diff --git a/src/applications/search/engine/PhabricatorDatasourceEngine.php b/src/applications/search/engine/PhabricatorDatasourceEngine.php
--- a/src/applications/search/engine/PhabricatorDatasourceEngine.php
+++ b/src/applications/search/engine/PhabricatorDatasourceEngine.php
@@ -34,4 +34,22 @@
     return null;
   }
 
+  public function newDatasourcesForCompositeDatasource(
+    PhabricatorTypeaheadCompositeDatasource $datasource) {
+    $viewer = $this->getViewer();
+    $extensions = PhabricatorDatasourceEngineExtension::getAllExtensions();
+
+    $sources = array();
+    foreach ($extensions as $extension) {
+      $extension_sources = id(clone $extension)
+        ->setViewer($viewer)
+        ->newDatasourcesForCompositeDatasource($datasource);
+      foreach ($extension_sources as $extension_source) {
+        $sources[] = $extension_source;
+      }
+    }
+
+    return $sources;
+  }
+
 }
diff --git a/src/applications/search/engineextension/PhabricatorDatasourceEngineExtension.php b/src/applications/search/engineextension/PhabricatorDatasourceEngineExtension.php
--- a/src/applications/search/engineextension/PhabricatorDatasourceEngineExtension.php
+++ b/src/applications/search/engineextension/PhabricatorDatasourceEngineExtension.php
@@ -21,6 +21,11 @@
     return null;
   }
 
+  public function newDatasourcesForCompositeDatasource(
+    PhabricatorTypeaheadCompositeDatasource $datasource) {
+    return array();
+  }
+
   final public static function getAllExtensions() {
     return id(new PhutilClassMapQuery())
       ->setAncestorClass(__CLASS__)
diff --git a/src/applications/typeahead/datasource/PhabricatorTypeaheadCompositeDatasource.php b/src/applications/typeahead/datasource/PhabricatorTypeaheadCompositeDatasource.php
--- a/src/applications/typeahead/datasource/PhabricatorTypeaheadCompositeDatasource.php
+++ b/src/applications/typeahead/datasource/PhabricatorTypeaheadCompositeDatasource.php
@@ -223,8 +223,17 @@
 
   private function getUsableDatasources() {
     if ($this->usable === null) {
+      $viewer = $this->getViewer();
+
       $sources = $this->getComponentDatasources();
 
+      $extension_sources = id(new PhabricatorDatasourceEngine())
+        ->setViewer($viewer)
+        ->newDatasourcesForCompositeDatasource($this);
+      foreach ($extension_sources as $extension_source) {
+        $sources[] = $extension_source;
+      }
+
       $usable = array();
       foreach ($sources as $source) {
         $application_class = $source->getDatasourceApplicationClass();
@@ -239,7 +248,7 @@
           }
         }
 
-        $source->setViewer($this->getViewer());
+        $source->setViewer($viewer);
         $usable[] = $source;
       }
       $this->usable = $usable;