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>>