diff --git a/dwds/lib/src/handlers/injected_handler.dart b/dwds/lib/src/handlers/injected_handler.dart
index fbe86bbd2..99164b016 100644
--- a/dwds/lib/src/handlers/injected_handler.dart
+++ b/dwds/lib/src/handlers/injected_handler.dart
@@ -24,8 +24,9 @@ const mainExtensionMarker = '/* MAIN_EXTENSION_MARKER */';
 const _clientScript = 'dwds/src/injected/client';
 
 Handler Function(Handler) createInjectedHandler(
-  ReloadConfiguration configuration,
-) =>
+        ReloadConfiguration configuration,
+        {String extensionHostname,
+        int extensionPort}) =>
     (innerHandler) {
       return (Request request) async {
         if (request.url.path == '$_clientScript.js') {
@@ -66,7 +67,9 @@ Handler Function(Handler) createInjectedHandler(
                 .replaceAll('(', '')
                 .replaceAll(')', '')
                 .trim();
-            body += _injectedClientJs(configuration, appId, mainFuntion);
+            body += _injectedClientJs(configuration, appId, mainFuntion,
+                extensionHostname: extensionHostname,
+                extensionPort: extensionPort);
             body += bodyLines.sublist(extensionIndex + 2).join('\n');
             // Change the hot restart handler to re-assign
             // `window.$dartRunMain` to the new main, instead of invoking it.
@@ -86,11 +89,20 @@ Handler Function(Handler) createInjectedHandler(
     };
 
 String _injectedClientJs(
-        ReloadConfiguration configuration, String appId, String mainFunction) =>
-    '''\n
-// Injected by webdev for build results support.
-window.\$dartAppId = "$appId";
-window.\$dartRunMain = $mainFunction;
-window.\$dartReloadConfiguration = "$configuration";
-window.\$dartLoader.forceLoadModule('$_clientScript');
-''';
+    ReloadConfiguration configuration, String appId, String mainFunction,
+    {String extensionHostname, int extensionPort}) {
+  var injectedBody = '''\n
+            // Injected by webdev for build results support.
+            window.\$dartAppId = "$appId";
+            window.\$dartRunMain = $mainFunction;
+            window.\$dartReloadConfiguration = "$configuration";
+            window.\$dartLoader.forceLoadModule('$_clientScript');
+            ''';
+  if (extensionPort != null && extensionHostname != null) {
+    injectedBody += '''
+      window.\$extensionHostname = "$extensionHostname";
+      window.\$extensionPort = "$extensionPort";
+      ''';
+  }
+  return injectedBody;
+}
diff --git a/dwds/test/handlers/injected_handler_test.dart b/dwds/test/handlers/injected_handler_test.dart
index 713d262c9..8f4e4fad5 100644
--- a/dwds/test/handlers/injected_handler_test.dart
+++ b/dwds/test/handlers/injected_handler_test.dart
@@ -16,7 +16,7 @@ void main() {
   const entryEtag = 'entry etag';
   const nonEntryEtag = 'some etag';
 
-  group('InjectedMiddelware', () {
+  group('InjectedHandlerWithoutExtension', () {
     setUp(() async {
       var pipeline = const Pipeline()
           .addMiddleware(createInjectedHandler(ReloadConfiguration.liveReload));
@@ -95,5 +95,41 @@ void main() {
           });
       expect(cachedResponse.statusCode, HttpStatus.notModified);
     });
+
+    test('Does not inject the extension backend port', () async {
+      var result = await http.get(
+          'http://localhost:${server.port}/entrypoint$bootstrapJsExtension');
+      expect(result.body.contains('extensionHostname'), isFalse);
+      expect(result.body.contains('extensionPort'), isFalse);
+    });
+  });
+
+  group('InjectedHandlerWithExtension', () {
+    setUp(() async {
+      var someExtensionHostname = 'localhost';
+      var someExtensionPort = 4000;
+      var pipeline = const Pipeline().addMiddleware(createInjectedHandler(
+          ReloadConfiguration.liveReload,
+          extensionHostname: someExtensionHostname,
+          extensionPort: someExtensionPort));
+      server = await shelf_io.serve(pipeline.addHandler((request) {
+        return Response.ok(
+            '$entrypointExtensionMarker\n'
+            '$mainExtensionMarker\n'
+            'app.main.main()',
+            headers: {HttpHeaders.etagHeader: entryEtag});
+      }), 'localhost', 0);
+    });
+
+    tearDown(() async {
+      await server.close();
+    });
+
+    test('Injects the extension backend port', () async {
+      var result = await http.get(
+          'http://localhost:${server.port}/entrypoint$bootstrapJsExtension');
+      expect(result.body.contains('extensionHostname'), isTrue);
+      expect(result.body.contains('extensionPort'), isTrue);
+    });
   });
 }