There is an extra " when calling MessagePackSerializer.ConvertToJson for extension code of 100 (typeless)
In MessagePackSerializer.Json.cs at line 435
writer.Write("{\"$type\":\"" + typeNameTokenBuilder.ToString() + "}");
There is an extra " if after type (when no data after type). This will breaks the JSON format. Should be:
writer.Write("{\"$type\":" + typeNameTokenBuilder.ToString() + "}");
Cheers
Tao