Security by construction is an approach to system development where security considerations are integrated into the design process from the very beginning. Threat modeling helps identify potential threats and vulnerabilities early in the system development process, assess the risk associated with each threat, and design appropriate mitigation actions. In this paper, we study threat model repair, a method to automatically suggest structural changes to the design that mitigate threats discovered by the analysis. This helps find a secure design early in the process by allowing a user to quickly iterate over different design variants.