Designing proper chemical synthesis reactions and routes towards target compounds is one of core tasks during both drug discovery and process development, requiring significant time and cost investments . Currently, due to the enormous design space and necessity of experimental validation, this process mainly relies on expert chemists and chemical engineers to go through iterative designmake-test-analyze cycles to identify an efficient synthesis route . The multifaceted and complex requirements for synthesis reaction design, such as efficiency, cost, sustainability, safety, scalability, and impurity control, make it hard to formulate this task into a well-defined problem that can be tackled algorithmically and autonomously without customized inputs and decisions from experts .