I trained SynthonGPT, a transformer which is trained in the "language" of chemical building blocks and reactions. Those are chosen based on an existing chemical space, so that any valid sequence of reaction and synthon tokens produces a real drug-like molecule you can actually order (or synthesize yourself in one or two steps of synthesis). Thanks to this trick, it has zero hallucinations: in contrast to GPT-5 which often produces invalid or nonexisting structures, as well as previous models (Chemberta, MolGPT etc.).