ethraid characterizes long-period bound companions using partial-orbit data by analyzing RV trends and curvature to constrain the properties, including mass and separation, of distant giant planets and other long-period companions. It jointly models radial velocity, astrometric, and direct imaging measurements in a Bayesian framework, using Doppler, astrometric, and imaging constraints to derive posterior distributions on companion parameters. A command-line interface runs orbit fits, generates plots, and reports 95% confidence intervals in a single step. The code returns raw and processed posterior samples and up to three plots: a joint two-dimensional posterior in mass–separation space, and marginalized one-dimensional PDFs and CDFs. ethraid includes fitting tools, with modules for astrometry fitting and radial-velocity fitting.