POST request from Retrofit works in unexpected way - http

Question

There is a third-party website that uses HTTPS and where the start page performs a POST upon login. I have inspected that POST request in my browser and then I have been able to manually create the request with Fiddler's composer. Thus, depending on the credentials, I could either successfully or unsuccessfully log in with Fiddler. The return code is always 302, which comes along with either a redirect (header "Location") to the user management page or a login failed page, respectively.
However, when I create that request using the Retrofit library, it does not work. I get response code 200, which in this specific case is not to be considered a success.
In order to inspect the POST request from Retrospect, I have directed it to Fiddler (http://localhost:8888) instead of the third-party URL. If I copy that request into the composer and adjust the URL to be the third-party one, the request does work. I.e., I could not find anything wrong with the request built by Retrofit.
Does anybody have an idea what could be wrong?
My code is written in Kotlin, but should be easily understandable if you know Java:
import okhttp3.ResponseBody
import retrofit2.Call
import retrofit2.Retrofit
import retrofit2.http.*
interface MyApi {
#POST("<relative login url>")
#FormUrlEncoded
#Headers(
//...
)
fun login(
#Field("username") username: String,
#Field("password") password: String
) : Call<ResponseBody>;
}
fun main(args: Array<String>) {
val baseUrl = "https://<url>"
val retrofit = Retrofit.Builder().baseUrl(baseUrl).build()
val myApi = retrofit.create(MyApi::class.java)
val code = myApi.login("<username>", "<password>").execute().code()
println(code)
}

Answer 1

As already stated in the comments, but to make it more understandable to others too here goes an answer.
When using retrofit with okhttp redirects will be followed by default. The reason for this is because the default okhttp client is set to follow redirects. This is why you never get a 302 - The redirect is followed automatically and you get the 200 from the followed url.
You can disable this behaviour by building your retrofit instance with a properly configured okhttp client:
OkHttpClient client = new OkHttpClient.Builder()
.followRedirects(false)
.followSslRedirects(false)
.build();
Retrofit retrofit = new Retrofit.Builder()
.client(client)
// other configurations for retrofit
.build();
Notice that here we create an instance of retrofit with a client configure not to follow redirects. This will effectively make you receive the 302 and other redirect codes.
(note that I didn't fully configure the retrofit instance here to focus the answer in the important part)