diff --git a/oauthex/oauthex.go b/oauthex/oauthex.go
index 34ed55b5..d843f2e2 100644
--- a/oauthex/oauthex.go
+++ b/oauthex/oauthex.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by an MIT-style
 // license that can be found in the LICENSE file.
 
+//go:build mcp_go_client_oauth
+
 // Package oauthex implements extensions to OAuth2.
 package oauthex