The FilterByRequest() works as a pre-filter that is called before the action executes, so you won't have the response on the filter delegate.

The option to discard events based on the response is to do a post-filter via a custom action, for example on your startup:

using Audit.WebApi;

Audit.Core.Configuration.AddCustomAction(ActionType.OnEventSaving, scope =>
{
    var ev = scope.GetWebApiAuditAction();
    if (ev?.ResponseStatusCode == 500 || ev?.ResponseStatusCode == 400)
    {
        scope.Discard();
    }
});