Automated unit test generation bears the promise of significantly reducing test cost and hence improving software quality. However, the maintenance cost of the automatically generated tests presents a significant barrier to adoption of this technology. To address this challenge, in previous work, we proposed a novel technique for automated and fine-grained incremental generation of unit tests through minimal augmentation of an existing test suite. In this paper we describe a tool FSX, implementing this technique. We describe the architecture, user-interface, and salient features of FSX, and specific practical use-cases of its technology. We also report on a real, large-scale deployment of FSX as a practical validation of the underlying research contribution and of automated test generation research in general. © 2016 ACM.