Homologous recombination deficiency (HRD) is a predictive biomarker for efficacy of PARP inhibition and platinum chemotherapy but remains challenging to detect from low tumor fraction samples such as liquid biopsies. Here, we describe DirectHRD, a whole-genome sequencing (WGS) scar-based classifier that is 10x more sensitive than state-of-the-art methods. DirectHRD can detect HRD at >=1% tumor fraction using 50x WGS of cell-free DNA.