Reframing ontology fact acquisition during large language model fine-tuning as a time-to-event process.

Introduction: Large language models can be fine-tuned to improve retrieval of biomedical facts that are not reliably accessible after pretraining. We used ontology term-identifier mappings from the Human Phenotype Ontology (HPO) and Gene Ontology (GO) as structured biomedical facts to study fact acquisition during fine-tuning. Methods: Each ontology fact consisted of a term and its corresponding machine-readable identifier. We reframed fact acquisition as a discrete time-to-event process indexed by training epoch. Llama-3.1-8B Instruct was fine-tuned on HPO and GO ontology facts. Deterministic retrieval accuracy was assessed at baseline and after each fine-tuning epoch. Repeated stochastic decoding at baseline was used to probe for latent parametric support. Baseline-incorrect facts that were recovered at least once under stochastic decoding were classified as Results: At baseline, the model correctly retrieved 1.1% (9/800) of HPO facts and 5.6% (44/802) of GO facts. After 20 epochs of supervised fine-tuning, correct retrieval increased to 71.9% (575/800) for HPO-trained facts, 61.8% (248/401) for GO-trained facts, and 11.2% (45/401) for GO-untrained facts. Latent-knowledge-positive facts were acquired more efficiently than latent-knowledge-negative facts. Corpus-based fact support in the biomedical literature had smaller positive effects on fact acquisition. Untrained-fact acquisition for withheld GO facts was uncommon, occurring in 5.8% (22/378) of baseline-incorrect withheld facts, but was more likely for latent-knowledge-positive facts. Among GO facts that were correct at baseline, fact loss was more frequent for untrained facts than for trained facts, suggesting that continued training exposure was associated with greater retention of already-correct mappings. Discussion: Ontology facts provide a useful experimental model for studying fact acquisition during large language model fine-tuning. A time-to-event framework reveals not only whether facts are acquired, but also when they are acquired and whether they are later lost. The findings suggest that pretraining-derived latent knowledge, detectable through stochastic decoding, influences the rate and stability of fact acquisition during fine-tuning. This framework may help evaluate fine-tuning strategies, curriculum design, and the durability of learned biomedical facts.