diff --git a/classes/form/context/render_context.php b/classes/form/context/render_context.php
index 7ca8a84d..a5bbf22a 100644
--- a/classes/form/context/render_context.php
+++ b/classes/form/context/render_context.php
@@ -146,7 +146,7 @@ abstract public function hide_if(string $dependant, string $dependency, string $
      * @return string name of the element qualified by this context's prefix
      */
     public function mangle_name(string $name): string {
-        if (utils::str_starts_with($name, $this->prefix)) {
+        if (str_starts_with($name, $this->prefix)) {
             // Already mangled, perhaps by an array_render_context.
             return $name;
         }
diff --git a/classes/question_ui_renderer.php b/classes/question_ui_renderer.php
index bddb7191..201c817f 100644
--- a/classes/question_ui_renderer.php
+++ b/classes/question_ui_renderer.php
@@ -249,7 +249,7 @@ private function mangle_ids_and_names(): void {
                 ") as $attr
         ) {
             $original = $attr->value;
-            if ($attr->name === "usemap" && utils::str_starts_with($original, "#")) {
+            if ($attr->name === "usemap" && str_starts_with($original, "#")) {
                 // See https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/useMap.
                 $attr->value = "#" . $this->attempt->get_qt_field_name(substr($original, 1));
             } else {
diff --git a/classes/utils.php b/classes/utils.php
index 8d4b61b2..39c7d85f 100644
--- a/classes/utils.php
+++ b/classes/utils.php
@@ -42,18 +42,6 @@ public static function &ensure_exists(array &$array, string $key): array {
         return $array[$key];
     }
 
-    /**
-     * Determines whether a string starts with another.
-     *
-     * @param string $haystack
-     * @param string $needle
-     * @return bool
-     */
-    public static function str_starts_with(string $haystack, string $needle): bool {
-        // From https://stackoverflow.com/a/10473026.
-        return substr_compare($haystack, $needle, 0, strlen($needle)) === 0;
-    }
-
     /**
      * Given an array and a key such as `abc[def]`, returns `$array["abc"]["def"]`.
      *
@@ -127,4 +115,15 @@ public static function reindex_integer_arrays(array &$array): void {
             $array = array_values($array);
         }
     }
+
+    /**
+     * Given an array as returned by {@see \question_attempt_step::get_qt_data()}, filters out vars starting with `_`.
+     *
+     * @param array $qtvars
+     * @return array
+     * @see question_attempt_step for the meaning of different step var prefixes
+     */
+    public static function filter_for_response(array $qtvars): array {
+        return array_filter($qtvars, fn($key) => !str_starts_with($key, '_'), ARRAY_FILTER_USE_KEY);
+    }
 }
diff --git a/question.php b/question.php
index 57a1da08..6201c6ea 100644
--- a/question.php
+++ b/question.php
@@ -24,8 +24,8 @@
 
 use qtype_questionpy\api\api;
 use qtype_questionpy\api\attempt_ui;
-use qtype_questionpy\constants;
 use qtype_questionpy\api\scoring_code;
+use qtype_questionpy\constants;
 use qtype_questionpy\question_ui_metadata_extractor;
 use qtype_questionpy\utils;
 
@@ -160,11 +160,7 @@ public function apply_attempt_state(question_attempt_step $step) {
         $this->scoringstate = $this->get_behaviour()->get_qa()->get_last_qt_var(constants::QT_VAR_SCORING_STATE);
 
         $lastqtdata = $this->get_behaviour()->get_qa()->get_last_qt_data(null);
-        $lastresponse = $lastqtdata === null ? null : array_filter(
-            $lastqtdata,
-            fn($key) => !utils::str_starts_with($key, "_"),
-            ARRAY_FILTER_USE_KEY
-        );
+        $lastresponse = $lastqtdata === null ? null : utils::filter_for_response($lastqtdata);
 
         /* TODO: This method is also called from question_attempt->regrade and
                  question_attempt->start_question_based_on, where we shouldn't need to get the UI. */
@@ -267,7 +263,7 @@ public function is_complete_response(array $response): bool {
     }
 
     /**
-     * Use by many of the behaviours to determine whether the student's
+     * Used by many of the behaviours to determine whether the student's
      * response has changed. This is normally used to determine that a new set
      * of responses can safely be discarded.
      *
@@ -277,8 +273,10 @@ public function is_complete_response(array $response): bool {
      * @return bool whether the two sets of responses are the same - that is
      *                            whether the new set of responses can safely be discarded.
      */
-    public function is_same_response(array $prevresponse, array $newresponse) {
-        return false;
+    public function is_same_response(array $prevresponse, array $newresponse): bool {
+        // The response has already been filtered against get_expected_data, we just need to filter out attempt state
+        // and scoring state before comparing.
+        return utils::filter_for_response($prevresponse) == utils::filter_for_response($newresponse);
     }
 
     /**