@BrennanConroy Although the issue title cites Swagger this doesn't seem to be OpenAPI related. Any hunch as to what is going on here?

Any hunch as to what is going on here?

Yeah, looks like no one is adding an Accept header on negotiate, and I'd guess other requests as well. We'd need to figure out what header value(s) are acceptable.

Looking at the browser it sends "*/*" for the negotiate request.

@BrennanConroy Although the issue title cites Swagger this doesn't seem to be OpenAPI related. Any hunch as to what is going on here?

My apologies, I was working on both SignalR and Swagger at the time and it seems I had a brain fart while naming the issue.

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@BrennanConroy Might the following changes within HttpConnection.cs be a viable solution to this ?

1. Adding httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("*/*")); to CreateHttpClient()

var httpClient = new HttpClient(httpMessageHandler);
httpClient.Timeout = HttpClientTimeout;
httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("*/*"));

2. Ensuring the Accept header is present in NegotiateAsync()

 // Set the Accept header to "*/*"
request.Headers.Accept.Clear();
request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("*/*"));

In negotiate yes (although Clear seems unnecessary).
Does having default headers on HttpClient also add them to requests if the HttpRequestMessage added a specific value for the header already? Might want to just set Accept on all call sites where we send requests if DefaultRequestHeaders appends.

@BrennanConroy I believe the HttpRequestMessage header value would override the DefaultRequestHeaders header value. So, I agree that we might want to just ensure all call sites set Accept to the appropriate value when sending requests. Other than NegotiateAsync() are there any other call sites or areas you'd liked checked?

There is the send from LongPolling and ServerSentEvents, and the get from LongPolling. The ServerSentEvents "get" already sets Accept.