Continued improvement in computing efficiency requires functional specialization of hardware designs. We present an agile design flow for custom SoCs using the Chipyard framework, an integrated SoC research and implementation environment for custom systems. Chipyard includes configurable, composable, open-source, generator-based designs that can be used across multiple stages of the hardware development flow while maintaining design intent and integration consistency. Through cloud FPGA simulation and rapid ASIC implementation, we demonstrate an iterative agile hardware design cycle which enables continuous validation of physically-realizable customized systems.