Working on F1-OPT in 17/18 in the US, thought I file a 1040NR, Sprintax tells me I am a resident. 2018 255 days (F1-OPT), 2017 326 days (F1-OPT), 2016 160 days (F1)

Summary of days in the US: 2018 255 days (F1-OPT) plus 20 days (ESTA), 2017 83 days (F1-OPT) plus 243 days (F1), 2016 160 days (F1), 2015 7 days (ESTA), 2013 25 days (F1), 2012 7 days (F1), 2011 20 days (F1), 2010 220 days (F1), 2009 120 days (F1)