Retrofit2 error java.io.EOFException: End of input at line 1 column 1

just return void instead, if the body is empty

@PATCH("alerts/{alert_id}/accept") Call<Void> accept_invited_alerts(@Header("X-Api-Token") String api_token, @Path("alert_id") int alert_id);

for retrofit with Rx java you can use something like this

@PATCH("alerts/{alert_id}/accept") Observable<Response<Void>> accept_invited_alerts(@Header("X-Api-Token") String api_token, @Path("alert_id") int alert_id);

EDIT: For kotlin

@PATCH("alerts/{alert_id}/accept")
fun accept_invited_alerts(@Header("X-Api-Token")  api_token: String, @Path("alert_id") alert_id: Int): Call<Unit>

and

@PATCH("alerts/{alert_id}/accept")
fun accept_invited_alerts(@Header("X-Api-Token") api_token: String, @Path("alert_id") alert_id: Int): Observable<Response<Unit>>

You can create NullOnEmptyConverterFactory.class :

import java.io.IOException;
import java.lang.annotation.Annotation;
import java.lang.reflect.Type;

import okhttp3.ResponseBody;
import retrofit2.Converter;
import retrofit2.Retrofit;


public class NullOnEmptyConverterFactory extends Converter.Factory {

    @Override
    public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[] annotations, Retrofit retrofit) {
        final Converter<ResponseBody, ?> delegate = retrofit.nextResponseBodyConverter(this, type, annotations);
        return new Converter<ResponseBody, Object>() {
            @Override
            public Object convert(ResponseBody body) throws IOException {
                if (body.contentLength() == 0) return null;
                return delegate.convert(body);               
            }
        };
    }
}

and add to code create. For ex:

 UploadImageNghiemThuApi uploadService = new Retrofit.Builder()
            .baseUrl(Config.URL+"/")
            .client(okHttpClient)
            // -----add here-------
            .addConverterFactory(new NullOnEmptyConverterFactory())
            //---------------------
            .addConverterFactory(GsonConverterFactory.create())
            .build()
            .create(UploadImageNghiemThuApi.class);

I hope it can help your problem. Thanks!

EDIT !!!

For a kotlin usecase you can check this

class NullOnEmptyConverterFactory : Converter.Factory() {
override fun responseBodyConverter(
    type: Type,
    annotations: Array<Annotation>,
    retrofit: Retrofit
): Converter<ResponseBody, *> {
    val delegate: Converter<ResponseBody, *> =
        retrofit.nextResponseBodyConverter<Any>(this, type, annotations)
    return Converter { body -> if (body.contentLength() == 0L) null else delegate.convert(body) }
}

}