The tin isotope ¹⁰⁰Sn is of singular interest for nuclear structure due to its closed-shell proton and neutron configurations. It is also the heaviest nucleus comprising protons and neutrons in equal numbers—a feature that enhances the contribution of the short-range proton–neutron pairing interaction and strongly influences its decay via the weak interaction. Decay studies in the region of ¹⁰⁰Sn have attempted to prove its doubly magic character¹ but few have studied it from an ab initio theoretical perspective^2,3, and none of these has addressed the odd-proton neighbours, which are inherently more difficult to describe but crucial for a complete test of nuclear forces. Here we present direct mass measurements of the exotic odd-proton nuclide ¹⁰⁰In, the beta-decay daughter of ¹⁰⁰Sn, and of ⁹⁹In, with one proton less than ¹⁰⁰Sn. We use advanced mass spectrometry techniques to measure ⁹⁹In, which is produced at a rate of only a few ions per second, and to resolve the ground and isomeric states in ¹⁰¹In. The experimental results are compared with ab initio many-body calculations. The 100-fold improvement in precision of the ¹⁰⁰In mass value highlights a discrepancy in the atomic-mass values of ¹⁰⁰Sn deduced from recent beta-decay results^4,5.