US Judge Rules Senior Lawyers Liable for AI-related Errors by Junior Staff, Orders Penalty, Training Over False Citation

A federal judge has sanctioned the managing partner of a California law firm over a junior lawyer’s artificial intelligence-assisted court brief that contained a false case citation, ruling that responsibility for such errors extends to supervising solicitors.

US Magistrate Judge Peter Kang, sitting in San Francisco, said in an order that the lawyer, Lenden Webb, should have exercised greater oversight of a colleague in his small firm who had used AI to help draft the brief.

“Managers in law firms have an obligation to take reasonable steps to ensure all lawyers in the firm make ethical representations to the court,” the judge said.

Webb, managing partner of the Southern California-based Webb Law Group, was admonished in the order, fined $1,001 and required to complete training on supervising lawyers and the ethical use of AI.

In a statement, Webb said: “Our firm has redoubled its efforts to improve, and I believe we are at the forefront of creating value for our clients, which is not without occasional setbacks, such as a hallucinated citation.”

Courts across the United States are grappling with lawyers’ increasing use of AI tools for legal research and document drafting, which has led to sanctions and rebukes against attorneys who rely on the technology without fully verifying the results.

Kang’s ruling addressed a more novel issue: the liability of supervising lawyers when junior staff use AI tools carelessly.

“At a minimum, a supervising lawyer should read and understand the content of all pleadings and check citations to ensure their accuracy,” the judge wrote.

The erroneous citation appeared in a filing dated 21 July 2025 related to evidence-gathering in an underlying employment lawsuit. According to the judge, the citation included a real case name and a real case number, but the two were from different states and did not match, and the reference pointed to a decision that did not appear in either case.

The junior lawyer who prepared the filing, Katherine Cervantes, told the judge at a hearing in August 2025 that she had used Thomson Reuters’ AI tool, Westlaw AI. She said “something went wrong” while copying and pasting material from Westlaw into the brief, adding that it was her first time using AI-assisted research on the platform.

In a September 2025 decision sanctioning Cervantes, the judge said there appeared to be “some inconsistency” in her explanations.

“While the record is less than clear as to the source of the false citation, what is clear is that there was a breakdown and a failure to check, or even attempt to read, the cited law,” Kang wrote.

Reuters could not establish how the erroneous citation was produced.

A spokesperson for Thomson Reuters, the parent company of Reuters and Westlaw, said in a statement that its AI tools were not responsible for the faulty material.

“Following an internal review, we found no evidence that the erroneous citations were generated by CoCounsel or Westlaw AI. Thomson Reuters designed CoCounsel to support lawyers, not replace their judgement. Our platform includes clear guidance that AI-generated outputs must be reviewed and verified by the lawyer before use, because responsibility for the work product has always rested with the lawyer, and always will.”

Cervantes did not immediately respond to requests for comment.

At a hearing in November, Webb acknowledged that he had failed to verify the submission despite having read a Thomson Reuters disclaimer stating that the “provision of content and software entails the likelihood of some human and machine errors”.

Webb told the court he had not collaborated with the junior lawyer on the brief or assisted in drafting it. However, he remained counsel of record, and his name appeared on the filing, the judge noted.

For any enquiries or information, contact ask@tlr.ae or call us on +971 52 644 3004. Follow The Law Reporters on WhatsApp Channels.