Here we present the first open-source radiative transfer model for computing the reflected light of exoplanets at any phase geometry, called PICASO: the planetary intensity code for atmospheric scattering observations. This code, written in Python, has heritage from a decades-old, well-known Fortran model used for several studies of planetary objects within the solar system and beyond. We have adopted it to include several methodologies for computing both direct and diffuse-scattering phase functions, and have added several updates including the ability to compute Raman scattering spectral features. Here we benchmark PICASO against two independent codes and discuss the degree to which the model is sensitive to a user’s specification for various phase functions. Then, we conduct a full information-content study of the model across a wide parameter space in temperature, cloud profile, signal-to-noise ratio, and resolving power.