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To get access to the raw response, use ResponseBody from okhttp as your call type.

Call<ResponseBody> login(...)

In your callback, you can check the response code with the code method of the response. This applies to any retrofit 2 return type, because your callback always gets a Response parameterized with your actual return type. For asynchronous --

Call<ResponseBody> myCall = myApi.login(...)
myCall.enqueue(new Callback<ResponseBody>() {
    @Override
    public void onResponse(Response<ResponseBody> response, Retrofit retrofit) {
        // access response code with response.code()
        // access string of the response with response.body().string()
    }

    @Override
    public void onFailure(Throwable t) {
        t.printStackTrace();
    }
});