Request Cancellation

MCP supports cancelling in-flight requests using the notifications/cancelled notification.

Client cancels a request

A client notifies the server that a request is cancelled via notify_cancellation():

use rust_mcp_sdk::schema::CancelledNotificationParams;

client
    .notify_cancellation(CancelledNotificationParams {
        request_id: Some(request_id),
        reason: Some("User cancelled".into()),
        meta: None,
    })
    .await?;

Server handles cancellation

The ServerHandler has a default handler for cancellation notifications that you can override:

async fn handle_cancelled_notification(
    &self,
    params: CancelledNotificationParams,
    _runtime: Arc<dyn McpServer>,
) -> Result<(), RpcError> {
    println!(
        "Request {:?} cancelled: {:?}",
        params.request_id, params.reason
    );
    Ok(())
}

Cancellation is cooperative - the server receives the notification and should stop processing if possible.