diff --git a/CHANGELOG.md b/CHANGELOG.md
index 479b7c9e..f9ca9cab 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## Unreleased
+- [#465](https://github.com/Shopify/shopify-api-php/pull/465) [Patch] Cast `Content-Length` header value to `string` in `Http::request()` to avoid a `guzzlehttp/psr7` 2.11 deprecation (`Passing int to MessageInterface::withHeader() is deprecated`) and stay compatible with `guzzlehttp/psr7` 3.0
 
 ## v6.1.1 - 2026-03-02
 - [#456](https://github.com/Shopify/shopify-api-php/pull/456) [Patch] Update firebase/php-jwt to ^7.0 to address security vulnerability (GHSA-2x45-7fc3-mxwq)
diff --git a/src/Clients/Http.php b/src/Clients/Http.php
index 4f03ab61..292661f1 100644
--- a/src/Clients/Http.php
+++ b/src/Clients/Http.php
@@ -207,7 +207,7 @@ protected function request(
             $request = $request
                 ->withBody($stream)
                 ->withHeader(HttpHeaders::CONTENT_TYPE, $dataType)
-                ->withHeader(HttpHeaders::CONTENT_LENGTH, mb_strlen($bodyString));
+                ->withHeader(HttpHeaders::CONTENT_LENGTH, (string) mb_strlen($bodyString));
         }
 
         $currentTries = 0;