diff --git a/src/applications/typeahead/controller/PhabricatorTypeaheadFunctionHelpController.php b/src/applications/typeahead/controller/PhabricatorTypeaheadFunctionHelpController.php
index 3084b2d434..27b30a6278 100644
--- a/src/applications/typeahead/controller/PhabricatorTypeaheadFunctionHelpController.php
+++ b/src/applications/typeahead/controller/PhabricatorTypeaheadFunctionHelpController.php
@@ -19,9 +19,25 @@ final class PhabricatorTypeaheadFunctionHelpController
       return new Aphront404Response();
     }
 
-    $source = $sources[$class];
+    $raw_parameters = $request->getStr('parameters');
+    if ($raw_parameters) {
+      $parameters = phutil_json_decode($raw_parameters);
+    } else {
+      $parameters = array();
+    }
+
+    $source = id(clone $sources[$class])
+      ->setParameters($parameters);
+
+    // This can fail for some types of datasources (like the custom field proxy
+    // datasources) if the "parameters" are wrong. Just fail cleanly instead
+    // of fataling.
+    try {
+      $application_class = $source->getDatasourceApplicationClass();
+    } catch (Exception $ex) {
+      return new Aphront404Response();
+    }
 
-    $application_class = $source->getDatasourceApplicationClass();
     if ($application_class) {
       $result = id(new PhabricatorApplicationQuery())
         ->setViewer($this->getViewer())
diff --git a/src/applications/typeahead/controller/PhabricatorTypeaheadModularDatasourceController.php b/src/applications/typeahead/controller/PhabricatorTypeaheadModularDatasourceController.php
index 2f55c26384..e0ba9a763b 100644
--- a/src/applications/typeahead/controller/PhabricatorTypeaheadModularDatasourceController.php
+++ b/src/applications/typeahead/controller/PhabricatorTypeaheadModularDatasourceController.php
@@ -226,6 +226,12 @@ final class PhabricatorTypeaheadModularDatasourceController
         if ($source->getAllDatasourceFunctions()) {
           $reference_uri = '/typeahead/help/'.get_class($source).'/';
 
+          $parameters = $source->getParameters();
+          if ($parameters) {
+            $reference_uri = (string)id(new PhutilURI($reference_uri))
+              ->setQueryParam('parameters', phutil_json_encode($parameters));
+          }
+
           $reference_link = phutil_tag(
             'a',
             array(
diff --git a/src/infrastructure/customfield/datasource/PhabricatorCustomFieldApplicationSearchAnyFunctionDatasource.php b/src/infrastructure/customfield/datasource/PhabricatorCustomFieldApplicationSearchAnyFunctionDatasource.php
index 9560620db4..31f189988c 100644
--- a/src/infrastructure/customfield/datasource/PhabricatorCustomFieldApplicationSearchAnyFunctionDatasource.php
+++ b/src/infrastructure/customfield/datasource/PhabricatorCustomFieldApplicationSearchAnyFunctionDatasource.php
@@ -22,7 +22,7 @@ final class PhabricatorCustomFieldApplicationSearchAnyFunctionDatasource
         'summary' => pht('Find results with any value.'),
         'description' => pht(
           "This function includes results which have any value. Use a query ".
-          "like this to find results with any value:\n\n%s".
+          "like this to find results with any value:\n\n%s",
           '> any()'),
       ),
     );