dotnet/yarp · error · InvalidOperationException

is missing.

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.

Solutions

Ensure the application is hosted on Http.sys (UseHttpSys) so IHttpSysRequestDelegationFeature is populated.
Confirm UseHttpSysDelegation is only in the pipeline when Http.sys is the server.
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
}

Defensive patterns

Strategy: type-guard

Validate before calling

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

Type guard

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

Try / catch

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

Prevention

Check IHttpSysRequestDelegationFeature before delegating.
Run delegation logic only within an Http.sys-hosted pipeline.
Provide a fallback (503 or normal proxy) when the feature is absent.

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

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

Appendix: 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)