diff --git a/src/Plugin.php b/src/Plugin.php
index cbc9592..99898b9 100644
--- a/src/Plugin.php
+++ b/src/Plugin.php
@@ -24,8 +24,8 @@ interface Plugin
      *
      * @see http://docs.php-http.org/en/latest/plugins/build-your-own.html
      *
-     * @param callable $next  Next middleware in the chain, the request is passed as the first argument
-     * @param callable $first First middleware in the chain, used to to restart a request
+     * @param callable(RequestInterface): Promise $next  Next middleware in the chain, the request is passed as the first argument
+     * @param callable(RequestInterface): Promise $first First middleware in the chain, used to to restart a request
      *
      * @return Promise Resolves a PSR-7 Response or fails with an Http\Client\Exception (The same as HttpAsyncClient)
      */