Abstract: Systems and methods are provided for co-processing of pyrolysis tar with pre-pyrolysis flash bottoms. In some aspects, the co-processing can correspond to solvent-assisted hydroprocessing. By combining pyrolysis tar and flash bottoms with a solvent, various difficulties associated with hydroprocessing of the fractions can be reduced or minimized, such as difficulties associated with hydroprocessing of high viscosity feeds and/or high sulfur feeds. Optionally, separate solvents and/or fluxes can be used for the pyrolysis tar and the flash bottoms. The resulting upgraded products can be suitable, for example, for inclusion in low sulfur fuel oils (LSFO).