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arXiv · 2608.14210

How Much Do Legal RAG Systems Still Hallucinate?

Souvick Das·Sallam Abualhaija·Domenico Bianculli
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Abstract

Hallucination is a major challenge for retrieval-augmented generation (RAG) systems in the legal domain, where ungrounded answers can lead to serious consequences. To better understand this problem, we conduct a fine-grained analysis of hallucination behavior in eight legal RAG systems across two legal corpora, the GDPR (in English) and a national civil law (in French). Using claim-level and answer-level evaluation, we report on hallucination density and severity, analyze performance across question categories and user personas, and validate our findings on an independent set of 142 legal-expert-authored questions. Our results show that hallucinations remain pervasive, ranging from less than 10% of responses for the best-performing systems to nearly half in the worst case. We further find that false-premise questions, containing incorrect assumptions that must be rejected, produce high hallucination rates on the manually-drafted questions.

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Souvick Das, Sallam Abualhaija, Domenico Bianculli. 2026-08-14. How Much Do Legal RAG Systems Still Hallucinate?. https://arxiv.org/abs/2608.14210

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