Context-Constrained Retrieval for Evidence-Grounded Retrieval-Augmented Generation

The Hong Kong University of Science and Technology
Department of Computer Science and Engineering


MPhil Thesis Defence


Title: "Context-Constrained Retrieval for Evidence-Grounded 
Retrieval-Augmented Generation"

By

Mr. Kwun Hang LAU


Abstract:

Retrieval-Augmented Generation (RAG) grounds large language models with 
external evidence, but reliable generation requires retrieval beyond semantic 
similarity. In multi-hop and temporal questions, a retriever must recover 
complete evidence chains and satisfy query-specific constraints. Existing 
dense retrievers and graph-based RAG systems often rely on static similarity 
scores or fixed graph transitions, causing partial retrieval, semantic drift, 
and hub-node bias.

This thesis studies context-constrained retrieval for evidence-grounded RAG. 
The main contribution is CatRAG, a query-adaptive graph retrieval framework 
that addresses the Static Graph Fallacy in graph-based RAG. CatRAG converts a 
fixed retrieval graph into a query-adaptive navigation structure through 
Symbolic Anchoring, Dynamic Edge Weighting, and Key-Fact Enhancement, 
allowing Personalized PageRank traversal to focus on query-relevant evidence 
paths. To support the broader thesis view, we also study Time-Aware RAG for 
temporal coverage and Interval-Predicate Approximate Nearest Neighbor Search 
with a Unified Dominance Graph for retrieval under interval predicates.

Experiments on multi-hop question answering and fact verification show that 
CatRAG improves retrieval quality, answer quality, and reasoning 
completeness, measured by Full Chain Retrieval and Joint Success Rate. 
Additional analysis confirms reduced hub bias and the effectiveness of the 
proposed modules. Overall, this thesis shows that reliable RAG should treat 
retrieval as constrained evidence selection rather than semantic 
nearest-neighbor search alone.


Date:                   Thursday, 30 July 2026

Time:                   2:00pm - 4:00pm

Venue:                  Room 4475
                        Lifts 25-26

Chairman:               Prof. Raymond WONG

Committee Members:      Prof. Xiaofang ZHOU (Supervisor)
                        Prof. Song GUO