diff --git a/src/applications/typeahead/controller/PhabricatorTypeaheadModularDatasourceController.php b/src/applications/typeahead/controller/PhabricatorTypeaheadModularDatasourceController.php
--- a/src/applications/typeahead/controller/PhabricatorTypeaheadModularDatasourceController.php
+++ b/src/applications/typeahead/controller/PhabricatorTypeaheadModularDatasourceController.php
@@ -35,7 +35,26 @@
 
     if (isset($sources[$class])) {
       $source = $sources[$class];
-      $source->setParameters($request->getRequestData());
+
+      $parameters = array();
+
+      $raw_parameters = $request->getStr('parameters');
+      if (strlen($raw_parameters)) {
+        try {
+          $parameters = phutil_json_decode($raw_parameters);
+        } catch (PhutilJSONParserException $ex) {
+          return $this->newDialog()
+            ->setTitle(pht('Invalid Parameters'))
+            ->appendParagraph(
+              pht(
+                'The HTTP parameter named "parameters" for this request is '.
+                'not a valid JSON parameter. JSON is required. Exception: %s',
+                $ex->getMessage()))
+            ->addCancelButton('/');
+        }
+      }
+
+      $source->setParameters($parameters);
       $source->setViewer($viewer);
 
       // NOTE: Wrapping the source in a Composite datasource ensures we perform
diff --git a/src/applications/typeahead/datasource/PhabricatorTypeaheadDatasource.php b/src/applications/typeahead/datasource/PhabricatorTypeaheadDatasource.php
--- a/src/applications/typeahead/datasource/PhabricatorTypeaheadDatasource.php
+++ b/src/applications/typeahead/datasource/PhabricatorTypeaheadDatasource.php
@@ -100,7 +100,7 @@
 
   public function getDatasourceURI() {
     $uri = new PhutilURI('/typeahead/class/'.get_class($this).'/');
-    $uri->setQueryParams($this->parameters);
+    $uri->setQueryParams($this->newURIParameters());
     return (string)$uri;
   }
 
@@ -110,10 +110,22 @@
     }
 
     $uri = new PhutilURI('/typeahead/browse/'.get_class($this).'/');
-    $uri->setQueryParams($this->parameters);
+    $uri->setQueryParams($this->newURIParameters());
     return (string)$uri;
   }
 
+  private function newURIParameters() {
+    if (!$this->parameters) {
+      return array();
+    }
+
+    $map = array(
+      'parameters' => phutil_json_encode($this->parameters),
+    );
+
+    return $map;
+  }
+
   abstract public function getPlaceholderText();
 
   public function getBrowseTitle() {