diff --git a/core/lib/Drupal/Core/EventSubscriber/FormAjaxSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/FormAjaxSubscriber.php
index 9bea727..2f10295 100644
--- a/core/lib/Drupal/Core/EventSubscriber/FormAjaxSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/FormAjaxSubscriber.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\Core\EventSubscriber;
 
-use Drupal\Core\Ajax\UpdateBuildIdCommand;
 use Drupal\Core\Form\FormAjaxException;
 use Drupal\Core\Form\FormAjaxResponseBuilderInterface;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
@@ -53,14 +52,11 @@ public function onException(GetResponseForExceptionEvent $event) {
       $request = $event->getRequest();
       $form = $exception->getForm();
       $form_state = $exception->getFormState();
-      $form_build_id = $request->get('form_build_id');
 
-      $commands = [];
-      if ($form_build_id !== $form['#build_id']) {
-        // If the form build ID has changed, issue an Ajax command to update it.
-        $commands[] = new UpdateBuildIdCommand($form_build_id, $form['#build_id']);
-      }
-      $response = $this->formAjaxResponseBuilder->buildResponse($request, $form, $form_state, $commands);
+      // Set the build ID from the request as the old build ID on the form.
+      $form['#build_id_old'] = $request->get('form_build_id');
+
+      $response = $this->formAjaxResponseBuilder->buildResponse($request, $form, $form_state, []);
       // Since this response is being set in place of an exception, explicitly
       // mark this as a 200 status.
       $response->headers->set('X-Status-Code', 200);
diff --git a/core/lib/Drupal/Core/Field/WidgetBase.php b/core/lib/Drupal/Core/Field/WidgetBase.php
index 1289596..cc13ae4 100644
--- a/core/lib/Drupal/Core/Field/WidgetBase.php
+++ b/core/lib/Drupal/Core/Field/WidgetBase.php
@@ -12,7 +12,6 @@
 use Drupal\Component\Utility\SortArray;
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Form\FormStateInterface;
-use Drupal\Core\Url;
 use Symfony\Component\Validator\ConstraintViolationInterface;
 use Symfony\Component\Validator\ConstraintViolationListInterface;
 
@@ -244,9 +243,6 @@ protected function formMultipleElements(FieldItemListInterface $items, array &$f
             'callback' => array(get_class($this), 'addMoreAjax'),
             'wrapper' => $wrapper_id,
             'effect' => 'fade',
-            // @todo Remove this.
-            'url' => Url::fromRoute('system.ajax'),
-            'cache_form' => TRUE,
           ),
         );
       }
diff --git a/core/lib/Drupal/Core/Form/FormAjaxResponseBuilder.php b/core/lib/Drupal/Core/Form/FormAjaxResponseBuilder.php
index c8b2940..4b826ff 100644
--- a/core/lib/Drupal/Core/Form/FormAjaxResponseBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormAjaxResponseBuilder.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\Form;
 
 use Drupal\Core\Ajax\AjaxResponse;
+use Drupal\Core\Ajax\UpdateBuildIdCommand;
 use Drupal\Core\Render\MainContent\MainContentRendererInterface;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Symfony\Component\HttpFoundation\Request;
@@ -52,6 +53,11 @@ public function __construct(MainContentRendererInterface $ajax_renderer, RouteMa
    * {@inheritdoc}
    */
   public function buildResponse(Request $request, array $form, FormStateInterface $form_state, array $commands) {
+    // If the form build ID has changed, issue an Ajax command to update it.
+    if (isset($form['#build_id_old']) && $form['#build_id_old'] !== $form['#build_id']) {
+      $commands[] = new UpdateBuildIdCommand($form['#build_id_old'], $form['#build_id']);
+    }
+
     // We need to return the part of the form (or some other content) that needs
     // to be re-rendered so the browser can update the page with changed
     // content. Since this is the generic menu callback used by many Ajax
diff --git a/core/lib/Drupal/Core/Form/FormBuilderInterface.php b/core/lib/Drupal/Core/Form/FormBuilderInterface.php
index 5f8b452..123c743 100644
--- a/core/lib/Drupal/Core/Form/FormBuilderInterface.php
+++ b/core/lib/Drupal/Core/Form/FormBuilderInterface.php
@@ -69,6 +69,14 @@ public function getForm($form_arg);
    *   The rendered form. This function may also perform a redirect and hence
    *   may not return at all depending upon the $form_state flags that were set.
    *
+   * @throws \Drupal\Core\Form\FormAjaxException
+   *   Thrown when a form is triggered via an AJAX submission. It will be
+   *   handled by \Drupal\Core\EventSubscriber\FormAjaxSubscriber.
+   * @throws \Drupal\Core\Form\EnforcedResponseException
+   *   Thrown when a form builder returns an exception directly, usually a
+   *   \Symfony\Component\HttpFoundation\RedirectResponse. It will be handled by
+   *   \Drupal\Core\EventSubscriber\EnforcedFormResponseSubscriber.
+   *
    * @see self::redirectForm()
    */
   public function buildForm($form_id, FormStateInterface &$form_state);
diff --git a/core/lib/Drupal/Core/Render/Element/RenderElement.php b/core/lib/Drupal/Core/Render/Element/RenderElement.php
index 0df954e..c5a2ae3 100644
--- a/core/lib/Drupal/Core/Render/Element/RenderElement.php
+++ b/core/lib/Drupal/Core/Render/Element/RenderElement.php
@@ -129,7 +129,6 @@ public static function preRenderGroup($element) {
    */
   public static function processAjaxForm(&$element, FormStateInterface $form_state, &$complete_form) {
     $element = static::preRenderAjaxForm($element);
-    // @todo Remove this.
     if (!empty($element['#ajax']['cache_form'])) {
       $form_state->setCached();
     }
diff --git a/core/modules/file/src/Element/ManagedFile.php b/core/modules/file/src/Element/ManagedFile.php
index 9719ea9..d6f7419 100644
--- a/core/modules/file/src/Element/ManagedFile.php
+++ b/core/modules/file/src/Element/ManagedFile.php
@@ -146,7 +146,7 @@ public static function processManagedFile(&$element, FormStateInterface $form_st
 
     $ajax_settings = [
       'url' => Url::fromRoute('file.ajax_upload'),
-      // @todo Remove this.
+      // @todo Remove this in https://www.drupal.org/node/2500527.
       'cache_form' => TRUE,
       'options' => [
         'query' => [
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index f296702..721fdd9 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -1504,8 +1504,6 @@ protected function drupalGetAjax($path, array $options = array(), array $headers
    *   is done by drupalPostAjaxForm(). This string is literally appended to the
    *   POST data, so it must already be urlencoded and contain a leading "&"
    *   (e.g., "&extra_var1=hello+world&extra_var2=you%26me").
-   *
-   * @return bool|string|null
    */
   protected function drupalPostForm($path, $edit, $submit, array $options = array(), array $headers = array(), $form_html_id = NULL, $extra_post = NULL) {
     $submit_matches = FALSE;
@@ -2624,11 +2622,7 @@ protected function prepareRequestForGenerator($clean_urls = TRUE, $override_serv
    */
   protected function buildUrl($path, array $options = array()) {
     if ($path instanceof Url) {
-      $absolute_path = clone $path;
-      return $absolute_path
-        ->setOptions($options)
-        ->setAbsolute()
-        ->toString();
+      return $path->setAbsolute()->toString();
     }
     // The URL generator service is not necessarily available yet; e.g., in
     // interactive installer tests.
diff --git a/core/modules/system/src/Controller/FormAjaxController.php b/core/modules/system/src/Controller/FormAjaxController.php
index 9f6ca98..edef325 100644
--- a/core/modules/system/src/Controller/FormAjaxController.php
+++ b/core/modules/system/src/Controller/FormAjaxController.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\system\Controller;
 
-use Drupal\Core\Ajax\UpdateBuildIdCommand;
 use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
 use Drupal\Core\Form\FormAjaxResponseBuilderInterface;
 use Drupal\Core\Form\FormState;
@@ -129,17 +128,6 @@ protected function getForm(Request $request) {
       throw new BadRequestHttpException();
     }
 
-    // When a page level cache is enabled, the form-build id might have been
-    // replaced from within \Drupal::formBuilder()->getCache(). If this is the
-    // case, it is also necessary to update it in the browser by issuing an
-    // appropriate Ajax command.
-    $commands = [];
-    if (isset($form['#build_id_old']) && $form['#build_id_old'] != $form['#build_id']) {
-      // If the form build ID has changed, issue an Ajax command to update it.
-      $commands[] = new UpdateBuildIdCommand($form['#build_id_old'], $form['#build_id']);
-      $form_build_id = $form['#build_id'];
-    }
-
     // Since some of the submit handlers are run, redirects need to be disabled.
     $form_state->disableRedirect();
 
@@ -156,7 +144,7 @@ protected function getForm(Request $request) {
     $form_state->setUserInput($request->request->all());
     $form_id = $form['#form_id'];
 
-    return new FileAjaxForm($form, $form_state, $form_id, $form_build_id, $commands);
+    return new FileAjaxForm($form, $form_state, $form_id, $form['#build_id'], $commands);
   }
 
 }
diff --git a/core/modules/system/src/Tests/Ajax/MultiFormTest.php b/core/modules/system/src/Tests/Ajax/MultiFormTest.php
index 449861c..860deca 100644
--- a/core/modules/system/src/Tests/Ajax/MultiFormTest.php
+++ b/core/modules/system/src/Tests/Ajax/MultiFormTest.php
@@ -81,7 +81,7 @@ function testMultiForm() {
     // page update, ensure the same as above.
     foreach ($field_xpaths as $form_html_id => $field_xpath) {
       for ($i = 0; $i < 2; $i++) {
-        $this->drupalPostAjaxForm(NULL, array(), array($button_name => $button_value), 'system/ajax', array(), array(), $form_html_id);
+        $this->drupalPostAjaxForm(NULL, array(), array($button_name => $button_value), NULL, array(), array(), $form_html_id);
         $this->assert(count($this->xpath($field_xpath . $field_items_xpath_suffix)) == $i+2, 'Found the correct number of field items after an AJAX submission.');
         $this->assertFieldByXPath($field_xpath . $button_xpath_suffix, NULL, 'Found the "add more" button after an AJAX submission.');
         $this->assertNoDuplicateIds(t('Updated page contains unique IDs'), 'Other');
diff --git a/core/modules/system/src/Tests/Form/RebuildTest.php b/core/modules/system/src/Tests/Form/RebuildTest.php
index e993b49..53de2f2 100644
--- a/core/modules/system/src/Tests/Form/RebuildTest.php
+++ b/core/modules/system/src/Tests/Form/RebuildTest.php
@@ -96,7 +96,7 @@ function testPreserveFormActionAfterAJAX() {
     // submission and verify it worked by ensuring the updated page has two text
     // field items in the field for which we just added an item.
     $this->drupalGet('node/add/page');
-    $this->drupalPostAjaxForm(NULL, array(), array('field_ajax_test_add_more' => t('Add another item')), 'system/ajax', array(), array(), 'node-page-form');
+    $this->drupalPostAjaxForm(NULL, array(), array('field_ajax_test_add_more' => t('Add another item')), NULL, array(), array(), 'node-page-form');
     $this->assert(count($this->xpath('//div[contains(@class, "field-name-field-ajax-test")]//input[@type="text"]')) == 2, 'AJAX submission succeeded.');
 
     // Submit the form with the non-Ajax "Save" button, leaving the title field
@@ -113,6 +113,9 @@ function testPreserveFormActionAfterAJAX() {
 
     // Ensure that the form's action is correct.
     $forms = $this->xpath('//form[contains(@class, "node-page-form")]');
-    $this->assert(count($forms) == 1 && $forms[0]['action'] == Url::fromRoute('node.add', ['node_type' => 'page'])->toString(), 'Re-rendered form contains the correct action value.');
+    $this->assertEqual(1, count($forms));
+    // Strip query params off the action before asserting.
+    $url = parse_url($forms[0]['action'])['path'];
+    $this->assertEqual(Url::fromRoute('node.add', ['node_type' => 'page'])->toString(), $url);
   }
 }
diff --git a/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestSimpleForm.php b/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestSimpleForm.php
index 73d5b90..5289bcd 100644
--- a/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestSimpleForm.php
+++ b/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestSimpleForm.php
@@ -71,9 +71,9 @@ public function buildForm(array $form, FormStateInterface $form_state) {
         '#options' => array('red' => 'red'),
         '#ajax' => array(
           'callback' => $value,
-          // @todo Remove this.
-          'url' => Url::fromRoute('system.ajax'),
-          'cache_form' => TRUE,
+          // If the callback is NULL, the form will not post to the original URL
+          // and will expect the form to be cached.
+          'cache_form' => $value === NULL,
         ),
       );
     }
diff --git a/core/modules/views_ui/admin.inc b/core/modules/views_ui/admin.inc
index b5cbda5..9cc4af8 100644
--- a/core/modules/views_ui/admin.inc
+++ b/core/modules/views_ui/admin.inc
@@ -53,7 +53,7 @@ function views_ui_add_ajax_trigger(&$wrapping_element, $trigger_key, $refresh_pa
   $triggering_element['#ajax']['callback'] = 'views_ui_ajax_update_form';
 
   // Specify the #ajax URL in order to retain form caching.
-  // @todo Remove this.
+  // @todo Remove this in https://www.drupal.org/node/2500523.
   $triggering_element['#ajax']['url'] = Url::fromRoute('system.ajax');
   $triggering_element['#ajax']['cache_form'] = TRUE;
 
diff --git a/core/tests/Drupal/Tests/Core/EventSubscriber/FormAjaxSubscriberTest.php b/core/tests/Drupal/Tests/Core/EventSubscriber/FormAjaxSubscriberTest.php
index 14ce121..21a1188 100644
--- a/core/tests/Drupal/Tests/Core/EventSubscriber/FormAjaxSubscriberTest.php
+++ b/core/tests/Drupal/Tests/Core/EventSubscriber/FormAjaxSubscriberTest.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\Tests\Core\EventSubscriber;
 
-use Drupal\Core\Ajax\UpdateBuildIdCommand;
 use Drupal\Core\EventSubscriber\FormAjaxSubscriber;
 use Drupal\Core\Form\FormAjaxException;
 use Drupal\Core\Form\FormState;
@@ -54,6 +53,9 @@ protected function setUp() {
    */
   public function testOnException() {
     $form = ['#type' => 'form', '#build_id' => 'the_build_id'];
+    $expected_form = $form + [
+      '#build_id_old' => 'the_build_id',
+    ];
     $form_state = new FormState();
     $exception = new FormAjaxException($form, $form_state);
 
@@ -63,7 +65,7 @@ public function testOnException() {
 
     $this->formAjaxResponseBuilder->expects($this->once())
       ->method('buildResponse')
-      ->with($request, $form, $form_state, $commands)
+      ->with($request, $expected_form, $form_state, $commands)
       ->willReturn($response);
 
     $event = new GetResponseForExceptionEvent($this->httpKernel, $request, HttpKernelInterface::MASTER_REQUEST, $exception);
@@ -78,17 +80,19 @@ public function testOnException() {
    */
   public function testOnExceptionNewBuildId() {
     $form = ['#type' => 'form', '#build_id' => 'the_build_id'];
+    $expected_form = $form + [
+      '#build_id_old' => 'a_new_build_id',
+    ];
     $form_state = new FormState();
     $exception = new FormAjaxException($form, $form_state);
 
     $request = new Request([], ['form_build_id' => 'a_new_build_id']);
     $commands = [];
-    $commands[] = new UpdateBuildIdCommand('a_new_build_id', 'the_build_id');
     $response = new Response('');
 
     $this->formAjaxResponseBuilder->expects($this->once())
       ->method('buildResponse')
-      ->with($request, $form, $form_state, $commands)
+      ->with($request, $expected_form, $form_state, $commands)
       ->willReturn($response);
 
 
diff --git a/core/tests/Drupal/Tests/Core/Form/FormAjaxResponseBuilderTest.php b/core/tests/Drupal/Tests/Core/Form/FormAjaxResponseBuilderTest.php
index ba09680..0698e28 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormAjaxResponseBuilderTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormAjaxResponseBuilderTest.php
@@ -173,4 +173,40 @@ public function testBuildResponseWithCommands() {
     $this->assertSame($commands_expected, $result->getCommands());
   }
 
+  /**
+   * @covers ::buildResponse
+   */
+  public function testBuildResponseWithUpdateCommand() {
+    $triggering_element = [
+      '#ajax' => [
+        'callback' => function (array $form, FormStateInterface $form_state) {
+          return new AjaxResponse([]);
+        }
+      ],
+    ];
+    $request = new Request();
+    $form = [
+      '#build_id' => 'the_build_id',
+      '#build_id_old' => 'a_new_build_id',
+      'test' => [
+        '#type' => 'textfield',
+      ],
+    ];
+    $form_state = new FormState();
+    $form_state->setTriggeringElement($triggering_element);
+    $commands = [
+      new AlertCommand('alert!'),
+    ];
+    $commands_expected = [];
+    $commands_expected[] = ['command' => 'update_build_id', 'old' => 'a_new_build_id', 'new' => 'the_build_id'];
+    $commands_expected[] = ['command' => 'alert', 'text' => 'alert!'];
+
+    $this->renderer->expects($this->never())
+      ->method('renderResponse');
+
+    $result = $this->formAjaxResponseBuilder->buildResponse($request, $form, $form_state, $commands);
+    $this->assertInstanceOf('\Drupal\Core\Ajax\AjaxResponse', $result);
+    $this->assertSame($commands_expected, $result->getCommands());
+  }
+
 }