diff --git a/src/wp-includes/interactivity-api/class-wp-interactivity-api.php b/src/wp-includes/interactivity-api/class-wp-interactivity-api.php
index 16f270be0f82d..1772d17c2fff8 100644
--- a/src/wp-includes/interactivity-api/class-wp-interactivity-api.php
+++ b/src/wp-includes/interactivity-api/class-wp-interactivity-api.php
@@ -12,7 +12,7 @@
  *
  * @since 6.5.0
  */
-final class WP_Interactivity_API {
+class WP_Interactivity_API {
 	/**
 	 * Holds the mapping of directive attribute names to their processor methods.
 	 *
@@ -1422,7 +1422,7 @@ private function data_wp_text_processor( WP_Interactivity_API_Directives_Process
 	 * @return string The CSS styles for the router's top loading bar animation.
 	 */
 	private function get_router_animation_styles(): string {
-		return <<<CSS
+		return <<<'CSS'
 			.wp-interactivity-router-loading-bar {
 				position: fixed;
 				top: 0;
@@ -1475,7 +1475,7 @@ public function print_router_loading_and_screen_reader_markup() {
 	 * @since 6.7.0
 	 */
 	public function print_router_markup() {
-		echo <<<HTML
+		echo <<<'HTML'
 			<div
 				class="wp-interactivity-router-loading-bar"
 				data-wp-interactive="core/router/private"
diff --git a/src/wp-includes/interactivity-api/interactivity-api.php b/src/wp-includes/interactivity-api/interactivity-api.php
index 105f3f6f29c18..db686db54a2f2 100644
--- a/src/wp-includes/interactivity-api/interactivity-api.php
+++ b/src/wp-includes/interactivity-api/interactivity-api.php
@@ -20,11 +20,27 @@
  * @return WP_Interactivity_API The main WP_Interactivity_API instance.
  */
 function wp_interactivity(): WP_Interactivity_API {
-	global $wp_interactivity;
+		global $wp_interactivity;
+
 	if ( ! ( $wp_interactivity instanceof WP_Interactivity_API ) ) {
-		$wp_interactivity = new WP_Interactivity_API();
+			$wp_interactivity = new WP_Interactivity_API();
+
+		if (
+					defined( 'WP_INTERACTIVITY_ALLOW_PRIVATE_API' )
+					&& WP_INTERACTIVITY_ALLOW_PRIVATE_API
+			) {
+				$instance = apply_filters(
+					'wp_interactivity_instance',
+					$wp_interactivity
+				);
+
+			if ( $instance instanceof WP_Interactivity_API ) {
+				$wp_interactivity = $instance;
+			}
+		}
 	}
-	return $wp_interactivity;
+
+		return $wp_interactivity;
 }
 
 /**
diff --git a/src/wp-settings.php b/src/wp-settings.php
index 1d55db83e2d85..0e70558cd7554 100644
--- a/src/wp-settings.php
+++ b/src/wp-settings.php
@@ -458,7 +458,12 @@
 require ABSPATH . WPINC . '/view-transitions.php';
 
 add_action( 'after_setup_theme', array( wp_script_modules(), 'add_hooks' ) );
-add_action( 'after_setup_theme', array( wp_interactivity(), 'add_hooks' ) );
+add_action(
+	'after_setup_theme',
+	function () {
+		wp_interactivity()->add_hooks();
+	}
+);
 
 /**
  * @since 3.3.0
