diff --git a/program/include/rcmail_output.php b/program/include/rcmail_output.php
index aeff78bcd..f34c147e7 100644
--- a/program/include/rcmail_output.php
+++ b/program/include/rcmail_output.php
@@ -36,6 +36,7 @@ abstract class rcmail_output extends rcube_output
 
     protected $pagetitle       = '';
     protected $object_handlers = array();
+    protected $devel_mode      = false;
 
 
     /**
@@ -44,6 +45,8 @@ abstract class rcmail_output extends rcube_output
     public function __construct($task = null, $framed = false)
     {
         parent::__construct();
+
+        $this->devel_mode = (bool) $this->config->get('devel_mode');
     }
 
     /**
diff --git a/program/include/rcmail_output_html.php b/program/include/rcmail_output_html.php
index 65302b179..7bca9561e 100644
--- a/program/include/rcmail_output_html.php
+++ b/program/include/rcmail_output_html.php
@@ -690,7 +690,7 @@ EOF;
             $parent = $this->framed || preg_match('/^parent\./', $method);
 
             foreach ($args as $i => $arg) {
-                $args[$i] = self::json_serialize($arg);
+                $args[$i] = self::json_serialize($arg, $this->devel_mode);
             }
 
             if ($parent) {
@@ -1923,7 +1923,7 @@ EOF;
             return;
         }
 
-        $this->add_script('var images = ' . self::json_serialize($images) .';
+        $this->add_script('var images = ' . self::json_serialize($images, $this->devel_mode) .';
             for (var i=0; i<images.length; i++) {
                 img = new Image();
                 img.src = images[i];
diff --git a/program/include/rcmail_output_json.php b/program/include/rcmail_output_json.php
index 0d7e278b8..5b559e2cd 100644
--- a/program/include/rcmail_output_json.php
+++ b/program/include/rcmail_output_json.php
@@ -232,7 +232,7 @@ class rcmail_output_json extends rcmail_output
         $response = $hook['response'];
         unset($hook['response']);
 
-        echo self::json_serialize($response);
+        echo self::json_serialize($response, $this->devel_mode);
     }
 
     /**
@@ -245,7 +245,7 @@ class rcmail_output_json extends rcmail_output
         foreach ($this->commands as $i => $args) {
             $method = array_shift($args);
             foreach ($args as $i => $arg) {
-                $args[$i] = self::json_serialize($arg);
+                $args[$i] = self::json_serialize($arg, $this->devel_mode);
             }
 
             $out .= sprintf(
diff --git a/program/lib/Roundcube/rcube_output.php b/program/lib/Roundcube/rcube_output.php
index 2d9c69521..905f8434e 100644
--- a/program/lib/Roundcube/rcube_output.php
+++ b/program/lib/Roundcube/rcube_output.php
@@ -265,16 +265,22 @@ abstract class rcube_output
     /**
      * Convert a variable into a javascript object notation
      *
-     * @param mixed Input value
+     * @param mixed   $input  Input value
+     * @param boolean $pretty Enable JSON formatting
      *
      * @return string Serialized JSON string
      */
-    public static function json_serialize($input)
+    public static function json_serialize($input, $pretty = false)
     {
-        $input = rcube_charset::clean($input);
+        $input   = rcube_charset::clean($input);
+        $options = 0;
+
+        if ($pretty) {
+            $options |= JSON_PRETTY_PRINT;
+        }
 
         // sometimes even using rcube_charset::clean() the input contains invalid UTF-8 sequences
         // that's why we have @ here
-        return @json_encode($input);
+        return @json_encode($input, $options);
     }
 }