
ACM conference paper (ICAIF): Understanding the Inner Workings of a State-Space Foundation Model for Limit Order Books
Autoregressive foundation models trained on limit order book messages offer state-of-the-art market simulation, yet remain black boxes. This paper presents the first mechanistic interpretability study of such a model: an 8.2M-parameter Mamba-3 state-space model trained only on the tokenised message stream of eight large-cap US equities. Much as sequence models build an internal board in games, it learns an internal representation of the market and its dynamics, held in its recurrent state. Order-flow imbalance decodes at R² ≈ 0.59 against 0.12 from a book snapshot, the representation extends to unseen stocks, and steering the state along a feature’s axis shifts the model’s next-message beliefs as that feature implies, monotonically in dose.



