Sending cancellation notification to server based on client anyio.CancelScope status

src/mcp/client/session.py

@@ -140,6 +140,12 @@ async def initialize(self) -> types.InitializeResult:
                 )
             ),
             types.InitializeResult,
+            # TODO should set a request_read_timeout_seconds as per
+            # guidance from BaseSession.send_request not obvious
+            # what subsequent process should be, refer the following
+            # specification for more details
+            # https://modelcontextprotocol.io/specification/2025-03-26/basic/utilities/cancellation
+            cancellable=False,
         )
 
         if result.protocolVersion not in SUPPORTED_PROTOCOL_VERSIONS:
@@ -271,6 +277,7 @@ async def call_tool(
         arguments: dict[str, Any] | None = None,
         read_timeout_seconds: timedelta | None = None,
         progress_callback: ProgressFnT | None = None,
+        cancellable: bool = True,
     ) -> types.CallToolResult:
         """Send a tools/call request with optional progress callback support."""
 
@@ -287,6 +294,7 @@ async def call_tool(
             types.CallToolResult,
             request_read_timeout_seconds=read_timeout_seconds,
             progress_callback=progress_callback,
+            cancellable=cancellable,
         )
 
     async def list_prompts(self, cursor: str | None = None) -> types.ListPromptsResult:

src/mcp/server/lowlevel/server.py

@@ -86,7 +86,7 @@ async def main():
 from mcp.shared.context import RequestContext
 from mcp.shared.exceptions import McpError
 from mcp.shared.message import SessionMessage
-from mcp.shared.session import RequestResponder
+from mcp.shared.session import RequestId, RequestResponder
 
 logger = logging.getLogger(__name__)
 
@@ -447,6 +447,20 @@ async def handler(req: types.ProgressNotification):
 
         return decorator
 
+    def cancel_notification(self):
+        def decorator(
+            func: Callable[[RequestId, str | None], Awaitable[None]],
+        ):
+            logger.debug("Registering handler for CancelledNotification")
+
+            async def handler(req: types.CancelledNotification):
+                await func(req.params.requestId, req.params.reason)
+
+            self.notification_handlers[types.CancelledNotification] = handler
+            return func
+
+        return decorator
+
     def completion(self):
         """Provides completions for prompts and resource templates"""
 

src/mcp/shared/session.py

@@ -14,7 +14,9 @@
 from mcp.shared.exceptions import McpError
 from mcp.shared.message import MessageMetadata, ServerMessageMetadata, SessionMessage
 from mcp.types import (
+    REQUEST_CANCELLED,
     CancelledNotification,
+    CancelledNotificationParams,
     ClientNotification,
     ClientRequest,
     ClientResult,
@@ -226,12 +228,25 @@ async def send_request(
         request_read_timeout_seconds: timedelta | None = None,
         metadata: MessageMetadata = None,
         progress_callback: ProgressFnT | None = None,
+        cancellable: bool = True,
     ) -> ReceiveResultT:
         """
         Sends a request and wait for a response. Raises an McpError if the
         response contains an error. If a request read timeout is provided, it
         will take precedence over the session read timeout.
 
+        If cancellable is set to False then the request will wait
+        request_read_timeout_seconds to complete and ignore any attempt to
+        cancel via the anyio.CancelScope within which this method was called.
+
+        If cancellable is set to True (default) if the anyio.CancelScope within
+        which this method was called is cancelled it will generate a
+        CancelationNotfication and send this to the server which should then abort
+        the task however the server is is not guaranteed to honour this request.
+
+        For further information on the CancelNotification flow refer to
+        https://modelcontextprotocol.io/specification/2025-03-26/basic/utilities/cancellation
+
         Do not use this method to emit notifications! Use send_notification()
         instead.
         """
@@ -275,20 +290,38 @@ async def send_request(
             elif self._session_read_timeout_seconds is not None:
                 timeout = self._session_read_timeout_seconds.total_seconds()
 
-            try:
-                with anyio.fail_after(timeout):
-                    response_or_error = await response_stream_reader.receive()
-            except TimeoutError:
-                raise McpError(
-                    ErrorData(
-                        code=httpx.codes.REQUEST_TIMEOUT,
-                        message=(
-                            f"Timed out while waiting for response to "
-                            f"{request.__class__.__name__}. Waited "
-                            f"{timeout} seconds."
-                        ),
+            with anyio.CancelScope(shield=not cancellable):
+                try:
+                    with anyio.fail_after(timeout) as scope:
+                        response_or_error = await response_stream_reader.receive()
+
+                        if scope.cancel_called:
+                            notification = CancelledNotification(
+                                method="notifications/cancelled",
+                                params=CancelledNotificationParams(
+                                    requestId=request_id, reason="cancelled"
+                                ),
+                            )
+                            await self._send_notification(  # type: ignore
+                                notification, request_id
+                            )
+                            raise McpError(
+                                ErrorData(
+                                    code=REQUEST_CANCELLED, message="Request cancelled"
+                                )
+                            )
+
+                except TimeoutError:
+                    raise McpError(
+                        ErrorData(
+                            code=httpx.codes.REQUEST_TIMEOUT,
+                            message=(
+                                f"Timed out while waiting for response to "
+                                f"{request.__class__.__name__}. Waited "
+                                f"{timeout} seconds."
+                            ),
+                        )
                     )
-                )
 
             if isinstance(response_or_error, JSONRPCError):
                 raise McpError(response_or_error.error)

src/mcp/types.py

@@ -146,6 +146,7 @@ class JSONRPCResponse(BaseModel):
 METHOD_NOT_FOUND = -32601
 INVALID_PARAMS = -32602
 INTERNAL_ERROR = -32603
+REQUEST_CANCELLED = -32604
 
 
 class ErrorData(BaseModel):

tests/shared/test_session.py

@@ -1,4 +1,5 @@
 from collections.abc import AsyncGenerator
+from datetime import timedelta
 
 import anyio
 import pytest
@@ -9,10 +10,6 @@
 from mcp.shared.exceptions import McpError
 from mcp.shared.memory import create_connected_server_and_client_session
 from mcp.types import (
-    CancelledNotification,
-    CancelledNotificationParams,
-    ClientNotification,
-    ClientRequest,
     EmptyResult,
 )
 
@@ -46,11 +43,11 @@ async def test_in_flight_requests_cleared_after_completion(
 @pytest.mark.anyio
 async def test_request_cancellation():
     """Test that requests can be cancelled while in-flight."""
-    # The tool is already registered in the fixture
 
     ev_tool_called = anyio.Event()
+    ev_tool_cancelled = anyio.Event()
     ev_cancelled = anyio.Event()
-    request_id = None
+    ev_cancel_notified = anyio.Event()
 
     # Start the request in a separate task so we can cancel it
     def make_server() -> Server:
@@ -59,14 +56,24 @@ def make_server() -> Server:
         # Register the tool handler
         @server.call_tool()
         async def handle_call_tool(name: str, arguments: dict | None) -> list:
-            nonlocal request_id, ev_tool_called
+            nonlocal ev_tool_called, ev_tool_cancelled
             if name == "slow_tool":
-                request_id = server.request_context.request_id
                 ev_tool_called.set()
-                await anyio.sleep(10)  # Long enough to ensure we can cancel
-                return []
+                with anyio.CancelScope():
+                    try:
+                        await anyio.sleep(10)  # Long enough to ensure we can cancel
+                        return []
+                    except anyio.get_cancelled_exc_class() as err:
+                        ev_tool_cancelled.set()
+                        raise err
+
             raise ValueError(f"Unknown tool: {name}")
 
+        @server.cancel_notification()
+        async def handle_cancel(requestId: str | int, reason: str | None):
+            nonlocal ev_cancel_notified
+            ev_cancel_notified.set()
+
         # Register the tool so it shows up in list_tools
         @server.list_tools()
         async def handle_list_tools() -> list[types.Tool]:
@@ -80,20 +87,10 @@ async def handle_list_tools() -> list[types.Tool]:
 
         return server
 
-    async def make_request(client_session):
+    async def make_request(client_session: ClientSession):
         nonlocal ev_cancelled
         try:
-            await client_session.send_request(
-                ClientRequest(
-                    types.CallToolRequest(
-                        method="tools/call",
-                        params=types.CallToolRequestParams(
-                            name="slow_tool", arguments={}
-                        ),
-                    )
-                ),
-                types.CallToolResult,
-            )
+            await client_session.call_tool("slow_tool")
             pytest.fail("Request should have been cancelled")
         except McpError as e:
             # Expected - request was cancelled
@@ -110,17 +107,87 @@ async def make_request(client_session):
             with anyio.fail_after(1):  # Timeout after 1 second
                 await ev_tool_called.wait()
 
-            # Send cancellation notification
-            assert request_id is not None
-            await client_session.send_notification(
-                ClientNotification(
-                    CancelledNotification(
-                        method="notifications/cancelled",
-                        params=CancelledNotificationParams(requestId=request_id),
-                    )
-                )
-            )
+            # Cancel the task via task group
+            tg.cancel_scope.cancel()
 
             # Give cancellation time to process
             with anyio.fail_after(1):
                 await ev_cancelled.wait()
+
+            # Check server cancel notification received
+            with anyio.fail_after(1):
+                await ev_cancel_notified.wait()
+
+            # Give cancellation time to process on server
+            with anyio.fail_after(1):
+                await ev_tool_cancelled.wait()
+
+
+@pytest.mark.anyio
+async def test_request_cancellation_uncancellable():
+    """Test that asserts a call with cancellable=False is not cancelled on
+    server when cancel scope on client is set."""
+
+    ev_tool_called = anyio.Event()
+    ev_tool_commplete = anyio.Event()
+    ev_cancelled = anyio.Event()
+
+    # Start the request in a separate task so we can cancel it
+    def make_server() -> Server:
+        server = Server(name="TestSessionServer")
+
+        # Register the tool handler
+        @server.call_tool()
+        async def handle_call_tool(name: str, arguments: dict | None) -> list:
+            nonlocal ev_tool_called, ev_tool_commplete
+            if name == "slow_tool":
+                ev_tool_called.set()
+                with anyio.CancelScope():
+                    with anyio.fail_after(10):  # Long enough to ensure we can cancel
+                        await ev_cancelled.wait()
+                    ev_tool_commplete.set()
+                    return []
+
+            raise ValueError(f"Unknown tool: {name}")
+
+        # Register the tool so it shows up in list_tools
+        @server.list_tools()
+        async def handle_list_tools() -> list[types.Tool]:
+            return [
+                types.Tool(
+                    name="slow_tool",
+                    description="A slow tool that takes 10 seconds to complete",
+                    inputSchema={},
+                )
+            ]
+
+        return server
+
+    async def make_request(client_session: ClientSession):
+        nonlocal ev_cancelled
+        try:
+            await client_session.call_tool(
+                "slow_tool",
+                cancellable=False,
+                read_timeout_seconds=timedelta(seconds=10),
+            )
+        except McpError:
+            pytest.fail("Request should not have been cancelled")
+
+    async with create_connected_server_and_client_session(
+        make_server()
+    ) as client_session:
+        async with anyio.create_task_group() as tg:
+            tg.start_soon(make_request, client_session)
+
+            # Wait for the request to be in-flight
+            with anyio.fail_after(1):  # Timeout after 1 second
+                await ev_tool_called.wait()
+
+            # Cancel the task via task group
+            tg.cancel_scope.cancel()
+            ev_cancelled.set()
+
+            # Check server completed regardless
+            with anyio.fail_after(1):
+                await ev_tool_commplete.wait()