diff --git a/src/workflow/ArcanistWorkflow.php b/src/workflow/ArcanistWorkflow.php
--- a/src/workflow/ArcanistWorkflow.php
+++ b/src/workflow/ArcanistWorkflow.php
@@ -611,6 +611,10 @@
     return idx($this->arguments, $key, $default);
   }
 
+  final public function setArgument($key, $value) {
+    return $this->arguments[$key] = $value;
+  }
+
   final public function getPassedArguments() {
     return $this->passedArguments;
   }