fix: auto-send notifications/cancelled on anyio task cancellation

src/mcp/shared/session.py

@@ -23,6 +23,7 @@
     INVALID_PARAMS,
     REQUEST_TIMEOUT,
     CancelledNotification,
+    CancelledNotificationParams,
     ClientNotification,
     ClientRequest,
     ClientResult,
@@ -269,6 +270,7 @@ async def send_request(
             # Store the callback for this request
             self._progress_callbacks[request_id] = progress_callback
 
+        request_sent = False
         try:
             target = request_data.get("params", {}).get("name")
             span_name = f"MCP send {request.method} {target}" if target else f"MCP send {request.method}"
@@ -284,6 +286,7 @@ async def send_request(
 
                 jsonrpc_request = JSONRPCRequest(jsonrpc="2.0", id=request_id, **request_data)
                 await self._write_stream.send(SessionMessage(message=jsonrpc_request, metadata=metadata))
+                request_sent = True
 
                 # request read timeout takes precedence over session read timeout
                 timeout = request_read_timeout_seconds or self._session_read_timeout_seconds
@@ -301,6 +304,26 @@ async def send_request(
                 else:
                     return result_type.model_validate(response_or_error.result, by_name=False)
 
+        except anyio.get_cancelled_exc_class():
+            # Automatically notify the other side when a task/scope is cancelled,
+            # so the peer can abort work for a request nobody is waiting for.
+            if request_sent:
+                with anyio.CancelScope(shield=True):
+                    try:
+                        # Add a short timeout to prevent deadlock if the transport buffer is full
+                        with anyio.move_on_after(2.0):
+                            await self.send_notification(
+                                CancelledNotification(  # type: ignore[arg-type]
+                                    params=CancelledNotificationParams(
+                                        request_id=request_id,
+                                        reason="Task cancelled",
+                                    )
+                                )
+                            )
+                    except Exception:
+                        pass  # Transport may already be closed
+            raise
+
         finally:
             self._response_streams.pop(request_id, None)
             self._progress_callbacks.pop(request_id, None)

tests/server/test_cancel_handling.py

@@ -248,3 +248,64 @@ async def run_server():
             # Without the fixes: RuntimeError (dict mutation) or ClosedResourceError
             # (respond after write-stream close) escapes run_server and this hangs.
             await server_run_returned.wait()
+
+
+@pytest.mark.anyio
+async def test_anyio_cancel_scope_sends_cancelled_notification() -> None:
+    """Cancelling a call_tool via anyio cancel scope should automatically
+    send notifications/cancelled to the server, causing it to abort the handler."""
+
+    tool_started = anyio.Event()
+    handler_cancelled = anyio.Event()
+
+    async def handle_list_tools(ctx: ServerRequestContext, params: PaginatedRequestParams | None) -> ListToolsResult:
+        return ListToolsResult(
+            tools=[
+                Tool(
+                    name="slow_tool",
+                    description="A slow tool for testing cancellation",
+                    input_schema={},
+                )
+            ]
+        )
+
+    async def handle_call_tool(ctx: ServerRequestContext, params: CallToolRequestParams) -> CallToolResult:
+        if params.name == "slow_tool":
+            tool_started.set()
+            try:
+                await anyio.sleep_forever()
+            except anyio.get_cancelled_exc_class():
+                handler_cancelled.set()
+                raise
+        raise ValueError(f"Unknown tool: {params.name}")  # pragma: no cover
+
+    server = Server(
+        "test-server",
+        on_list_tools=handle_list_tools,
+        on_call_tool=handle_call_tool,
+    )
+
+    async with Client(server) as client:
+        # Cancel the call_tool via anyio scope cancellation.
+        # send_request should automatically send notifications/cancelled.
+        async with anyio.create_task_group() as tg:
+
+            async def do_call() -> None:
+                with anyio.CancelScope() as scope:
+                    # Store scope so the outer task can cancel it
+                    do_call.scope = scope  # type: ignore[attr-defined]
+                    await client.call_tool("slow_tool", {})
+
+            tg.start_soon(do_call)
+
+            # Wait for the server handler to start
+            await tool_started.wait()
+
+            # Cancel the client-side scope — this should trigger auto-notification
+            do_call.scope.cancel()  # type: ignore[attr-defined]
+
+        # Give the server a moment to process the cancellation
+        await anyio.sleep(0.1)
+
+        # The server handler should have been cancelled via the notification
+        assert handler_cancelled.is_set(), "Server handler was not cancelled — notifications/cancelled was not sent"