Cancellation

MCP supports cancellation of in-flight requests. Either side can cancel a previously issued request, and CancellationToken parameters on MCP methods are wired to send and receive notifications/cancelled notifications over the protocol.

How cancellation maps to MCP notifications

When a CancellationToken passed to a client method (such as CallToolAsync) is cancelled, a notifications/cancelled notification is sent to the server with the request ID. On the server side, the CancellationToken provided to the tool method is then triggered, allowing the handler to stop work gracefully. This same mechanism works in reverse for server-to-client requests.

Server-side cancellation handling

Server tool methods receive a CancellationToken that is triggered when the client sends a cancellation notification. Pass this token through to any async operations so they stop promptly:

[McpServerTool, Description("A long-running computation")]
public static async Task<string> LongComputation(
    [Description("Number of iterations")] int iterations,
    CancellationToken cancellationToken)
{
    for (int i = 0; i < iterations; i++)
    {
        await Task.Delay(1000, cancellationToken);
    }

    return $"Completed {iterations} iterations.";
}

When the client sends a cancellation notification, the OperationCanceledException propagates back to the client as a cancellation response.

Cancellation notification details

The cancellation notification includes:

• RequestId: The ID of the request to cancel, allowing the receiver to correlate the cancellation with the correct in-flight request.
• Reason: An optional human-readable reason for the cancellation.

Cancellation notifications can be observed by registering a handler. For broader interception of notifications and other messages, McpMessageFilter delegates can be added to the IncomingFilters collection in Filters.

mcpClient.RegisterNotificationHandler(
    NotificationMethods.CancelledNotification,
    (notification, ct) =>
    {
        var cancelled = notification.Params?.Deserialize<CancelledNotificationParams>(
            McpJsonUtilities.DefaultOptions);
        if (cancelled is not null)
        {
            Console.WriteLine($"Request {cancelled.RequestId} cancelled: {cancelled.Reason}");
        }
        return default;
    });