diff --git a/course/externallib.php b/course/externallib.php
index 7054241db69..ea687c6385e 100644
--- a/course/externallib.php
+++ b/course/externallib.php
@@ -2179,6 +2179,7 @@ class core_course_external extends external_api {
             'requiredcapabilities' => $requiredcapabilities
         );
         $params = self::validate_parameters(self::search_courses_parameters(), $parameters);
+        self::validate_context(context_system::instance());
 
         $allowedcriterianames = array('search', 'modulelist', 'blocklist', 'tagid');
         if (!in_array($params['criterianame'], $allowedcriterianames)) {
diff --git a/lib/ajax/service.php b/lib/ajax/service.php
index d3d23ab9d77..19a43b68762 100644
--- a/lib/ajax/service.php
+++ b/lib/ajax/service.php
@@ -41,6 +41,13 @@ if ($requests === null) {
 }
 $responses = array();
 
+// Defines the external settings required for Ajax processing.
+$settings = external_settings::get_instance();
+$settings->set_file('pluginfile.php');
+$settings->set_fileurl(true);
+$settings->set_filter(true);
+$settings->set_raw(false);
+
 foreach ($requests as $request) {
     $response = array();
     $methodname = clean_param($request['methodname'], PARAM_ALPHANUMEXT);
diff --git a/lib/externallib.php b/lib/externallib.php
index 7c0e838e2cb..befa4e6ed32 100644
--- a/lib/externallib.php
+++ b/lib/externallib.php
@@ -853,6 +853,11 @@ function external_validate_format($format) {
  * The caller can change the format (raw) with the external_settings singleton
  * All web service servers must set this singleton when parsing the $_GET and $_POST.
  *
+ * <pre>
+ * Options are the same that in {@link format_string()} with some changes:
+ *      filter      : Can be set to false to force filters off, else observes {@link external_settings}.
+ * </pre>
+ *
  * @param string $str The string to be filtered. Should be plain text, expect
  * possibly for multilang tags.
  * @param boolean $striplinks To strip any link in the result text. Moodle 1.8 default changed from false to true! MDL-8713
@@ -872,7 +877,7 @@ function external_format_string($str, $contextid, $striplinks = true, $options =
     if (!$settings->get_raw()) {
         $context = context::instance_by_id($contextid);
         $options['context'] = $context;
-        $options['filter'] = $settings->get_filter();
+        $options['filter'] = isset($options['filter']) && !$options['filter'] ? false : $settings->get_filter();
         $str = format_string($str, $striplinks, $options);
     }
 
@@ -890,8 +895,7 @@ function external_format_string($str, $contextid, $striplinks = true, $options =
  *      trusted     :   If true the string won't be cleaned. Default false.
  *      noclean     :   If true the string won't be cleaned only if trusted is also true. Default false.
  *      nocache     :   If true the string will not be cached and will be formatted every call. Default false.
- *      filter      :   If true the string will be run through applicable filters as well. Default (different from format_text)
- *                      got form settings.
+ *      filter      :   Can be set to false to force filters off, else observes {@link external_settings}.
  *      para        :   If true then the returned string will be wrapped in div tags. Default (different from format_text) false.
  *                      Default changed because div tags are not commonly needed.
  *      newlines    :   If true then lines newline breaks will be converted to HTML newline breaks. Default true.
@@ -935,7 +939,7 @@ function external_format_text($text, $textformat, $contextid, $component, $filea
             }
         }
 
-        $options['filter'] = isset($options['filter']) ? $options['filter'] : $settings->get_filter();
+        $options['filter'] = isset($options['filter']) && !$options['filter'] ? false : $settings->get_filter();
         $options['para'] = isset($options['para']) ? $options['para'] : false;
         $options['context'] = context::instance_by_id($contextid);
         $options['allowid'] = isset($options['allowid']) ? $options['allowid'] : true;
diff --git a/lib/tests/externallib_test.php b/lib/tests/externallib_test.php
index c018617b2c6..39796cc351f 100644
--- a/lib/tests/externallib_test.php
+++ b/lib/tests/externallib_test.php
@@ -96,6 +96,13 @@ class core_externallib_testcase extends advanced_testcase {
 </span></span>', FORMAT_HTML);
         $this->assertSame(external_format_text($test, $testformat, $context->id, 'core', '', 0), $correct);
 
+        // Filters can be opted out from by the developer.
+        $test = '$$ \pi $$';
+        $testformat = FORMAT_MARKDOWN;
+        $correct = array('<p>$$ \pi $$</p>
+', FORMAT_HTML);
+        $this->assertSame(external_format_text($test, $testformat, $context->id, 'core', '', 0, ['filter' => false]), $correct);
+
         $test = '<p><a id="test"></a><a href="#test">Text</a></p>';
         $testformat = FORMAT_HTML;
         $correct = array($test, FORMAT_HTML);
@@ -135,23 +142,47 @@ class core_externallib_testcase extends advanced_testcase {
     }
 
     public function test_external_format_string() {
+        $this->resetAfterTest();
         $settings = external_settings::get_instance();
-
         $currentraw = $settings->get_raw();
         $currentfilter = $settings->get_filter();
 
+        // Enable multilang filter to on content and heading.
+        filter_set_global_state('multilang', TEXTFILTER_ON);
+        filter_set_applies_to_strings('multilang', 1);
+        $filtermanager = filter_manager::instance();
+        $filtermanager->reset_caches();
+
         $settings->set_raw(true);
+        $settings->set_filter(true);
         $context = context_system::instance();
 
-        $test = '$$ \pi $$ <script>hi</script> <h3>there</h3>';
+        $test = '<span lang="en" class="multilang">EN</span><span lang="fr" class="multilang">FR</span> ' .
+            '<script>hi</script> <h3>there</h3>!';
         $correct = $test;
-        $this->assertSame(external_format_string($test, $context->id), $correct);
+        $this->assertSame($correct, external_format_string($test, $context->id));
 
         $settings->set_raw(false);
+        $settings->set_filter(false);
+
+        $test = '<span lang="en" class="multilang">EN</span><span lang="fr" class="multilang">FR</span> ' .
+            '<script>hi</script> <h3>there</h3>?';
+        $correct = 'ENFR hi there?';
+        $this->assertSame($correct, external_format_string($test, $context->id));
+
+        $settings->set_filter(true);
+
+        $test = '<span lang="en" class="multilang">EN</span><span lang="fr" class="multilang">FR</span> ' .
+            '<script>hi</script> <h3>there</h3>@';
+        $correct = 'EN hi there@';
+        $this->assertSame($correct, external_format_string($test, $context->id));
+
+        // Filters can be opted out.
+        $test = '<span lang="en" class="multilang">EN</span><span lang="fr" class="multilang">FR</span> ' .
+            '<script>hi</script> <h3>there</h3>%';
+        $correct = 'ENFR hi there%';
+        $this->assertSame($correct, external_format_string($test, $context->id, false, ['filter' => false]));
 
-        $test = '$$ \pi $$<script>hi</script> <h3>there</h3>';
-        $correct = '$$ \pi $$hi there';
-        $this->assertSame(external_format_string($test, $context->id), $correct);
 
         $settings->set_raw($currentraw);
         $settings->set_filter($currentfilter);