typeSpecs.any { it.name == "Unknown" }
+            }.map { ClassName(context.authority, it.name!!) }
+          }
         }
         .addAnnotation(
           AnnotationSpec.builder(TypeNames.Suppress)
@@ -130,4 +145,29 @@ class LexiconClassFileCreator(
       .build()
       .writeTo(environment.outputDirectory)
   }
+
+  fun generateSerializerModule(namespace: String) {
+    if (unionTypes.isEmpty()) return
+
+    val xrpcSerializersModuleMemberName = MemberName(namespace, "XrpcSerializersModule")
+
+    FileSpec.builder(xrpcSerializersModuleMemberName)
+      .addProperty(
+        PropertySpec.builder(xrpcSerializersModuleMemberName.simpleName, TypeNames.SerializersModule)
+          .initializer(
+            buildCodeBlock {
+              beginControlFlow("SerializersModule {")
+              unionTypes.forEach { unionType ->
+                beginControlFlow("%M(%T::class) {", polymorphic, unionType)
+                addStatement("defaultDeserializer { %T.Unknown.serializer() }", unionType)
+                endControlFlow()
+              }
+              endControlFlow()
+            }
+          )
+          .build()
+      )
+      .build()
+      .writeTo(environment.outputDirectory)
+  }
 }
diff --git a/generator/src/main/kotlin/sh/christian/ozone/api/generator/LexiconProcessingEnvironment.kt b/generator/src/main/kotlin/sh/christian/ozone/api/generator/LexiconProcessingEnvironment.kt
index 8289cb6..e970c3a 100644
--- a/generator/src/main/kotlin/sh/christian/ozone/api/generator/LexiconProcessingEnvironment.kt
+++ b/generator/src/main/kotlin/sh/christian/ozone/api/generator/LexiconProcessingEnvironment.kt
@@ -10,6 +10,7 @@ import java.io.File
 
 class LexiconProcessingEnvironment(
   allLexiconSchemaJsons: List<String>,
+  val defaults: DefaultsConfiguration,
   val outputDirectory: File,
 ) : Iterable<String> {
   private val schemasById: Map<String, String>
diff --git a/generator/src/main/kotlin/sh/christian/ozone/api/generator/TypeNames.kt b/generator/src/main/kotlin/sh/christian/ozone/api/generator/TypeNames.kt
index 0b95cfc..c592ae3 100644
--- a/generator/src/main/kotlin/sh/christian/ozone/api/generator/TypeNames.kt
+++ b/generator/src/main/kotlin/sh/christian/ozone/api/generator/TypeNames.kt
@@ -26,6 +26,7 @@ object TypeNames {
   val RKey by classOfPackage("sh.christian.ozone.api")
   val SerialName by classOfPackage("kotlinx.serialization")
   val Serializable by classOfPackage("kotlinx.serialization")
+  val SerializersModule by classOfPackage("kotlinx.serialization.modules")
   val Suppress by classOfPackage("kotlin")
   val Tid by classOfPackage("sh.christian.ozone.api")
   val Timestamp by classOfPackage("sh.christian.ozone.api.model")
diff --git a/generator/src/main/kotlin/sh/christian/ozone/api/generator/builder/LexiconDataClassesGenerator.kt b/generator/src/main/kotlin/sh/christian/ozone/api/generator/builder/LexiconDataClassesGenerator.kt
index b5899e8..1ffbd7c 100644
--- a/generator/src/main/kotlin/sh/christian/ozone/api/generator/builder/LexiconDataClassesGenerator.kt
+++ b/generator/src/main/kotlin/sh/christian/ozone/api/generator/builder/LexiconDataClassesGenerator.kt
@@ -357,6 +357,19 @@ class LexiconDataClassesGenerator(
       }
     }
 
+    if (environment.defaults.generateUnknownsForSealedTypes) {
+      sealedInterface.addTypes(
+        createValueClass(
+          className = name.nestedClass("Unknown"),
+          innerType = TypeNames.JsonContent,
+          serialName = null,
+          additionalConfiguration = {
+            addSuperinterface(name)
+          }
+        ),
+      )
+    }
+
     context.addType(sealedInterface.build())
 
     return name
diff --git a/generator/src/main/kotlin/sh/christian/ozone/api/generator/builder/util.kt b/generator/src/main/kotlin/sh/christian/ozone/api/generator/builder/util.kt
index 87c732d..9f085e4 100644
--- a/generator/src/main/kotlin/sh/christian/ozone/api/generator/builder/util.kt
+++ b/generator/src/main/kotlin/sh/christian/ozone/api/generator/builder/util.kt
@@ -145,45 +145,53 @@ fun createDataClass(
 
 fun createValueClass(
   className: ClassName,
-  serialName: String,
+  serialName: String?,
   innerType: TypeName,
   additionalConfiguration: TypeSpec.Builder.() -> Unit = {},
 ): List<TypeSpec> {
   val serializerClassName = className.peerClass(className.simpleName + "Serializer")
-  val serializerTypeSpec = TypeSpec.classBuilder(serializerClassName)
-    .addSuperinterface(
-      TypeNames.KSerializer.parameterizedBy(className),
-      CodeBlock.of(
-        """
-        %M(
-        ⇥serialName = %S,⇤
-        ⇥constructor = ::%T,⇤
-        ⇥valueProvider = %T::value,⇤
-        ⇥valueSerializerProvider = { %T.serializer() },⇤
+  val serializerTypeSpec = serialName?.let {
+    TypeSpec.classBuilder(serializerClassName)
+      .addSuperinterface(
+        TypeNames.KSerializer.parameterizedBy(className),
+        CodeBlock.of(
+          """
+          %M(
+          ⇥serialName = %S,⇤
+          ⇥constructor = ::%T,⇤
+          ⇥valueProvider = %T::value,⇤
+          ⇥valueSerializerProvider = { %T.serializer() },⇤
+          )
+          """.trimIndent(),
+          valueClassSerializer,
+          serialName,
+          className,
+          className,
+          innerType,
         )
-        """.trimIndent(),
-        valueClassSerializer,
-        serialName,
-        className,
-        className,
-        innerType,
       )
-    )
-    .build()
+      .build()
+  }
 
   val valueClassTypeSpec = TypeSpec.classBuilder(className)
-    .addAnnotation(
-      AnnotationSpec.builder(TypeNames.Serializable)
-        .addMember("with = %T::class", serializerClassName)
-        .build()
-    )
     .addModifiers(KModifier.VALUE)
     .addAnnotation(JvmInline::class)
-    .addAnnotation(
-      AnnotationSpec.builder(TypeNames.SerialName)
-        .addMember("%S", serialName)
-        .build()
-    )
+    .apply {
+      if (serialName == null) {
+        addAnnotation(TypeNames.Serializable)
+      } else {
+        addAnnotation(
+          AnnotationSpec.builder(TypeNames.Serializable)
+            .addMember("with = %T::class", serializerClassName)
+            .build()
+        )
+        addAnnotation(
+          AnnotationSpec.builder(TypeNames.SerialName)
+            .addMember("%S", serialName)
+            .build()
+        )
+      }
+    }
     .primaryConstructor(
       FunSpec.constructorBuilder()
         .addParameter(
@@ -202,7 +210,7 @@ fun createValueClass(
     .apply(additionalConfiguration)
     .build()
 
-  return listOf(serializerTypeSpec, valueClassTypeSpec)
+  return listOfNotNull(serializerTypeSpec, valueClassTypeSpec)
 }
 
 fun createObjectClass(
diff --git a/generator/src/main/kotlin/sh/christian/ozone/api/generator/memberNames.kt b/generator/src/main/kotlin/sh/christian/ozone/api/generator/memberNames.kt
index dbaf70b..4dde0b2 100644
--- a/generator/src/main/kotlin/sh/christian/ozone/api/generator/memberNames.kt
+++ b/generator/src/main/kotlin/sh/christian/ozone/api/generator/memberNames.kt
@@ -9,6 +9,8 @@ val findSubscriptionSerializer by memberOfPackage("sh.christian.ozone.api.xrpc")
 
 val persistentListOf by memberOfPackage("kotlinx.collections.immutable")
 
+val polymorphic by extensionMemberOfPackage("kotlinx.serialization.modules")
+
 val procedure by extensionMemberOfPackage("sh.christian.ozone.api.xrpc")
 
 val query by extensionMemberOfPackage("sh.christian.ozone.api.xrpc")
diff --git a/generator/src/main/kotlin/sh/christian/ozone/api/gradle/LexiconGeneratorExtension.kt b/generator/src/main/kotlin/sh/christian/ozone/api/gradle/LexiconGeneratorExtension.kt
index abbd029..a09e8fb 100644
--- a/generator/src/main/kotlin/sh/christian/ozone/api/gradle/LexiconGeneratorExtension.kt
+++ b/generator/src/main/kotlin/sh/christian/ozone/api/gradle/LexiconGeneratorExtension.kt
@@ -11,6 +11,7 @@ import org.intellij.lang.annotations.Language
 import sh.christian.ozone.api.generator.ApiConfiguration
 import sh.christian.ozone.api.generator.ApiReturnType
 import sh.christian.ozone.api.generator.ApiReturnType.Raw
+import sh.christian.ozone.api.generator.DefaultsConfiguration
 import javax.inject.Inject
 
 abstract class LexiconGeneratorExtension
@@ -21,6 +22,11 @@ abstract class LexiconGeneratorExtension
   internal val apiConfigurations: ListProperty<ApiConfiguration> =
     objects.listProperty<ApiConfiguration>().convention(emptyList())
 
+  val namespace: Property<String> =
+    objects.property<String>().convention("sh.christian.ozone")
+
+  internal val defaults = GeneratorDefaults(objects)
+
   val outputDirectory: DirectoryProperty =
     objects.directoryProperty().convention(
       projectLayout.buildDirectory
@@ -28,6 +34,10 @@ abstract class LexiconGeneratorExtension
         .map { it.dir("lexicons") }
     )
 
+  fun defaults(configure: GeneratorDefaults.() -> Unit) {
+    defaults.configure()
+  }
+
   fun generateApi(
     name: String,
     configure: ApiGeneratorExtension.() -> Unit = {},
@@ -35,10 +45,23 @@ abstract class LexiconGeneratorExtension
     apiConfigurations.add(
       ApiGeneratorExtension(name, objects)
         .apply(configure)
-        .apiConfiguration
+        .buildApiConfiguration(namespace.readFinalizedValue())
     )
   }
 
+  class GeneratorDefaults internal constructor(
+    objects: ObjectFactory,
+  ) {
+    val generateUnknownsForSealedTypes: Property<Boolean> =
+      objects.property<Boolean>().convention(false)
+
+    internal fun buildDefaultsConfiguration(): DefaultsConfiguration {
+      return DefaultsConfiguration(
+        generateUnknownsForSealedTypes = generateUnknownsForSealedTypes.readFinalizedValue(),
+      )
+    }
+  }
+
   class ApiGeneratorExtension internal constructor(
     internal val name: String,
     objects: ObjectFactory,
@@ -73,8 +96,9 @@ abstract class LexiconGeneratorExtension
       implementationName.set(name)
     }
 
-    internal val apiConfiguration by lazy {
-      ApiConfiguration(
+    internal fun buildApiConfiguration(namespace: String): ApiConfiguration {
+      return ApiConfiguration(
+        namespace = namespace,
         packageName = packageName.readFinalizedValue(),
         interfaceName = name,
         implementationName = implementationName.readFinalizedValueOrNull(),
diff --git a/generator/src/main/kotlin/sh/christian/ozone/api/gradle/LexiconGeneratorPlugin.kt b/generator/src/main/kotlin/sh/christian/ozone/api/gradle/LexiconGeneratorPlugin.kt
index 2040eb8..e645f49 100644
--- a/generator/src/main/kotlin/sh/christian/ozone/api/gradle/LexiconGeneratorPlugin.kt
+++ b/generator/src/main/kotlin/sh/christian/ozone/api/gradle/LexiconGeneratorPlugin.kt
@@ -26,6 +26,8 @@ private fun Project.applyPlugin() {
 
   val generateLexicons = tasks.register<LexiconGeneratorTask>("generateLexicons") {
     schemasClasspath.from(configuration)
+    namespace.set(extension.namespace)
+    defaults.set(extension.defaults.buildDefaultsConfiguration())
     apiConfigurations.set(extension.apiConfigurations)
     outputDirectory.set(extension.outputDirectory)
   }
diff --git a/generator/src/main/kotlin/sh/christian/ozone/api/gradle/LexiconGeneratorTask.kt b/generator/src/main/kotlin/sh/christian/ozone/api/gradle/LexiconGeneratorTask.kt
index 481fb36..46112be 100644
--- a/generator/src/main/kotlin/sh/christian/ozone/api/gradle/LexiconGeneratorTask.kt
+++ b/generator/src/main/kotlin/sh/christian/ozone/api/gradle/LexiconGeneratorTask.kt
@@ -4,6 +4,7 @@ import org.gradle.api.DefaultTask
 import org.gradle.api.file.ConfigurableFileCollection
 import org.gradle.api.file.DirectoryProperty
 import org.gradle.api.provider.ListProperty
+import org.gradle.api.provider.Property
 import org.gradle.api.tasks.CacheableTask
 import org.gradle.api.tasks.Input
 import org.gradle.api.tasks.InputFiles
@@ -13,6 +14,7 @@ import org.gradle.api.tasks.PathSensitivity.RELATIVE
 import org.gradle.api.tasks.SkipWhenEmpty
 import org.gradle.api.tasks.TaskAction
 import sh.christian.ozone.api.generator.ApiConfiguration
+import sh.christian.ozone.api.generator.DefaultsConfiguration
 import sh.christian.ozone.api.generator.LexiconApiGenerator
 import sh.christian.ozone.api.generator.LexiconClassFileCreator
 import sh.christian.ozone.api.generator.LexiconProcessingEnvironment
@@ -31,6 +33,12 @@ abstract class LexiconGeneratorTask : DefaultTask() {
   @get:PathSensitive(RELATIVE)
   abstract val schemasClasspath: ConfigurableFileCollection
 
+  @get:Input
+  abstract val namespace: Property<String>
+
+  @get:Input
+  abstract val defaults: Property<DefaultsConfiguration>
+
   @get:Input
   abstract val apiConfigurations: ListProperty<ApiConfiguration>
 
@@ -48,6 +56,7 @@ abstract class LexiconGeneratorTask : DefaultTask() {
       allLexiconSchemaJsons = schemasClasspath.flatMap { inputFile ->
         inputFile.toPath().findJsonFiles()
       },
+      defaults = defaults.get(),
       outputDirectory = outputDir,
     )
 
@@ -65,6 +74,7 @@ abstract class LexiconGeneratorTask : DefaultTask() {
     }
 
     lexiconClassFileCreator.generateSealedRelationshipMapping()
+    lexiconClassFileCreator.generateSerializerModule(namespace.get())
     lexiconApiGenerator.generateApis()
   }
 
diff --git a/jetstream/build.gradle.kts b/jetstream/build.gradle.kts
index 7021640..7fae6d9 100644
--- a/jetstream/build.gradle.kts
+++ b/jetstream/build.gradle.kts
@@ -24,7 +24,7 @@ kotlin {
   sourceSets {
     val commonMain by getting {
       dependencies {
-        implementation(libs.bluesky)
+        api(libs.bluesky)
       }
     }
     val jvmMain by getting {