diff --git a/libs/sdk-py/langgraph_sdk/auth/__init__.py b/libs/sdk-py/langgraph_sdk/auth/__init__.py
index 3379affac..3cfd0574e 100644
--- a/libs/sdk-py/langgraph_sdk/auth/__init__.py
+++ b/libs/sdk-py/langgraph_sdk/auth/__init__.py
@@ -259,7 +259,7 @@ class Auth:
         """
         if self._authenticate_handler is not None:
             raise ValueError(
-                "Authentication handler already set as {self._authenticate_handler}."
+                f"Authentication handler already set as {self._authenticate_handler}."
             )
         self._authenticate_handler = fn
         return fn