diff --git a/src/unit/engine/base/ArcanistBaseUnitTestEngine.php b/src/unit/engine/base/ArcanistBaseUnitTestEngine.php
index a8f5b41d..c3dee67d 100644
--- a/src/unit/engine/base/ArcanistBaseUnitTestEngine.php
+++ b/src/unit/engine/base/ArcanistBaseUnitTestEngine.php
@@ -26,6 +26,7 @@ abstract class ArcanistBaseUnitTestEngine {
   private $workingCopy;
   private $paths;
   private $arguments = array();
+  protected $diffID;
 
   final public function __construct() {
 
@@ -62,4 +63,12 @@ abstract class ArcanistBaseUnitTestEngine {
 
   abstract public function run();
 
+  /**
+   * This function is called after run() when the diff is created
+   * Child classes should override this function if they want to
+   * do more with the diff ID.
+   */
+  public function setDifferentialDiffID($id) {
+    $this->diffID = $id;
+  }
 }
diff --git a/src/workflow/diff/ArcanistDiffWorkflow.php b/src/workflow/diff/ArcanistDiffWorkflow.php
index b86ecc73..f21e5f67 100644
--- a/src/workflow/diff/ArcanistDiffWorkflow.php
+++ b/src/workflow/diff/ArcanistDiffWorkflow.php
@@ -27,6 +27,7 @@ class ArcanistDiffWorkflow extends ArcanistBaseWorkflow {
   private $unresolvedLint;
   private $unresolvedTests;
   private $diffID;
+  private $unitWorkflow;
 
   public function getCommandHelp() {
     return phutil_console_format(<<<EOTEXT
@@ -295,6 +296,10 @@ EOTEXT
       'differential.creatediff',
       $diff);
 
+    if ($this->unitWorkflow) {
+      $this->unitWorkflow->setDifferentialDiffID($diff_info['diffid']);
+    }
+
     if ($this->unresolvedLint) {
       $data = array();
       foreach ($this->unresolvedLint as $message) {
@@ -1107,8 +1112,8 @@ EOTEXT
       if ($repository_api instanceof ArcanistSubversionAPI) {
         $argv = array_merge($argv, array_keys($paths));
       }
-      $unit_workflow = $this->buildChildWorkflow('unit', $argv);
-      $unit_result   = $unit_workflow->run();
+      $this->unitWorkflow = $this->buildChildWorkflow('unit', $argv);
+      $unit_result = $this->unitWorkflow->run();
       switch ($unit_result) {
         case ArcanistUnitWorkflow::RESULT_OKAY:
           echo phutil_console_format(
@@ -1133,7 +1138,7 @@ EOTEXT
           break;
       }
 
-      $this->unresolvedTests = $unit_workflow->getUnresolvedTests();
+      $this->unresolvedTests = $this->unitWorkflow->getUnresolvedTests();
 
       return $unit_result;
     } catch (ArcanistNoEngineException $ex) {
diff --git a/src/workflow/unit/ArcanistUnitWorkflow.php b/src/workflow/unit/ArcanistUnitWorkflow.php
index c706ce9e..0a53487e 100644
--- a/src/workflow/unit/ArcanistUnitWorkflow.php
+++ b/src/workflow/unit/ArcanistUnitWorkflow.php
@@ -29,6 +29,7 @@ class ArcanistUnitWorkflow extends ArcanistBaseWorkflow {
   const RESULT_SKIP     = 3;
 
   private $unresolvedTests;
+  private $engine;
 
   public function getCommandHelp() {
     return phutil_console_format(<<<EOTEXT
@@ -92,12 +93,12 @@ EOTEXT
 
 
     PhutilSymbolLoader::loadClass($engine_class);
-    $engine = newv($engine_class, array());
-    $engine->setWorkingCopy($working_copy);
-    $engine->setPaths($paths);
-    $engine->setArguments($this->getPassthruArgumentsAsMap('unit'));
+    $this->engine = newv($engine_class, array());
+    $this->engine->setWorkingCopy($working_copy);
+    $this->engine->setPaths($paths);
+    $this->engine->setArguments($this->getPassthruArgumentsAsMap('unit'));
 
-    $results = $engine->run();
+    $results = $this->engine->run();
 
     $status_codes = array(
       ArcanistUnitTestResult::RESULT_PASS => phutil_console_format(
@@ -156,4 +157,10 @@ EOTEXT
     return $this->unresolvedTests;
   }
 
+  public function setDifferentialDiffID($id) {
+    if ($this->engine) {
+      $this->engine->setDifferentialDiffID($id);
+    }
+  }
+
 }