System-Prompt Anchoring with Cross-Attention Layers

arXiv:2605.09737v3 Announce Type: replace Abstract: Cross-attention provides a dedicated route from a selected information source into a model's computation, but the effect of where that route is inserted remains underexplored. We study this question when the source is a privileged system-prompt span. We insert Cross-Attention Layer (CAL) blocks between the system prompt and text while keeping the causal-decoder backbone frozen. A ten-configuration sweep on a 1.5B backbone shows that performance is task-dependent and strongly affected by placement: later placements are generally more effective and parameter-efficient. In an 8B scaling study, we train only the overall best configuration and compare it with parameter-matched adaptation baselines. Across the evaluated benchmarks, the effects remain task-dependent: cross-attention changes instruction-following and security behavior while largely preserving general-task performance. Together, these experiments characterize placement as an important design variable when injecting system-prompt information through cross-attention.