The early phases of bridge design are particularly demanding, as engineers must evaluate a wide range of feasible solutions while balancing cost efficiency, sustainability targets, and tight project schedules. This article presents EFESTO, an innovative and user-friendly tool developed to optimize the design of composite girder bridge decks through a genetic-algorithm-based approach. The tool identifies the optimal geometric configuration of the longitudinal beams that minimize overall cost while ensuring full compliance with all structural verifications. By integrating Courbon’s theory with an analytical parametric model, EFESTO offers a fast and reliable decision-support environment capable of guiding engineers across several stages of the design workflow. Input parameters can be adjusted through an interactive Excel interface linked to Python scripts, enabling real-time evaluation of different structural configurations and facilitating the exploration of innovative, cost-effective solutions. The application of EFESTO significantly reduces design time and supports decision-making. Overall, it improves both structural performance and resource utilization, making it a powerful tool for modern composite bridge engineering.