I leverage the staggered roll out of VAT in Pakistan to document the role of withholding mechanism in the self-enforcement of a VAT. Focusing on firms already in the tax net, I see how their outcomes respond when the tax is extended upward to intermediates used by them. I find that the upward extension of VAT, which triggers the withholding mechanism, causes an immediate and large (more than 40 log-points) surge in sales reported by firms. The evolution of bunching above the zero-liability point and of input costs reported by firms suggest that this large response is indeed driven by the withholding mechanism. I also explore the role of withholding in the extensive margin compliance choices of firms.