diff --git a/core/includes/menu.inc b/core/includes/menu.inc
index de98776..eedd3a2 100644
--- a/core/includes/menu.inc
+++ b/core/includes/menu.inc
@@ -1858,14 +1858,13 @@ function menu_navigation_links($menu_name, $level = 0) {
  *
  * @return
  *   An array containing
- *   - tabs: Local tasks for the requested level:
- *     - count: The number of local tasks.
- *     - output: The themed output of local tasks.
- *   - actions: Action links for the requested level:
- *     - count: The number of action links.
- *     - output: The themed output of action links.
+ *   - tabs: Local tasks for the requested level.
+ *   - actions: Action links for the requested level.
  *   - root_path: The router path for the current page. If the current page is
  *     a default local task, then this corresponds to the parent tab.
+ *
+ * @see hook_menu_local_tasks()
+ * @see hook_menu_local_tasks_alter()
  */
 function menu_local_tasks($level = 0) {
   $data = &drupal_static(__FUNCTION__);
@@ -1957,7 +1956,7 @@ function menu_local_tasks($level = 0) {
             if ($link['href'] != current_path()) {
               $link['localized_options']['attributes']['class'][] = 'active';
             }
-            $tabs_current[] = array(
+            $tabs_current[$link['href']] = array(
               '#theme' => 'menu_local_task',
               '#link' => $link,
               '#active' => TRUE,
@@ -1971,7 +1970,7 @@ function menu_local_tasks($level = 0) {
             // MENU_IS_LOCAL_ACTION before checking.
             if (($item['type'] & MENU_IS_LOCAL_ACTION) == MENU_IS_LOCAL_ACTION) {
               // The item is an action, display it as such.
-              $actions_current[] = array(
+              $actions_current[$link['href']] = array(
                 '#theme' => 'menu_local_action',
                 '#link' => $link,
                 '#weight' => isset($link['weight']) ? $link['weight'] : NULL,
@@ -1980,7 +1979,7 @@ function menu_local_tasks($level = 0) {
             }
             else {
               // Otherwise, it's a normal tab.
-              $tabs_current[] = array(
+              $tabs_current[$link['href']] = array(
                 '#theme' => 'menu_local_task',
                 '#link' => $link,
                 '#weight' => isset($link['weight']) ? $link['weight'] : NULL,
@@ -2034,7 +2033,7 @@ function menu_local_tasks($level = 0) {
             if ($link['href'] != current_path()) {
               $link['localized_options']['attributes']['class'][] = 'active';
             }
-            $tabs_current[] = array(
+            $tabs_current[$link['href']] = array(
               '#theme' => 'menu_local_task',
               '#link' => $link,
               '#active' => TRUE,
@@ -2046,7 +2045,7 @@ function menu_local_tasks($level = 0) {
             }
           }
           else {
-            $tabs_current[] = array(
+            $tabs_current[$link['href']] = array(
               '#theme' => 'menu_local_task',
               '#link' => $link,
               '#weight' => isset($link['weight']) ? $link['weight'] : NULL,
@@ -2194,7 +2193,7 @@ function menu_contextual_links($module, $parent_path, $args) {
 function menu_primary_local_tasks() {
   $links = menu_local_tasks(0);
   // Do not display single tabs.
-  return count($links['tabs']) > 1 ? $links['tabs'] : '';
+  return count(element_get_visible_children($links['tabs'])) > 1 ? $links['tabs'] : '';
 }
 
 /**
@@ -2203,7 +2202,7 @@ function menu_primary_local_tasks() {
 function menu_secondary_local_tasks() {
   $links = menu_local_tasks(1);
   // Do not display single tabs.
-  return count($links['tabs']) > 1 ? $links['tabs'] : '';
+  return count(element_get_visible_children($links['tabs'])) > 1 ? $links['tabs'] : '';
 }
 
 /**
@@ -2226,11 +2225,12 @@ function menu_tab_root_path() {
  * Returns a renderable element for the primary and secondary tabs.
  */
 function menu_local_tabs() {
-  return array(
+  $build = array(
     '#theme' => 'menu_local_tasks',
     '#primary' => menu_primary_local_tasks(),
     '#secondary' => menu_secondary_local_tasks(),
   );
+  return !empty($build['#primary']) ? $build : array();
 }
 
 /**
diff --git a/core/modules/system/lib/Drupal/system/Tests/Menu/LocalTasksTest.php b/core/modules/system/lib/Drupal/system/Tests/Menu/LocalTasksTest.php
new file mode 100644
index 0000000..5f695c0
--- /dev/null
+++ b/core/modules/system/lib/Drupal/system/Tests/Menu/LocalTasksTest.php
@@ -0,0 +1,143 @@
+<?php
+
+/**
+ * @file
+ * Contains Drupal\system\Tests\Menu\LocalTasksTest.
+ */
+
+namespace Drupal\system\Tests\Menu;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests local tasks derived from router and added/altered via hooks.
+ */
+class LocalTasksTest extends WebTestBase {
+
+  public static $modules = array('menu_test');
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Local tasks',
+      'description' => 'Tests local tasks derived from router and added/altered via hooks.',
+      'group' => 'Menu',
+    );
+  }
+
+  /**
+   * Tests appearance of local tasks.
+   *
+   * @see menu_test_menu()
+   * @see menu_test_menu_local_tasks()
+   * @see menu_test_menu_local_tasks_alter()
+   */
+  function testLocalTasks() {
+    // Verify that there is no local tasks markup if none are defined in the
+    // router and no module adds any dynamically.
+    $this->drupalGet('menu-test/tasks/empty');
+    $this->assertNoRaw('tabs');
+    $this->drupalGet('menu-test/tasks/default');
+    $this->assertNoRaw('tabs');
+
+    // Verify that local tasks appear as defined in the router.
+    $this->drupalGet('menu-test/tasks/tasks');
+    $this->assertLocalTasks(array(
+      // MENU_DEFAULT_LOCAL_TASK is expected to get a default weight of -10
+      // (without having to define it manually), so it should appear first,
+      // despite that its label is "View".
+      'menu-test/tasks/tasks',
+      'menu-test/tasks/tasks/edit',
+      'menu-test/tasks/tasks/settings',
+    ));
+
+    // Enable addition of tasks in menu_test_menu_local_tasks().
+    config('menu_test.settings')->set('tasks.add', TRUE)->save();
+
+    // Verify that the added tasks appear even if there are no tasks normally.
+    $this->drupalGet('menu-test/tasks/empty');
+    $this->assertLocalTasks(array(
+      'task/foo',
+      'task/bar',
+    ));
+
+    // Verify that the default local task appears before the added tasks.
+    $this->drupalGet('menu-test/tasks/default');
+    $this->assertLocalTasks(array(
+      'menu-test/tasks/default',
+      'task/foo',
+      'task/bar',
+    ));
+
+    // Verify that the added tasks appear within normal tasks.
+    $this->drupalGet('menu-test/tasks/tasks');
+    $this->assertLocalTasks(array(
+      'menu-test/tasks/tasks',
+      // The Edit task defines no weight, which is expected to sort as 0.
+      'menu-test/tasks/tasks/edit',
+      'task/foo',
+      'task/bar',
+      'menu-test/tasks/tasks/settings',
+    ));
+
+    // Enable manipulation of tasks in menu_test_menu_local_tasks_alter().
+    config('menu_test.settings')->set('tasks.alter', TRUE)->save();
+
+    // Verify that the added tasks appear even if there are no tasks normally.
+    $this->drupalGet('menu-test/tasks/empty');
+    $this->assertLocalTasks(array(
+      'task/bar',
+      'task/foo',
+    ));
+    $this->assertNoText('Show it');
+    $this->assertText('Advanced settings');
+
+    // Verify that the default local task appears before the added tasks.
+    $this->drupalGet('menu-test/tasks/default');
+    $this->assertLocalTasks(array(
+      'menu-test/tasks/default',
+      'task/bar',
+      'task/foo',
+    ));
+    $this->assertText('Show it');
+    $this->assertText('Advanced settings');
+
+    // Verify that the added tasks appear within normal tasks.
+    $this->drupalGet('menu-test/tasks/tasks');
+    $this->assertLocalTasks(array(
+      'menu-test/tasks/tasks',
+      'menu-test/tasks/tasks/edit',
+      'task/bar',
+      'menu-test/tasks/tasks/settings',
+      'task/foo',
+    ));
+    $this->assertText('Show it');
+    $this->assertText('Advanced settings');
+  }
+
+  /**
+   * Asserts local tasks in the page output.
+   *
+   * @param array $hrefs
+   *   A list of expected link hrefs of local tasks to assert on the page (in
+   *   the given order).
+   * @param int $level
+   *   (optional) The local tasks level to assert; 0 for primary, 1 for
+   *   secondary. Defaults to 0.
+   */
+  protected function assertLocalTasks(array $hrefs, $level = 0) {
+    $elements = $this->xpath('//*[contains(@class, :class)]//a', array(
+      ':class' => $level == 0 ? 'tabs primary' : 'tabs secondary',
+    ));
+    $this->assertTrue(count($elements), 'Local tasks found.');
+    foreach ($hrefs as $index => $element) {
+      $expected = url($hrefs[$index]);
+      $method = ($elements[$index]['href'] == $expected ? 'pass' : 'fail');
+      $this->{$method}(format_string('Task @number href @value equals @expected.', array(
+        '@number' => $index + 1,
+        '@value' => (string) $elements[$index]['href'],
+        '@expected' => $expected,
+      )));
+    }
+  }
+
+}
diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index 5dd8534..f646c70 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -1054,10 +1054,10 @@ function hook_menu_link_delete($link) {
  *
  * @param array $data
  *   An associative array containing:
- *   - actions: A list of of actions, each one being an associative array
- *     as described above.
- *   - tabs: A list of (up to 2) tab levels that contain a list of of tabs, each
- *     one being an associative array as described above.
+ *   - actions: A list of of actions keyed by their href, each one being an
+ *     associative array as described above.
+ *   - tabs: A list of (up to 2) tab levels that contain a list of of tabs keyed
+ *     by their href, each one being an associative array as described above.
  * @param array $router_item
  *   The menu system router item of the page.
  * @param string $root_path
@@ -1065,7 +1065,7 @@ function hook_menu_link_delete($link) {
  */
 function hook_menu_local_tasks(&$data, $router_item, $root_path) {
   // Add an action linking to node/add to all pages.
-  $data['actions'][] = array(
+  $data['actions']['node/add'] = array(
     '#theme' => 'menu_local_action',
     '#link' => array(
       'title' => t('Add new content'),
@@ -1079,7 +1079,7 @@ function hook_menu_local_tasks(&$data, $router_item, $root_path) {
   );
 
   // Add a tab linking to node/add to all pages.
-  $data['tabs'][0][] = array(
+  $data['tabs'][0]['node/add'] = array(
     '#theme' => 'menu_local_task',
     '#link' => array(
       'title' => t('Example tab'),
@@ -1090,9 +1090,7 @@ function hook_menu_local_tasks(&$data, $router_item, $root_path) {
         ),
       ),
     ),
-    // Define whether this link is active. This can be omitted for
-    // implementations that add links to pages outside of the current page
-    // context.
+    // Define whether this link is active. This can usually be omitted.
     '#active' => ($router_item['path'] == $root_path),
   );
 }
diff --git a/core/modules/system/tests/modules/menu_test/menu_test.module b/core/modules/system/tests/modules/menu_test/menu_test.module
index b0bcdc5..1b0ada5 100644
--- a/core/modules/system/tests/modules/menu_test/menu_test.module
+++ b/core/modules/system/tests/modules/menu_test/menu_test.module
@@ -236,6 +236,38 @@ function menu_test_menu() {
     'type' => MENU_LOCAL_TASK,
   ) + $base;
 
+  // Menu local tasks tests.
+  // @see Drupal\system\Tests\Menu\LocalTasksTest
+  $items['menu-test/tasks'] = array(
+    'title' => 'Local tasks',
+  ) + $base;
+  $items['menu-test/tasks/empty'] = array(
+    'title' => 'Empty',
+  ) + $base;
+  $items['menu-test/tasks/default'] = array(
+    'title' => 'Default only',
+  ) + $base;
+  $items['menu-test/tasks/default/view'] = array(
+    'title' => 'View',
+    'type' => MENU_DEFAULT_LOCAL_TASK,
+  ) + $base;
+  $items['menu-test/tasks/tasks'] = array(
+    'title' => 'With tasks',
+  ) + $base;
+  $items['menu-test/tasks/tasks/view'] = array(
+    'title' => 'View',
+    'type' => MENU_DEFAULT_LOCAL_TASK,
+  ) + $base;
+  $items['menu-test/tasks/tasks/edit'] = array(
+    'title' => 'Edit',
+    'type' => MENU_LOCAL_TASK,
+  ) + $base;
+  $items['menu-test/tasks/tasks/settings'] = array(
+    'title' => 'Settings',
+    'type' => MENU_LOCAL_TASK,
+    'weight' => 100,
+  ) + $base;
+
   // Menu trail tests.
   // @see MenuTrailTestCase
   $items['menu-test/menu-trail'] = array(
@@ -364,6 +396,56 @@ function menu_test_menu() {
 }
 
 /**
+ * Implements hook_menu_local_tasks().
+ */
+function menu_test_menu_local_tasks(&$data, $router_item, $root_path) {
+  if (!config('menu_test.settings')->get('tasks.add')) {
+    return;
+  }
+  if (strpos($router_item['tab_root'], 'menu-test/tasks/') === 0) {
+    $data['tabs'][0]['foo'] = array(
+      '#theme' => 'menu_local_task',
+      '#link' => array(
+        'title' => 'Task 1',
+        'href' => 'task/foo',
+      ),
+      '#weight' => 10,
+    );
+    $data['tabs'][0]['bar'] = array(
+      '#theme' => 'menu_local_task',
+      '#link' => array(
+        'title' => 'Task 2',
+        'href' => 'task/bar',
+      ),
+      '#weight' => 20,
+    );
+  }
+}
+
+/**
+ * Implements hook_menu_local_tasks_alter().
+ */
+function menu_test_menu_local_tasks_alter(&$data, $router_item, $root_path) {
+  if (!config('menu_test.settings')->get('tasks.alter')) {
+    return;
+  }
+  if (strpos($router_item['tab_root'], 'menu-test/tasks/') === 0) {
+    // Rename the default local task from 'View' to 'Show'.
+    // $data['tabs'] is expected to be keyed by link hrefs.
+    // The default local task always links to its parent path, which means that
+    // if the tab root path appears as key in $data['tabs'], then that key is
+    // the default local task.
+    $key = $router_item['tab_root'];
+    if (isset($data['tabs'][0][$key])) {
+      $data['tabs'][0][$key]['#link']['title'] = 'Show it';
+    }
+    // Rename the 'foo' task to "Advanced settings" and put it last.
+    $data['tabs'][0]['foo']['#link']['title'] = 'Advanced settings';
+    $data['tabs'][0]['foo']['#weight'] = 110;
+  }
+}
+
+/**
  * Dummy argument loader for hook_menu() to point to.
  */
 function menu_test_argument_load($arg1) {
diff --git a/core/themes/seven/template.php b/core/themes/seven/template.php
index b92d064..4921748 100644
--- a/core/themes/seven/template.php
+++ b/core/themes/seven/template.php
@@ -34,7 +34,7 @@ function seven_preprocess_page(&$vars) {
   unset($vars['primary_local_tasks']['#secondary']);
   $vars['secondary_local_tasks'] = array(
     '#theme' => 'menu_local_tasks',
-    '#secondary' => $vars['tabs']['#secondary'],
+    '#secondary' => isset($vars['tabs']['#secondary']) ? $vars['tabs']['#secondary'] : '',
   );
 }