From 59b19eb549a94e8fd9189074c2786e544f4c6240 Mon Sep 17 00:00:00 2001
From: William Easton <bill.easton@elastic.co>
Date: Fri, 27 Mar 2026 18:31:05 -0500
Subject: [PATCH] Fix error classification using __cause__ instead of raised
 exception type

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 src/fastmcp/server/middleware/error_handling.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/fastmcp/server/middleware/error_handling.py b/src/fastmcp/server/middleware/error_handling.py
index 5b235e804..122642250 100644
--- a/src/fastmcp/server/middleware/error_handling.py
+++ b/src/fastmcp/server/middleware/error_handling.py
@@ -89,7 +89,7 @@ class ErrorHandlingMiddleware(Middleware):
             return error
 
         # Map common exceptions to appropriate MCP error codes
-        error_type = type(error.__cause__) if error.__cause__ else type(error)
+        error_type = type(error)
 
         if error_type in (ValueError, TypeError):
             return McpError(