Retrosynthesis Planning via Worst-path Policy Optimisation in Tree-structured MDPs

Retrosynthesis planning aims to decompose target molecules into available building blocks, forming a synthetic tree where each internal node represents an intermediate compound and each leaf ideally corresponds to a purchasable reactant. However, this tree becomes invalid if any leaf node is not a valid building block, making the planning process vulnerable to the weakest link in the synthetic route. Existing methods often optimise for average performance across branches, failing to account for this worst-case sensitivity.