fix: improve error handling and request cancellation for issue #88

src/mcp/shared/session.py

@@ -1,3 +1,4 @@
+import logging
 from contextlib import AbstractAsyncContextManager
 from datetime import timedelta
 from typing import Any, Callable, Generic, TypeVar
@@ -273,19 +274,28 @@ class BaseSession(
                         await self._incoming_message_stream_writer.send(responder)
 
                 elif isinstance(message.root, JSONRPCNotification):
-                    notification = self._receive_notification_type.model_validate(
+                    try:
-                        message.root.model_dump(
+                        notification = self._receive_notification_type.model_validate(
-                            by_alias=True, mode="json", exclude_none=True
+                            message.root.model_dump(
+                                by_alias=True, mode="json", exclude_none=True
+                            )
+                        )
+                        # Handle cancellation notifications
+                        if isinstance(notification.root, CancelledNotification):
+                            cancelled_id = notification.root.params.requestId
+                            if cancelled_id in self._in_flight:
+                                await self._in_flight[cancelled_id].cancel()
+                        else:
+                            await self._received_notification(notification)
+                            await self._incoming_message_stream_writer.send(
+                                notification
+                            )
+                    except Exception as e:
+                        # For other validation errors, log and continue
+                        logging.warning(
+                            f"Failed to validate notification: {e}. "
+                            f"Message was: {message.root}"
                         )
-                    )
-                    # Handle cancellation notifications
-                    if isinstance(notification.root, CancelledNotification):
-                        cancelled_id = notification.root.params.requestId
-                        if cancelled_id in self._in_flight:
-                            await self._in_flight[cancelled_id].cancel()
-                    else:
-                        await self._received_notification(notification)
-                        await self._incoming_message_stream_writer.send(notification)
                 else:  # Response or error
                     stream = self._response_streams.pop(message.root.id, None)
                     if stream:

tests/issues/test_88_random_error.py

@@ -30,19 +30,23 @@ async def test_notification_validation_error(tmp_path: Path):
 
     server = Server(name="test")
     request_count = 0
-    slow_request_complete = False
+    slow_request_started = anyio.Event()
+    slow_request_complete = anyio.Event()
 
     @server.call_tool()
     async def slow_tool(
         name: str, arg
     ) -> Sequence[TextContent | ImageContent | EmbeddedResource]:
-        nonlocal request_count, slow_request_complete
+        nonlocal request_count
         request_count += 1
 
         if name == "slow":
+            # Signal that slow request has started
+            slow_request_started.set()
             # Long enough to ensure timeout
             await anyio.sleep(0.2)
-            slow_request_complete = True
+            # Signal completion
+            slow_request_complete.set()
             return [TextContent(type="text", text=f"slow {request_count}")]
         elif name == "fast":
             # Fast enough to complete before timeout
@@ -71,7 +75,7 @@ async def test_notification_validation_error(tmp_path: Path):
             # First call should work (fast operation)
             result = await session.call_tool("fast")
             assert result.content == [TextContent(type="text", text="fast 1")]
-            assert not slow_request_complete
+            assert not slow_request_complete.is_set()
 
             # Second call should timeout (slow operation)
             with pytest.raises(McpError) as exc_info:
@@ -79,8 +83,8 @@ async def test_notification_validation_error(tmp_path: Path):
             assert "Timed out while waiting" in str(exc_info.value)
 
             # Wait for slow request to complete in the background
-            await anyio.sleep(0.3)
+            with anyio.fail_after(1):  # Timeout after 1 second
-            assert slow_request_complete
+                await slow_request_complete.wait()
 
             # Third call should work (fast operation),
             # proving server is still responsive
@@ -91,10 +95,17 @@ async def test_notification_validation_error(tmp_path: Path):
     server_writer, server_reader = anyio.create_memory_object_stream(1)
     client_writer, client_reader = anyio.create_memory_object_stream(1)
 
+    server_ready = anyio.Event()
+
+    async def wrapped_server_handler(read_stream, write_stream):
+        server_ready.set()
+        await server_handler(read_stream, write_stream)
+
     async with anyio.create_task_group() as tg:
-        tg.start_soon(server_handler, server_reader, client_writer)
+        tg.start_soon(wrapped_server_handler, server_reader, client_writer)
         # Wait for server to start and initialize
-        await anyio.sleep(0.1)
+        with anyio.fail_after(1):  # Timeout after 1 second
+            await server_ready.wait()
         # Run client in a separate task to avoid cancellation
         async with anyio.create_task_group() as client_tg:
             client_tg.start_soon(client, client_reader, server_writer)