Hongwei Ruan

Seth Z. Zhao, Luobin Wang, Hongwei Ruan et al.

Open-loop (OL) to closed-loop (CL) gap (OL-CL gap) exists when OL-pretrained policies scoring high in OL evaluations fail to transfer effectively in closed-loop (CL) deployment. In this paper, we unveil the root causes of this systemic failure and propose a practical remedy. Specifically, we demonstrate that OL policies suffer from Observational Domain Shift and Objective Mismatch. We show that while the former is largely recoverable with adaptation techniques, the latter creates a structural inability to model complex reactive behaviors, which forms the primary OL-CL gap. We find that a wide range of OL policies learn a biased Q-value estimator that neglects both the reactive nature of CL simulations and the temporal awareness needed to reduce compounding errors. To this end, we propose a Test-Time Adaptation (TTA) framework that calibrates observational shift, reduces state-action biases, and enforces temporal consistency. Extensive experiments show that TTA effectively mitigates planning biases and yields superior scaling dynamics than its baseline counterparts. Furthermore, our analysis highlights the existence of blind spots in standard OL evaluation protocols that fail to capture the realities of closed-loop deployment.

Zichen Zhang, Kunlong Zhang, Hongwei Ruan et al.

Transformer-based models have advanced the field of question answering, but multi-hop reasoning, where answers require combining evidence across multiple passages, remains difficult. This paper presents a comprehensive evaluation of retrieval strategies for multi-hop question answering within a retrieval-augmented generation framework. We compare cosine similarity, maximal marginal relevance, and a hybrid method that integrates dense embeddings with lexical overlap and re-ranking. To further improve retrieval, we adapt the EfficientRAG pipeline for query optimization, introducing token labeling and iterative refinement while maintaining efficiency. Experiments on the HotpotQA dataset show that the hybrid approach substantially outperforms baseline methods, achieving a relative improvement of 50 percent in exact match and 47 percent in F1 score compared to cosine similarity. Error analysis reveals that hybrid retrieval improves entity recall and evidence complementarity, while remaining limited in handling distractors and temporal reasoning. Overall, the results suggest that hybrid retrieval-augmented generation provides a practical zero-shot solution for multi-hop question answering, balancing accuracy, efficiency, and interpretability.