diff --git a/mod/assign/externallib.php b/mod/assign/externallib.php
index cd7be3042dd..7510f81cc59 100644
--- a/mod/assign/externallib.php
+++ b/mod/assign/externallib.php
@@ -1639,9 +1639,11 @@ class mod_assign_external extends external_api {
         $pluginsubmissionparams = array();
 
         foreach ($instance->get_submission_plugins() as $plugin) {
-            $pluginparams = $plugin->get_external_parameters();
-            if (!empty($pluginparams)) {
-                $pluginsubmissionparams = array_merge($pluginsubmissionparams, $pluginparams);
+            if ($plugin->is_visible()) {
+                $pluginparams = $plugin->get_external_parameters();
+                if (!empty($pluginparams)) {
+                    $pluginsubmissionparams = array_merge($pluginsubmissionparams, $pluginparams);
+                }
             }
         }
 
@@ -1716,9 +1718,11 @@ class mod_assign_external extends external_api {
         $pluginfeedbackparams = array();
 
         foreach ($instance->get_feedback_plugins() as $plugin) {
-            $pluginparams = $plugin->get_external_parameters();
-            if (!empty($pluginparams)) {
-                $pluginfeedbackparams = array_merge($pluginfeedbackparams, $pluginparams);
+            if ($plugin->is_visible()) {
+                $pluginparams = $plugin->get_external_parameters();
+                if (!empty($pluginparams)) {
+                    $pluginfeedbackparams = array_merge($pluginfeedbackparams, $pluginparams);
+                }
             }
         }
 
@@ -1856,9 +1860,11 @@ class mod_assign_external extends external_api {
         $pluginfeedbackparams = array();
 
         foreach ($instance->get_feedback_plugins() as $plugin) {
-            $pluginparams = $plugin->get_external_parameters();
-            if (!empty($pluginparams)) {
-                $pluginfeedbackparams = array_merge($pluginfeedbackparams, $pluginparams);
+            if ($plugin->is_visible()) {
+                $pluginparams = $plugin->get_external_parameters();
+                if (!empty($pluginparams)) {
+                    $pluginfeedbackparams = array_merge($pluginfeedbackparams, $pluginparams);
+                }
             }
         }
 
diff --git a/mod/assign/feedback/comments/locallib.php b/mod/assign/feedback/comments/locallib.php
index 6ba2770ab82..ab5a7c09aeb 100644
--- a/mod/assign/feedback/comments/locallib.php
+++ b/mod/assign/feedback/comments/locallib.php
@@ -513,7 +513,7 @@ class assign_feedback_comments extends assign_feedback_plugin {
     public function get_external_parameters() {
         $editorparams = array('text' => new external_value(PARAM_RAW, 'The text for this feedback.'),
                               'format' => new external_value(PARAM_INT, 'The format for this feedback'));
-        $editorstructure = new external_single_structure($editorparams);
+        $editorstructure = new external_single_structure($editorparams, 'Editor structure', VALUE_OPTIONAL);
         return array('assignfeedbackcomments_editor' => $editorstructure);
     }
 
diff --git a/mod/assign/feedback/file/locallib.php b/mod/assign/feedback/file/locallib.php
index 27d1b344e29..859701141d9 100644
--- a/mod/assign/feedback/file/locallib.php
+++ b/mod/assign/feedback/file/locallib.php
@@ -682,7 +682,8 @@ class assign_feedback_file extends assign_feedback_plugin {
         return array(
             'files_filemanager' => new external_value(
                 PARAM_INT,
-                'The id of a draft area containing files for this feedback.'
+                'The id of a draft area containing files for this feedback.',
+                VALUE_OPTIONAL
             )
         );
     }
diff --git a/mod/assign/submission/file/locallib.php b/mod/assign/submission/file/locallib.php
index 506eeaa7c0e..cfc4558e975 100644
--- a/mod/assign/submission/file/locallib.php
+++ b/mod/assign/submission/file/locallib.php
@@ -528,7 +528,8 @@ class assign_submission_file extends assign_submission_plugin {
         return array(
             'files_filemanager' => new external_value(
                 PARAM_INT,
-                'The id of a draft area containing files for this submission.'
+                'The id of a draft area containing files for this submission.',
+                VALUE_OPTIONAL
             )
         );
     }
diff --git a/mod/assign/submission/onlinetext/locallib.php b/mod/assign/submission/onlinetext/locallib.php
index ce31a25021d..063849014f1 100644
--- a/mod/assign/submission/onlinetext/locallib.php
+++ b/mod/assign/submission/onlinetext/locallib.php
@@ -618,7 +618,7 @@ class assign_submission_onlinetext extends assign_submission_plugin {
         $editorparams = array('text' => new external_value(PARAM_TEXT, 'The text for this submission.'),
                               'format' => new external_value(PARAM_INT, 'The format for this submission'),
                               'itemid' => new external_value(PARAM_INT, 'The draft area id for files attached to the submission'));
-        $editorstructure = new external_single_structure($editorparams);
+        $editorstructure = new external_single_structure($editorparams, 'Editor structure', VALUE_OPTIONAL);
         return array('onlinetext_editor' => $editorstructure);
     }
 
diff --git a/mod/assign/tests/externallib_test.php b/mod/assign/tests/externallib_test.php
index 78fa73e5565..07339c470a5 100644
--- a/mod/assign/tests/externallib_test.php
+++ b/mod/assign/tests/externallib_test.php
@@ -1557,4 +1557,47 @@ class mod_assign_external_testcase extends externallib_advanced_testcase {
 
     }
 
+    /**
+     * Test subplugins availability
+     */
+    public function test_subplugins_availability() {
+        global $CFG;
+
+        require_once($CFG->dirroot . '/mod/assign/adminlib.php');
+        $this->resetAfterTest(true);
+
+        // Hide assignment file submissiong plugin.
+        $pluginmanager = new assign_plugin_manager('assignsubmission');
+        $pluginmanager->hide_plugin('file');
+        $parameters = mod_assign_external::save_submission_parameters();
+
+        $this->assertTrue(!isset($parameters->keys['plugindata']->keys['files_filemanager']));
+
+        // Show it again and check that the value is returned as optional.
+        $pluginmanager->show_plugin('file');
+        $parameters = mod_assign_external::save_submission_parameters();
+        $this->assertTrue(isset($parameters->keys['plugindata']->keys['files_filemanager']));
+        $this->assertEquals(VALUE_OPTIONAL, $parameters->keys['plugindata']->keys['files_filemanager']->required);
+
+        // Hide feedback file submissiong plugin.
+        $pluginmanager = new assign_plugin_manager('assignfeedback');
+        $pluginmanager->hide_plugin('file');
+
+        $parameters = mod_assign_external::save_grade_parameters();
+
+        $this->assertTrue(!isset($parameters->keys['plugindata']->keys['files_filemanager']));
+
+        // Show it again and check that the value is returned as optional.
+        $pluginmanager->show_plugin('file');
+        $parameters = mod_assign_external::save_grade_parameters();
+
+        $this->assertTrue(isset($parameters->keys['plugindata']->keys['files_filemanager']));
+        $this->assertEquals(VALUE_OPTIONAL, $parameters->keys['plugindata']->keys['files_filemanager']->required);
+
+        // Check a different one.
+        $pluginmanager->show_plugin('comments');
+        $this->assertTrue(isset($parameters->keys['plugindata']->keys['assignfeedbackcomments_editor']));
+        $this->assertEquals(VALUE_OPTIONAL, $parameters->keys['plugindata']->keys['assignfeedbackcomments_editor']->required);
+    }
+
 }