From 5ce67f756ba07c185e0037dcac601768ca6ff30d Mon Sep 17 00:00:00 2001
From: Omer Aharonian <43315856+omeraha@users.noreply.github.com>
Date: Sat, 18 Oct 2025 11:46:57 -0400
Subject: [PATCH] transform NotFoundError to standard MCP error (#2133)

---
 src/fastmcp/server/middleware/error_handling.py |  4 +++-
 tests/server/middleware/test_error_handling.py  | 12 ++++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/src/fastmcp/server/middleware/error_handling.py b/src/fastmcp/server/middleware/error_handling.py
index 0a71a24ea..176240584 100644
--- a/src/fastmcp/server/middleware/error_handling.py
+++ b/src/fastmcp/server/middleware/error_handling.py
@@ -9,6 +9,8 @@ from typing import Any
 from mcp import McpError
 from mcp.types import ErrorData
 
+from fastmcp.exceptions import NotFoundError
+
 from .middleware import CallNext, Middleware, MiddlewareContext
 
 
@@ -90,7 +92,7 @@ class ErrorHandlingMiddleware(Middleware):
             return McpError(
                 ErrorData(code=-32602, message=f"Invalid params: {str(error)}")
             )
-        elif error_type in (FileNotFoundError, KeyError):
+        elif error_type in (FileNotFoundError, KeyError, NotFoundError):
             return McpError(
                 ErrorData(code=-32001, message=f"Resource not found: {str(error)}")
             )
diff --git a/tests/server/middleware/test_error_handling.py b/tests/server/middleware/test_error_handling.py
index 6c7c3cde3..1ed534709 100644
--- a/tests/server/middleware/test_error_handling.py
+++ b/tests/server/middleware/test_error_handling.py
@@ -6,6 +6,7 @@ from unittest.mock import AsyncMock, MagicMock
 import pytest
 from mcp import McpError
 
+from fastmcp.exceptions import NotFoundError
 from fastmcp.server.middleware.error_handling import (
     ErrorHandlingMiddleware,
     RetryMiddleware,
@@ -146,6 +147,17 @@ class TestErrorHandlingMiddleware:
         assert result.error.code == -32001
         assert "Resource not found: test error" in result.error.message
 
+    def test_transform_error_not_found_error(self):
+        """Test transforming NotFoundError."""
+        middleware = ErrorHandlingMiddleware()
+        error = NotFoundError("test error")
+
+        result = middleware._transform_error(error)
+
+        assert isinstance(result, McpError)
+        assert result.error.code == -32001
+        assert "Resource not found: test error" in result.error.message
+
     def test_transform_error_permission_error(self):
         """Test transforming PermissionError."""
         middleware = ErrorHandlingMiddleware()