Next month Las Vegas will host the Final Event of the DARPA Cyber grand Challenge as an all-computer cyber-defence Capture the Flag tournament. From an initial field of over 100 applicant seven teams will compete for the $3.5 million prize pool. Reproducibility is a key aspect of a sound scientific design. While perfect system state replay is impossible without a full system event recorder, DECREE has been designed to allow high determinism and reproducibility given a record of software and inputs. This reproducibility property has been built into DECREE from kernel modifications up through the entire platform stack.