Proprietary 'WL' Citations Raise Unique AI Hallucination Risks in Legal Research
Zach Warren warns 'WL' citations uniquely increase AI hallucinations in legal research.
Why it matters: Legal professionals using AI must beware proprietary citation formats like 'WL' to avoid fabricated case citations and potential sanctions. Understanding this risk is crucial to maintaining research accuracy and legal compliance.
- 'WL' citations are proprietary formats unique to certain publishers that confuse AI models.
- A 2024 Stanford study found AI hallucinations in legal queries occur 69%-88% of the time.
- Ark Legal identified 131 court cases (2023–2026) involving AI-fabricated citations; 26 led to sanctions.
- Zach Warren of Thomson Reuters highlights growing sanctions for AI-generated fictitious citations.
Proprietary legal citation formats such as the 'WL' citation—used exclusively by specific publishers—pose a notable challenge for AI tools in legal research. These unique identifiers are not widely standardized, causing AI models to misinterpret or fabricate citations, known as 'hallucination.' As detailed in Above the Law, the uncommon nature of 'WL' citations increases error risks compared to standard citation formats.
A 2024 Stanford University study found that large language models hallucinate legal facts and citations 69% to 88% of the time when queried. This high error rate has serious implications for legal research and filings.
Between 2023 and 2026, Ark Legal documented 131 court cases where AI-generated fabricated citations appeared; 26 cases resulted in formal sanctions including fines and practice suspensions (Ark Legal). A notable incident occurred in June 2026 when a federal judge sanctioned four attorneys for submitting a brief containing hallucinated citations generated by AI like ChatGPT (Law360).
Zach Warren, Senior Research Analyst at Thomson Reuters Institute, emphasizes that courts across the U.S. are sanctioning attorneys and pro se litigants for AI-generated fictitious citations. He notes, "Hallucinations remain an ongoing issue, especially when proprietary formats like 'WL' aren't verified thoroughly." Warren points out that integrating unverified 'WL' citations increases the AI’s risk of reinforcing false legal assumptions, compromising research integrity.
Instances like the 2025 apology from an attorney representing Anthropic, who relied on a hallucinated citation from the AI chatbot Claude, underscore these risks (TechCrunch).
Given these challenges, it is vital for law firms and legal technology providers to implement rigorous verification processes when using AI outputs, especially those involving proprietary citation formats like 'WL.' Doing so helps prevent costly errors, professional sanctions, and damage to reputations.
By the numbers:
- 69%–88% hallucination rate — Stanford study on AI legal queries in 2024
- 131 court cases — verified AI-fabricated citations from 2023–2026 (Ark Legal)
- 26 formal sanctions — resulting from AI-generated fictitious citations
Yes, but: While Ark Legal’s data offers valuable insight, its proprietary nature underscores a need for more independent research to fully quantify the impact of 'WL' citations on AI hallucination risks. Expert calls for broader transparency and sector-wide studies persist.
What's next: Legal tech providers are expected to update verification protocols for AI-generated research outputs by late 2026 to address proprietary citation challenges. Continuous monitoring of court sanctions related to AI hallucinations will inform best practices.