fix: check the cause of the tool error (#2674)

src/fastmcp/server/middleware/error_handling.py

@@ -87,7 +87,7 @@ class ErrorHandlingMiddleware(Middleware):
             return error
 
         # Map common exceptions to appropriate MCP error codes
-        error_type = type(error)
+        error_type = type(error.__cause__) if error.__cause__ else type(error)
 
         if error_type in (ValueError, TypeError):
             return McpError(

tests/server/middleware/test_error_handling.py

@@ -6,7 +6,7 @@ from unittest.mock import AsyncMock, MagicMock
 import pytest
 from mcp import McpError
 
-from fastmcp.exceptions import NotFoundError
+from fastmcp.exceptions import NotFoundError, ToolError
 from fastmcp.server.middleware.error_handling import (
     ErrorHandlingMiddleware,
     RetryMiddleware,
@@ -215,6 +215,23 @@ class TestErrorHandlingMiddleware:
         assert "Invalid params: test error" in exc_info.value.error.message
         assert "Error in test_method: ValueError: test error" in caplog.text
 
+    async def test_on_message_error_transform_tool_error(self, mock_context, caplog):
+        """Test error handling with transformation and cause type."""
+        middleware = ErrorHandlingMiddleware()
+        tool_error = ToolError("test error")
+        tool_error.__cause__ = ValueError()
+        mock_call_next = AsyncMock(side_effect=tool_error)
+
+        with caplog_for_fastmcp(caplog):
+            with caplog.at_level(logging.ERROR):
+                with pytest.raises(McpError) as exc_info:
+                    await middleware.on_message(mock_context, mock_call_next)
+
+        assert isinstance(exc_info.value, McpError)
+        assert exc_info.value.error.code == -32602
+        assert "Invalid params: test error" in exc_info.value.error.message
+        assert "Error in test_method: ToolError: test error" in caplog.text
+
     def test_get_error_stats(self, mock_context):
         """Test getting error statistics."""
         middleware = ErrorHandlingMiddleware()