Sub-Token Routing in LoRA for Adaptation and Query-Aware KV Compression

Sub-token routing provides a finer compression axis for transformer efficiency than the coarse units used in most prior work, such as tokens, pages, heads, or layers. In this paper, we study routing within a token representation itself in LoRA-adapted transformers. We consider two settings. In the query-independent setting, we combine routed subspace LoRA with value-group routing on the KV path for compression-aware language modeling. In the query-aware setting, we use a predictor-based selector to allocate a global retention budget over context-token/value-group pairs using query-conditioned relevance. Experiments show that the query-independent design improves language-model quality under reduced KV budgets, while the query-aware design preserves downstream behavior well under KV compression. We further show that sub-token routing is most effective as a complementary compression axis to token-level query-aware selection: token-level methods decide which tokens survive globally, while sub-token routing determines how the surviving tokens are compressed internally. Their combination enables deeper KV compression at nearly unchanged task accuracy.