diff -u b/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
--- b/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -2456,16 +2456,21 @@
 /**
  * Returns the test prefix if this is an internal request from SimpleTest.
  *
+ * @param string $new_prefix
+ *   Internal use only. A new prefix to be stored.
+ *
  * @return
  *   Either the simpletest prefix (the string "simpletest" followed by any
  *   number of digits) or FALSE if the user agent does not contain a valid
  *   HMAC and timestamp.
  */
-function drupal_valid_test_ua() {
+function drupal_valid_test_ua($new_prefix = NULL) {
   global $drupal_hash_salt;
-  // No reason to reset this.
   static $test_prefix;
 
+  if (isset($new_prefix)) {
+    $test_prefix = $new_prefix;
+  }
   if (isset($test_prefix)) {
     return $test_prefix;
   }
diff -u b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
--- b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -122,11 +122,11 @@
   protected $verboseDirectory;
 
   /**
-   * The original value of $_SERVER['HTTP_USER_AGENT']
+   * The original database prefix when running inside Simpletest.
    *
    * @var string
    */
-  protected $originalUserAgent;
+  protected $originalPrefix;
 
   /**
    * Constructor for Test.
@@ -748,8 +748,8 @@
       $GLOBALS['config_directories'][$type]['path'] = 'simpletest/' . substr($this->databasePrefix, 10) . '/config_' . $type;
     }
     if (drupal_valid_test_ua()) {
-      $this->originalUserAgent = $_SERVER['HTTP_USER_AGENT'];
-      $_SERVER['HTTP_USER_AGENT'] = drupal_generate_test_ua($this->databasePrefix);
+      $this->originalPrefix = drupal_valid_test_ua();
+      drupal_valid_test_ua($this->databasePrefix);
     }
 
     // Reset and create a new service container.
@@ -838,7 +838,9 @@
     drupal_container($this->originalContainer);
     $language_interface = $this->originalLanguage;
     $GLOBALS['config_directories'] = $this->originalConfigDirectories;
-    $_SERVER['HTTP_USER_AGENT'] = $this->originalUserAgent;
+    if (isset($this->originalPrefix)) {
+      drupal_valid_test_ua($this->originalPrefix);
+    }
 
     // Restore original shutdown callbacks.
     $callbacks = &drupal_register_shutdown_function();