This paper assesses the sensitivity of the risk buffers, or capital requirements, of central counterparties clearing over-the-counter derivatives trades to a range of model inputs. It finds capital requirements to be highly sensitive to whether key model parameters are calibrated on a point-in-time versus stress-period basis, whether the risk tolerance metric adequately captures tail-risk events, and the ability – or lack thereof – to define exposures on the basis of netting sets spanning multiple risk factors. Our results suggest that there are considerable benefits from prudential authorities adopting a more prescriptive approach to central counterparties’ risk buffers, in line with recent enhancement of the capital regime for banks’ trading books.