No MediaTypeFormatter is available to read an object of type 'String' from content with media type 'text/plain'

Try using ReadAsStringAsync() instead.

 var foo = resp.Content.ReadAsStringAsync().Result;

The reason why it ReadAsAsync<string>() doesn't work is because ReadAsAsync<> will try to use one of the default MediaTypeFormatter (i.e. JsonMediaTypeFormatter, XmlMediaTypeFormatter, ...) to read the content with content-type of text/plain. However, none of the default formatter can read the text/plain (they can only read application/json, application/xml, etc).

By using ReadAsStringAsync(), the content will be read as string regardless of the content-type.

Or you can just create your own MediaTypeFormatter. I use this for text/html. If you add text/plain to it, it'll work for you too:

public class TextMediaTypeFormatter : MediaTypeFormatter
{
    public TextMediaTypeFormatter()
    {
        SupportedMediaTypes.Add(new MediaTypeHeaderValue("text/html"));
    }

    public override Task<object> ReadFromStreamAsync(Type type, Stream readStream, HttpContent content, IFormatterLogger formatterLogger)
    {
        return ReadFromStreamAsync(type, readStream, content, formatterLogger, CancellationToken.None);
    }

    public override async Task<object> ReadFromStreamAsync(Type type, Stream readStream, HttpContent content, IFormatterLogger formatterLogger, CancellationToken cancellationToken)
    {
        using (var streamReader = new StreamReader(readStream))
        {
            return await streamReader.ReadToEndAsync();
        }
    }

    public override bool CanReadType(Type type)
    {
        return type == typeof(string);
    }

    public override bool CanWriteType(Type type)
    {
        return false;
    }
}

Finally you have to assign this to the HttpMethodContext.ResponseFormatter property.

I know this is an older question, but I felt the answer from t3chb0t led me to the best path and felt like sharing. You don't even need to go so far as implementing all the formatter's methods. I did the following for the content-type "application/vnd.api+json" being returned by an API I was using:

public class VndApiJsonMediaTypeFormatter : JsonMediaTypeFormatter
{
    public VndApiJsonMediaTypeFormatter()
    {
        SupportedMediaTypes.Add(new MediaTypeHeaderValue("application/vnd.api+json"));
    }
}

Which can be used simply like the following:

HttpClient httpClient = new HttpClient("http://api.someaddress.com/");
HttpResponseMessage response = await httpClient.GetAsync("person");

List<System.Net.Http.Formatting.MediaTypeFormatter> formatters = new List<System.Net.Http.Formatting.MediaTypeFormatter>();
formatters.Add(new System.Net.Http.Formatting.JsonMediaTypeFormatter());
formatters.Add(new VndApiJsonMediaTypeFormatter());

var responseObject = await response.Content.ReadAsAsync<Person>(formatters);

Super simple and works exactly as I expected.