you may want to complete this IRS tool as the proof you can certainly claim your partner (and show it to your partner or have them fill it out from YOUR perspective); otherwise all you have is a bunch of anonymous experts which might not win over your partner
https://www.irs.gov/help/ita/whom-may-i-claim-as-a-dependent
if the partner files and fails to check the 'CAN be claimed by someone else' button, that would be fraud.
