dotnet/yarp · error · InvalidOperationException

{typeof(IHttpSysRequestDelegationFeature).FullName} is missi

Error message

{typeof(IHttpSysRequestDelegationFeature).FullName} is missing.

What it means

Thrown by HttpSysDelegator.DelegateRequest when the per-request IHttpSysRequestDelegationFeature is not present on the HttpContext features. This feature is supplied by the Http.sys server per connection; its absence means the request is not running on Http.sys or the feature was stripped. The delegator cannot forward the request to an Http.sys queue without it.

Source

Thrown at src/ReverseProxy/Delegation/HttpSysDelegator.cs:63

    {
        if (_serverDelegationFeature is not null)
        {
            var key = new QueueKey(queueName, urlPrefix);
            if (_queues.TryGetValue(key, out var queueWeakRef) && queueWeakRef.TryGetTarget(out var queue))
            {
                var detachedQueueState = queue.Detach();
                Log.QueueReset(_logger, queueName, urlPrefix, detachedQueueState);
            }
        }
    }

    public void DelegateRequest(HttpContext context, DestinationState destination)
    {
        ArgumentNullException.ThrowIfNull(context);
        ArgumentNullException.ThrowIfNull(destination);

        var requestDelegationFeature = context.Features.Get<IHttpSysRequestDelegationFeature>()
                    ?? throw new InvalidOperationException($"{typeof(IHttpSysRequestDelegationFeature).FullName} is missing.");

        if (!requestDelegationFeature.CanDelegate)
        {
            throw new InvalidOperationException(
                "Current request can't be delegated. Either the request body has started to be read or the response has started to be sent.");
        }

        if (_serverDelegationFeature is null || !_queuesPerDestination.TryGetValue(destination, out var queue))
        {
            Log.QueueNotFound(_logger, destination);
            context.Response.StatusCode = StatusCodes.Status503ServiceUnavailable;
            context.Features.Set<IForwarderErrorFeature>(new ForwarderErrorFeature(ForwarderError.NoAvailableDestinations, ex: null));
            return;
        }

        Delegate(context, destination, _serverDelegationFeature, requestDelegationFeature, queue, _logger, shouldRetry: true);

        static void Delegate(

View on GitHub (pinned to bd11867bee)

Solutions

1. Ensure the application is hosted on Http.sys (UseHttpSys) so IHttpSysRequestDelegationFeature is populated.
2. Confirm UseHttpSysDelegation is only in the pipeline when Http.sys is the server.
3. Add a server-feature guard before calling DelegateRequest to skip non-Http.sys requests.

Example fix

// before
_delegator.DelegateRequest(context, destination); // throws if feature missing
// after
if (context.Features.Get<IHttpSysRequestDelegationFeature>() is { } feat && feat.CanDelegate)
{
    _delegator.DelegateRequest(context, destination);
}
else
{
    // fall back to normal proxying or 503
}

var feat = context.Features.Get<IHttpSysRequestDelegationFeature>();
if (feat is null)
{
    context.Response.StatusCode = 503;
    return;
}

static bool HasRequestDelegationFeature(HttpContext ctx) =>
    ctx.Features.Get<IHttpSysRequestDelegationFeature>() is not null;

try { _delegator.DelegateRequest(context, destination); }
catch (InvalidOperationException ex) when (ex.Message.Contains("is missing"))
{ context.Response.StatusCode = 503; await context.Response.WriteAsync("Delegation unavailable"); }

When it happens

Trigger: DelegateRequest is invoked for a request that is not handled by the Http.sys server, or where the delegation feature was not initialized for that connection. This can happen if delegation middleware runs for a non-Http.sys pipeline.

Common situations: Mixing Kestrel-hosted requests with HttpSysDelegator. A misconfigured pipeline where delegation logic runs for requests not backed by Http.sys. Feature removed by another middleware.

Related errors

• {typeof(IHttpSysRequestDelegationFeature).FullName} is not a
• Current request can't be delegated. Either the request body
• The IReverseProxyFeature Destinations collection was not set
• The IReverseProxyFeature Cluster was not set.
• Chosen destination has no model set: '{destination.Destinati

AI-assisted analysis of dotnet/yarp@bd11867bee (2026-08-13). Data as JSON: /api/errors/c67b6b59ff446767. Report an issue: GitHub.