Many of these generative chemistry 8-12 models represent molecules through SMILES strings that provide a text encoding of molecular graphs. Because molecule generation is thus rendered as a text generation task, syntax errors lead to invalid SMILES strings that cannot be converted to a molecular structure 14 . Other models represent molecules directly through a weighted graph. Molecule generation in this case is cast as a graph generation task. Graph generation schemes where a single-atom is added at a time 15 (atom-by-atom generation) also lead to chemically invalid intermediate graphs requiring further correction. Models such as the junction tree variational autoencoder (JT-VAE) obviate this issue by generating graphs in a sub-structure by sub-structure manner thus consistently yielding valid molecules. The ability of generative chemistry models to consistently propose novel valid molecules is key towards gaining the confidence and uptake of medicinal chemistry teams.