diff --git a/codegen/gql_code_builder/lib/serializer.dart b/codegen/gql_code_builder/lib/serializer.dart
index 12cc076d..cadb2432 100644
--- a/codegen/gql_code_builder/lib/serializer.dart
+++ b/codegen/gql_code_builder/lib/serializer.dart
@@ -1,42 +1,39 @@
 import "package:analyzer/dart/element/element.dart";
 import "package:code_builder/code_builder.dart";
 
-Expression withCustomSerializers(
-  Expression serializersExpression,
-  Set<Expression> customSerializers,
-) =>
-    customSerializers.fold(
-      serializersExpression,
-      (exp, serializer) => exp.cascade("add").call([serializer]),
-    );
-
 Library buildSerializerLibrary(
   Set<ClassElement> builtClasses,
   String partDirectiveUrl,
-  Set<Expression> additionalSerializers,
-) =>
+  Set<Expression> additionalSerializers, {
+  Expression? externalSerializers,
+}) =>
     Library(
       (b) => b
         ..directives.add(Directive.part(partDirectiveUrl))
         ..body.addAll([
-          withCustomSerializers(
-            declareFinal(
-              "_serializersBuilder",
-              type: refer(
-                  "SerializersBuilder", "package:built_value/serializer.dart"),
-            )
-                .assign(
-                  refer(r"_$serializers"),
-                )
-                .property("toBuilder")
-                .call([]),
-            additionalSerializers,
-          ).cascade("addPlugin").call([
-            refer(
-              "StandardJsonPlugin",
-              "package:built_value/standard_json_plugin.dart",
-            ).call([])
-          ]).statement,
+          declareFinal(
+            "_serializersBuilder",
+            type: refer(
+                "SerializersBuilder", "package:built_value/serializer.dart"),
+          )
+              .assign(
+                refer(r"_$serializers"),
+              )
+              .property("toBuilder")
+              .call([])
+              .withCustomSerializers(additionalSerializers)
+              .applyIf(
+                externalSerializers != null,
+                (expr) => expr.cascade("addAll").call([externalSerializers!]),
+              )
+              .cascade("addPlugin")
+              .call([
+                refer(
+                  "StandardJsonPlugin",
+                  "package:built_value/standard_json_plugin.dart",
+                ).call([])
+              ])
+              .statement,
           refer("@SerializersFor", "package:built_value/serializer.dart").call([
             literalList(
               builtClasses
@@ -55,3 +52,19 @@ Library buildSerializerLibrary(
               .call([]).statement,
         ]),
     );
+
+extension on Expression {
+  Expression applyIf(
+    bool condition,
+    Expression Function(Expression) wrap,
+  ) =>
+      condition ? wrap(this) : this;
+
+  Expression withCustomSerializers(
+    Set<Expression> customSerializers,
+  ) =>
+      customSerializers.fold(
+        this,
+        (exp, serializer) => exp.cascade("add").call([serializer]),
+      );
+}
diff --git a/codegen/gql_code_builder/pubspec.yaml b/codegen/gql_code_builder/pubspec.yaml
index 27f683c0..f4c68b61 100644
--- a/codegen/gql_code_builder/pubspec.yaml
+++ b/codegen/gql_code_builder/pubspec.yaml
@@ -1,5 +1,5 @@
 name: gql_code_builder
-version: 0.12.0
+version: 0.13.0
 description: Dart code builders taking *.graphql documents and SDL to build useful classes.
 repository: https://github.com/gql-dart/gql
 environment: