This paper proposes a response compactor, named ChiYun compactor, to compact scan-out responses in the presence of unknown values. By adding storage elements into an Xor network, a ChiYun compactor can offer multiple chances for a scan-out response to be observed at ATE channels in one to several scan-shift cycles. We also develop a mathematical analysis to predict the percent-age of scan-out responses masked by the unknown values for the ChiYun compactor. With this analysis, we can derive the optimal, configuration of a ChiYun compactor for minimizing the masking of scan-out responses. We further propose a selection scheme for the ChiYun compactor to selectively observe partial Xor results for improving the fault coverage. The experimental results demonstrate the effectiveness of the proposed mathematical analysis and the selection scheme. We also demonstrate that the unknown tolerance of a ChiYun compactor is higher than that of a state-of-the-art response compactor proposed in .