Update

Author: Shane32

src/Transports.AspNetCore/WebSockets/SubscriptionsTransportWs/SubscriptionServer.cs

@@ -126,9 +126,14 @@ protected override Task OnConnectionInitAsync(OperationMessage message, bool sma
     /// as this protocol does not support the other modes. Override this method to support your own implementation.
     /// </remarks>
     protected override Task OnKeepAliveLoopAsync(TimeSpan keepAliveTimeout, KeepAliveMode keepAliveMode)
-        => base.OnKeepAliveLoopAsync(
+    {
+        if (keepAliveMode == KeepAliveMode.TimeoutWithPayload)
+            throw new NotSupportedException("The 'subscription-transport-ws' protocol does not support the 'TimeoutWithPayload' keep-alive mode.");
+
+        return base.OnKeepAliveLoopAsync(
             keepAliveTimeout,
             KeepAliveMode.Interval);
+    }
 
     private static readonly OperationMessage _keepAliveMessage = new() { Type = MessageType.GQL_CONNECTION_KEEP_ALIVE };
     /// <inheritdoc/>