We model the fastest moving (vtot > 300 km s−1) local (D 3 kpc) halo stars using cosmological simulations and six-dimensional Gaia data. Our approach is to use our knowledge of the assembly history and phase-space distribution of halo stars to constrain the form of the high-velocity tail of the stellar halo. Using simple analytical models and cosmological simulations, we find that the shape of the high-velocity tail is strongly dependent on the velocity anisotropy and number density profile of the halo stars - highly eccentric orbits and/or shallow density profiles have more extended high-velocity tails. The halo stars in the solar vicinity are known to have a strongly radial velocity anisotropy, and it has recently been shown the origin of these highly eccentric orbits is the early accretion of a massive (Mstar ∼ 109 M☉) dwarf satellite. We use this knowledge to construct a prior on the shape of the high-velocity tail. Moreover, we use the simulations to define an appropriate outer boundary of 2r200, beyond which stars can escape. After applying our methodology to the Gaia data, we find a local (r0 = 8.3 kpc) escape speed of vesc(r0) = 528+−2425 km s−1. We use our measurement of the escape velocity to estimate the total Milky Way mass, and dark halo concentration: M200,tot = 1.00+−003124 × 1012 M☉, c200 = 10.9+−4343. Our estimated mass agrees with recent results in the literature that seem to be converging on a Milky Way mass of M200,tot ∼ 1012 M☉
