fix: Improve exception handling in lowlevel server

- Handle exceptions in match statement as suggested by TODO
- Use proper dict structure for error logging instead of ErrorData
- Add comprehensive tests with parameterization
- Include exception type, module, and args in log data

Reported-by: AishwaryaKalloli
Github-Issue: modelcontextprotocol#786

Author: felixweinberger

src/mcp/server/lowlevel/server.py

@@ -525,8 +525,7 @@ async def _handle_message(
         raise_exceptions: bool = False,
     ):
         with warnings.catch_warnings(record=True) as w:
-            # TODO(Marcelo): We should be checking if message is Exception here.
-            match message:  # type: ignore[reportMatchNotExhaustive]
+            match message:
                 case (
                     RequestResponder(request=types.ClientRequest(root=req)) as responder
                 ):
@@ -537,16 +536,18 @@ async def _handle_message(
                 case types.ClientNotification(root=notify):
                     await self._handle_notification(notify)
                 case Exception():
-                    logger.error(f"Received error message: {message}")
+                    logger.error(f"Received exception from stream: {message}")
                     if raise_exceptions:
                         raise message
-                    # Send the error as a notification
-                    # as we don't have a request context
                     await session.send_log_message(
                         level="error",
-                        data=types.ErrorData(
-                            code=types.INTERNAL_ERROR, message=str(message), data=None
-                        ),
+                        data={
+                            "message": str(message),
+                            "type": type(message).__name__,
+                            "module": type(message).__module__,
+                            "args": getattr(message, "args", None),
+                        },
+                        logger="mcp.server.exception_handler",
                     )
 
             for warning in w:

tests/server/test_lowlevel_exception_handling.py

@@ -0,0 +1,76 @@
+from unittest.mock import AsyncMock, Mock
+
+import pytest
+
+import mcp.types as types
+from mcp.server.lowlevel.server import Server
+from mcp.server.session import ServerSession
+from mcp.shared.session import RequestResponder
+
+
+@pytest.mark.anyio
+async def test_exception_handling_with_raise_exceptions_true():
+    """Test that exceptions are re-raised when raise_exceptions=True"""
+    server = Server("test-server")
+    session = Mock(spec=ServerSession)
+    session.send_log_message = AsyncMock()
+
+    test_exception = RuntimeError("Test error")
+
+    with pytest.raises(RuntimeError, match="Test error"):
+        await server._handle_message(test_exception, session, {}, raise_exceptions=True)
+
+    # Should not send log message when re-raising
+    session.send_log_message.assert_not_called()
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize(
+    "exception_class,message",
+    [
+        (ValueError, "Test validation error"),
+        (RuntimeError, "Test runtime error"),
+        (KeyError, "Test key error"),
+        (Exception, "Basic error"),
+    ],
+)
+async def test_exception_handling_with_raise_exceptions_false(exception_class, message):
+    """Test that exceptions are logged when raise_exceptions=False"""
+    server = Server("test-server")
+    session = Mock(spec=ServerSession)
+    session.send_log_message = AsyncMock()
+
+    test_exception = exception_class(message)
+
+    await server._handle_message(test_exception, session, {}, raise_exceptions=False)
+
+    # Should send log message
+    session.send_log_message.assert_called_once()
+    call_args = session.send_log_message.call_args
+
+    assert call_args.kwargs["level"] == "error"
+    assert call_args.kwargs["data"]["message"] == str(test_exception)
+    assert call_args.kwargs["data"]["type"] == exception_class.__name__
+    assert call_args.kwargs["logger"] == "mcp.server.exception_handler"
+
+
+@pytest.mark.anyio
+async def test_normal_message_handling_not_affected():
+    """Test that normal messages still work correctly"""
+    server = Server("test-server")
+    session = Mock(spec=ServerSession)
+
+    # Create a mock RequestResponder
+    responder = Mock(spec=RequestResponder)
+    responder.request = types.ClientRequest(root=types.PingRequest(method="ping"))
+    responder.__enter__ = Mock(return_value=responder)
+    responder.__exit__ = Mock(return_value=None)
+
+    # Mock the _handle_request method to avoid complex setup
+    server._handle_request = AsyncMock()
+
+    # Should handle normally without any exception handling
+    await server._handle_message(responder, session, {}, raise_exceptions=False)
+
+    # Verify _handle_request was called
+    server._handle_request.assert_called_once()