Fixes protocol compliance gap described in issue #1419

src/mcp/shared/session.py

@@ -147,6 +147,20 @@ async def cancel(self) -> None:
             response=ErrorData(code=0, message="Request cancelled", data=None),
         )
 
+    async def mark_cancelled_without_response(self) -> None:
+        """Cancel this request and mark it as completed without sending a response.
+
+        This is used when cancellation is initiated by a cancellation notification,
+        where the receiver SHOULD NOT send a response per the MCP spec.
+        """
+        if not self._entered:
+            raise RuntimeError("RequestResponder must be used as a context manager")
+        if not self._cancel_scope:
+            raise RuntimeError("No active cancel scope")
+
+        self._cancel_scope.cancel()
+        self._completed = True
+
     @property
     def in_flight(self) -> bool:
         return not self._completed and not self.cancelled
@@ -314,6 +328,24 @@ async def send_notification(
         )
         await self._write_stream.send(session_message)
 
+        # If we are emitting a cancellation notification for a request that we
+        # originally sent, proactively cancel the local waiter so callers of
+        # send_request() are unblocked without relying on a peer response.
+        try:
+            from mcp.types import CancelledNotification as _CancelledNotification  # local import to avoid cycle
+
+            root = getattr(notification, "root", None)
+            if isinstance(root, _CancelledNotification):
+                cancelled_id = root.params.requestId
+                stream = self._response_streams.pop(cancelled_id, None)
+                if stream is not None:
+                    error = ErrorData(code=0, message="Request cancelled", data=None)
+                    await stream.send(JSONRPCError(jsonrpc="2.0", id=cancelled_id, error=error))
+                    await stream.aclose()
+        except Exception:
+            # Never let local cancellation propagation break notification sending
+            pass
+
     async def _send_response(self, request_id: RequestId, response: SendResultT | ErrorData) -> None:
         if isinstance(response, ErrorData):
             jsonrpc_error = JSONRPCError(jsonrpc="2.0", id=request_id, error=response)
@@ -383,7 +415,8 @@ async def _receive_loop(self) -> None:
                             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()
+                                    # Silent cancellation in response to a cancellation notification
+                                    await self._in_flight[cancelled_id].mark_cancelled_without_response()
                             else:
                                 # Handle progress notifications callback
                                 if isinstance(notification.root, ProgressNotification):