Disclosed are compounds, compositions, and methods involving theranostic capture agent for a target where the capture agent is (a) a precursor that can be loaded with a detectable moiety, a therapeutic moiety, or both, (b) loaded with a detectable moiety, (c) loaded with a therapeutic moiety, or (d) loaded with both a detectable moiety and a therapeutic moiety. Also disclosed are stable peptide-based PSMA capture agents and methods of use as detection agents.