Gist showing serialization issue.

ServiceStackDictionarySerialize

[Route("/tmp")]
public class Tmp:IReturn<TmpResponse>
{
    public string Param { get; set; }
}

public class TmpResponse
{
  public Dictionary<KeyClass, List<ResponseData>> ResponseOptions { get; set; }
}

public class KeyClass
{
  public string FirstString { get; set; }
  public string SecondString { get; set; }
}

public class ResponseData
{
  public string FirstProperty { get; set; }
  public int SecondProperty { get; set; }
}

public class TmpService : Service
{
  public object Get(Tmp request)
  {
    //return on client:
    // {"{\"FirstString\":"first",\"SecondString\":"second"}":[{"FirstProperty":"first","SecondProperty":1}]}
    //which is not deserializable to object due to quotes
    return new Dictionary<KeyClass, List<ResponseData>>
    {
      {
        new KeyClass() {FirstString = "first", SecondString = "second"},
        new List<ResponseData>() {new ResponseData() {FirstProperty = "first", SecondProperty = 1}}
      }
    };
  }
}