It's hardly news that you shouldn't file briefs with AI-hallucinated cases. But should you check all of your opponent's citations to see if they're hallucinated, so that you can alert the court to that?

I at first thought not. Naturally, if a citation is critical to the opponent's argument, you'll want to read the case the opponent is citing, and alert the court if the case doesn't actually exist. But many citations are on tangential points, cited for uncontroversial matters; there isn't much reason to try to track them down.

Indeed, even using a tool such as Westlaw's document analysis tool, which can do a good job checking citations in bulk takes at least some time and therefore client money: One needs to not just run the tool but closely analyze any errors it claims to find. And then one would have to spent time and money confirming that they are genuine errors and not typos, since one doesn't want to seem to be making a mountain out of a molehill, and writing up the analysis.

But it now seems clear that quite a few judges do get annoyed not just at the lawyers whose briefs contain hallucinations, but also at opposing counsel who don't warn the court about those hallucinations. I saw this most recently in Landberg v. City of N.Y., where appellate court judges admonished the opposing counsel (to be sure, much less harshly than they admonished the directly offending lawyer); see this video, starting roughly at the 19-minute mark. Here's an excerpt from an article at 404 Media (Samantha Cole):

[T]he attorney [Friscia] representing the owner of the property that faces the sidewalk, stood up before the judges next. He started to speak, but [Presiding Justice Hector] LaSalle wasn't finished with the dressing-down. "He's raising a court of appeal standard that doesn't exist," LaSalle said, interrupting Friscia. "He was using it as a component of his argument, and you didn't think you should bring it to our attention?"

"I didn't notice in particular that the principle of law that he was citing was incorrect," Friscia said.

"I'm sorry, I'm going to give you every opportunity to make your argument," LaSalle said. "But I'm befuddled. I honestly am. I'm absolutely—and I'm not here to—lawyers make mistakes. It's not an easy profession. I don't want to sit here beating up on lawyers, but we rely on the bar so much in what we do. So the first thing that I did, I don't want to speak for my colleagues, but after seeing what he wrote, when I went to your papers, I expected to see something referencing […] It wasn't one case, counsel, it was several cases, and you didn't see fit to bring it to our attention either. It's just striking to me."

Friscia, now with the fear of the bar in him, apologized profusely. "Your honor, I apologize to the court. I will do further due diligence going forward from this point on."

"I hope so," LaSalle said. "You should apologize to your client, not to me."

"Yes, I apologize for that," Friscia said. "And I will, going forward, check every single case, even if it stands for, you know, general principles of law, like the construed liberally to effectuate remedial purpose, and things like that. I will bring them to the court's attention." …

And the judges then turned to the lawyer (Freedman) representing the City of New York:

She got the same questioning from [Justice] Nelson: "So, how do you explain your failure to bring to the attention of this court that a brief was filed with this court by appellant's counsel with apparent fabrications and misrepresentations?"

Freedman tried to explain. "I certainly read the briefs," she said. "I certainly read all of the briefs here, but I certainly didn't focus on it, because it was not our issue. And I do apologize to the court for not catching that, but I tended to focus more on the issue of prior written notice."

In January, I noted three trial court cases taking the same view:

[1.] From Nuvola, LLC v. Wright, decided Nov. 21 by Hennepin County (Minn.) Judge Laurie Miller:

For the reasons the Court has outlined above, the Court finds Mr. Knaak's failure to check the legal authority cited in his brief and his reliance on non-existent case law in his advocacy before this Court raises a question as to whether he breached his duty of competence to Ms. Wright. The Court will refer Mr. Knaak to the Office of Lawyer's Professional Responsibility (OLPR) for appropriate action. Any mitigating factors proffered by Mr. Knaak, including his compliance with his obligation to educate other lawyers about this issue under this Order, may be relevant to a future ethics investigation and any subsequent recommendations by the OLPR, but the Court will not analyze them here as the Court does not make explicit findings on or enforce ethics rules.

The Court also finds troubling Mr. Braun's [i.e., opposing counsel's] failure to identify or bring the non-existent case citations to the Court's attention before the hearing on the motion to compel arbitration. TheCourt should not be left as the last line of defense against citations to fictional cases in briefs filed with the court.

While Mr. Braun did not create or rely on the fake citations, he also did not detect them. Instead, he admitted he did not review the cases cited by his opponent. If he had checked out the citations in the brief to which he was responding, he no doubt would have brought the issue to the Court's attention by the time of the motion hearing, and that would have allowed the Court to take the non-existence of the cited cases into consideration as it heard the argument on the merits of Defendant's motion to compel arbitration, instead of leaving the Court to discover that issue on its own, after the hearing was concluded.

The Court does not find Mr. Braun's conduct to be sanctionable, as he did not cite any non-existent cases to the Court. [Nonetheless], the Court reminds counsel that it is the obligation of counsel on both sides to respond to each other's arguments, including completing a basic cite-check of the cases cited by the other side.

The Court urges all lawyers to take seriously their obligation to ensure that the legal arguments being made and considered by theCourt rest upon good law, not fictional cases dreamed up by a computer. The development of the common law relies upon the accurate citation of existing caselaw, as lawyers and courts analyze new disputes. Infection of the body of caselaw by fake Al-generated citations threatens the integrity of the common law.

The court earlier in the opinion noted (focusing Mr. Knaak's failure to check his own work) that:

Westlaw now provides a tool called "Quick Check" that allows a user to upload a lawyer's own work or the work of an adverse party and quickly identify a list of authorities cited. The Court did not use this tool to discover the non-existent citations, and does not endorse it as a substitute for traditional citation checks. The Court followed the old-fashioned process of looking up the key cases cited by the lawyers, to read them and decide whether it agreed with the parties' reading of pertinent caselaw. However, to the extent a lawyer may wish to conduct a faster citation-check process, the availability of the "Quick Check" tool demonstrates that the investigation required of a lawyer by Rule 11 can be accomplished efficiently through readily available methods.

[2.] Judge Sharion Aycock (N.D. Miss.) reached a similar conclusion in Billups v. Louisville Municipal School Dist.:

The Court also observes that the Defendant … could have flagged the fictious citation and misrepresentation of case law [by Plaintiff's counsel] in a reply brief or supplemental filing. The Court takes this opportunity to issue a charge. Going forward, the Court expects all parties to assist in maintaining the integrity of the judicial process and to be diligent in flagging AI misuse. "[O]therwise, the risk is too great that such errors will persist undetected, potentially leading to an outcome unsupported by law." Elizondo v. City of Laredo (S.D. Tex. 2025).

[3.] And Judge Marina Garcia Marmolejo's order in Elizondo did indeed take the same view:

The Court also observes that Defendant, the City of Laredo, could have flagged these fictitious citations in a reply brief or supplemental filing. Although this oversight does not rise to the level of sanctionable conduct, the Court expects all parties to assist in maintaining the integrity of the judicial process by alerting the Court to such errors. The Court encourages greater diligence in flagging citation errors in the future—otherwise, the risk is too great that such errors will persist undetected, potentially leading to an outcome unsupported by law.

I doubt you'll get sanctioned for not spotting hallucinations in your opponent's work. But it looks like you might get the judge annoyed at you, which is not what you want. So if you have Westlaw, use its Litigation Document Analyzer tool on opponents' filings and not just on your own.