Add and augment conceptual docs (#1375)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Mike Kistler <mikekistler@microsoft.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>
Co-authored-by: Stephen Halter <halter73@gmail.com>

docs/concepts/cancellation/cancellation.md

@@ -0,0 +1,62 @@
+---
+title: Cancellation
+author: jeffhandley
+description: How to cancel in-flight MCP requests using cancellation tokens and notifications.
+uid: cancellation
+---
+
+## 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.
+
+[cancellation]: https://modelcontextprotocol.io/specification/2025-11-25/basic/utilities/cancellation
+[task cancellation]: https://learn.microsoft.com/dotnet/standard/parallel-programming/task-cancellation
+
+### How cancellation maps to MCP notifications
+
+When a `CancellationToken` passed to a client method (such as <xref:ModelContextProtocol.Client.McpClient.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:
+
+```csharp
+[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 $"Completed {iterations} iterations.";
+}
+```
+
+When the client sends a cancellation notification, the `OperationCanceledException` propagates back to the client as a cancellation response.
+
+### Cancellation notification details
+
+The cancellation notification includes:
+
+- **RequestId**: The ID of the request to cancel, allowing the receiver to correlate the cancellation with the correct in-flight request.
+- **Reason**: An optional human-readable reason for the cancellation.
+
+Cancellation notifications can be observed by registering a handler. For broader interception of notifications and other messages, <xref:ModelContextProtocol.Server.McpMessageFilter> delegates can be added to the <xref:ModelContextProtocol.Server.McpMessageFilters.IncomingFilters> collection in <xref:ModelContextProtocol.Server.McpServerOptions.Filters>.
+
+```csharp
+mcpClient.RegisterNotificationHandler(
+    NotificationMethods.CancelledNotification,
+    (notification, ct) =>
+    {
+        var cancelled = notification.Params?.Deserialize<CancelledNotificationParams>(
+            McpJsonUtilities.DefaultOptions);
+        if (cancelled is not null)
+        {
+            Console.WriteLine($"Request {cancelled.RequestId} cancelled: {cancelled.Reason}");
+        }
+        return default;
+    });
+```