diff -r 48c4eec2b7e6 -r 8c2e4d02f4ef wp/wp-includes/rest-api.php
--- a/wp/wp-includes/rest-api.php	Fri Sep 05 18:40:08 2025 +0200
+++ b/wp/wp-includes/rest-api.php	Fri Sep 05 18:52:52 2025 +0200
@@ -38,26 +38,55 @@
 		 * and namespace indexes. If you really need to register a
 		 * non-namespaced route, call `WP_REST_Server::register_route` directly.
 		 */
-		_doing_it_wrong( 'register_rest_route', __( 'Routes must be namespaced with plugin or theme name and version.' ), '4.4.0' );
+		_doing_it_wrong(
+			__FUNCTION__,
+			sprintf(
+				/* translators: 1: string value of the namespace, 2: string value of the route. */
+				__( 'Routes must be namespaced with plugin or theme name and version. Instead there seems to be an empty namespace \'%1$s\' for route \'%2$s\'.' ),
+				'<code>' . $route_namespace . '</code>',
+				'<code>' . $route . '</code>'
+			),
+			'4.4.0'
+		);
 		return false;
 	} elseif ( empty( $route ) ) {
-		_doing_it_wrong( 'register_rest_route', __( 'Route must be specified.' ), '4.4.0' );
+		_doing_it_wrong(
+			__FUNCTION__,
+			sprintf(
+				/* translators: 1: string value of the namespace, 2: string value of the route. */
+				__( 'Route must be specified. Instead within the namespace \'%1$s\', there seems to be an empty route \'%2$s\'.' ),
+				'<code>' . $route_namespace . '</code>',
+				'<code>' . $route . '</code>'
+			),
+			'4.4.0'
+		);
 		return false;
 	}
 
 	$clean_namespace = trim( $route_namespace, '/' );
 
 	if ( $clean_namespace !== $route_namespace ) {
-		_doing_it_wrong( __FUNCTION__, __( 'Namespace must not start or end with a slash.' ), '5.4.2' );
+		_doing_it_wrong(
+			__FUNCTION__,
+			sprintf(
+				/* translators: 1: string value of the namespace, 2: string value of the route. */
+				__( 'Namespace must not start or end with a slash. Instead namespace \'%1$s\' for route \'%2$s\' seems to contain a slash.' ),
+				'<code>' . $route_namespace . '</code>',
+				'<code>' . $route . '</code>'
+			),
+			'5.4.2'
+		);
 	}
 
 	if ( ! did_action( 'rest_api_init' ) ) {
 		_doing_it_wrong(
-			'register_rest_route',
+			__FUNCTION__,
 			sprintf(
-				/* translators: %s: rest_api_init */
-				__( 'REST API routes must be registered on the %s action.' ),
-				'<code>rest_api_init</code>'
+				/* translators: 1: rest_api_init, 2: string value of the route, 3: string value of the namespace. */
+				__( 'REST API routes must be registered on the %1$s action. Instead route \'%2$s\' with namespace \'%3$s\' was not registered on this action.' ),
+				'<code>rest_api_init</code>',
+				'<code>' . $route . '</code>',
+				'<code>' . $route_namespace . '</code>'
 			),
 			'5.1.0'
 		);
@@ -401,6 +430,16 @@
 		return;
 	}
 
+	// Return an error message if query_var is not a string.
+	if ( ! is_string( $GLOBALS['wp']->query_vars['rest_route'] ) ) {
+		$rest_type_error = new WP_Error(
+			'rest_path_invalid_type',
+			__( 'The REST route parameter must be a string.' ),
+			array( 'status' => 400 )
+		);
+		wp_die( $rest_type_error );
+	}
+
 	/**
 	 * Whether this is a REST Request.
 	 *
@@ -789,9 +828,6 @@
 		}
 
 		foreach ( $endpoints as $endpoint ) {
-			// Remove the redundant preg_match() argument.
-			unset( $args[0] );
-
 			$request->set_url_params( $args );
 			$request->set_attributes( $endpoint );
 		}
@@ -1275,6 +1311,9 @@
 /**
  * Parses an RFC3339 time into a Unix timestamp.
  *
+ * Explicitly check for `false` to detect failure, as zero is a valid return
+ * value on success.
+ *
  * @since 4.4.0
  *
  * @param string $date      RFC3339 timestamp.
@@ -1340,7 +1379,7 @@
 
 	$date = rest_parse_date( $date );
 
-	if ( empty( $date ) ) {
+	if ( false === $date ) {
 		return null;
 	}
 
@@ -2230,7 +2269,7 @@
 				break;
 
 			case 'date-time':
-				if ( ! rest_parse_date( $value ) ) {
+				if ( false === rest_parse_date( $value ) ) {
 					return new WP_Error( 'rest_invalid_date', __( 'Invalid date.' ) );
 				}
 				break;
@@ -2906,6 +2945,7 @@
 		}
 	}
 
+	// Remove trailing slashes at the end of the REST API path (query part).
 	$path = untrailingslashit( $path );
 	if ( empty( $path ) ) {
 		$path = '/';
@@ -2916,6 +2956,14 @@
 		return $memo;
 	}
 
+	if ( isset( $path_parts['path'] ) && '/' !== $path_parts['path'] ) {
+		// Remove trailing slashes from the "path" part of the REST API path.
+		$path_parts['path'] = untrailingslashit( $path_parts['path'] );
+		$path               = str_contains( $path, '?' ) ?
+			$path_parts['path'] . '?' . ( $path_parts['query'] ?? '' ) :
+			$path_parts['path'];
+	}
+
 	$request = new WP_REST_Request( $method, $path_parts['path'] );
 	if ( ! empty( $path_parts['query'] ) ) {
 		parse_str( $path_parts['query'], $query_params );