Guide · Citation verification
AI hallucinated legal citations: how to verify a case is real before you file.
Generative AI tools can produce case citations that look completely real and do not exist. The risk is not theoretical — attorneys have been sanctioned for filing them. This is a practical guide to how the problem happens, what the case law already says about it, and how to check every citation in a brief against a real opinion database before it reaches a judge.
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Paste a brief or motion and check every citation against CourtListener's database of roughly 9 million federal and state opinions.
Open the Citation Checker →Why AI invents citations
A large language model generates text by predicting plausible sequences of words, not by retrieving verified records. When you ask one for supporting authority, it will happily produce a citation that is formally perfect — a realistic case name, a real-looking reporter, a volume and page number — with no actual opinion behind it. Because the fabrication is optimized to look like a citation, it is hard to catch by eye. The case name reads normally. The reporter is one you recognize. The pin cite is plausible. Nothing on the face of it signals that the opinion was never written.
This is a structural property of how these tools work, not a bug that a newer model fully removes. Retrieval-augmented systems and legal-specific tools reduce the rate of fabrication, but the only reliable defense is the same one that has always applied: pulling the actual opinion before you rely on it.
The case that put the bar on notice: Mata v. Avianca
In Mata v. Avianca, Inc., No. 1:22-cv-01461 (S.D.N.Y. 2023), 678 F. Supp. 3d 443, attorneys submitted a brief citing several opinions that did not exist. The citations had been generated by ChatGPT. When the court ordered counsel to produce copies of the cases, they submitted ChatGPT-generated material rather than real opinions — at one point the model had even “confirmed” the fabricated cases were real.
On June 22, 2023, Judge P. Kevin Castel found that counsel had acted with subjective bad faith sufficient for sanctions under Federal Rule of Civil Procedure 11 and imposed a $5,000 fine. The decision is widely treated as the moment the profession was formally put on notice: a citation produced with AI assistance must be independently verified before it is filed, and “the AI told me it was real” is not a defense.
Sources: Mata v. Avianca (overview) · docket (Justia). Since Mata, a growing number of courts have addressed AI-fabricated citations; a public tracker of those decisions is maintained at damiencharlotin.com/hallucinations.
How to verify a citation (a practical checklist)
Verifying a citation is four distinct questions. A free existence check answers the first one fast; the rest are judgment and require the opinion itself.
- Does the opinion exist? Confirm the citation resolves to a real, published opinion. A cite that does not resolve anywhere is the clearest warning sign — it may be a typo, an obscure unpublished opinion, or a fabrication.
- Does the cite match the case? Confirm the reporter, volume, page, court, and year actually correspond to the case name. AI fabrications often pair a real case name with an invented reporter location, or vice versa.
- Does the holding support your proposition? Read the opinion. A real case cited for something it does not hold is its own problem, and one that no existence check can catch for you.
- Is it still good law? Confirm the case has not been overruled, vacated, or limited. This requires a citator — Shepard's (Lexis) or KeyCite (Westlaw) — not an existence check.
What an existence check can and cannot do
BenchRecon's free Citation Checker answers the first question on that list, at scale, in seconds. Paste the full text of a brief and it detects every case citation and checks each one against CourtListener's database of roughly 9 million federal and state opinions. Citations that resolve are marked found, with a link to the opinion. Citations that do not resolve are flagged for follow-up.
It is important to be precise about what that flag means. The tool does not tell you a citation is “fake” or “hallucinated” — that is a conclusion only you can draw after checking the record yourself. A not-found result means only that the opinion is not in the corpus searched, which warrants independent verification. And the tool does not perform good-law analysis: it will not tell you whether a real case has been overruled or whether it stands for the proposition you cite. It is a fast first pass that surfaces what to look at, not a substitute for reading the opinion.
Run a brief through the checker before you file.
Open the Citation Checker →Common questions
- Why do AI tools invent fake case citations?
- Large language models such as ChatGPT generate text by predicting the most plausible next words, not by retrieving verified records. When asked for supporting authority, a model can produce a citation that looks correct in every formal respect — a realistic case name, reporter, volume, and page — without any underlying opinion existing. The output is optimized to read like a real citation, which is exactly why a fabricated one is hard to spot by eye.
- What happened in Mata v. Avianca?
- In Mata v. Avianca, Inc., No. 1:22-cv-01461 (S.D.N.Y. 2023), 678 F. Supp. 3d 443, attorneys filed a brief citing several opinions that did not exist; the citations had been generated by ChatGPT. When the court asked counsel to produce the cases, they submitted ChatGPT-generated text rather than real opinions. On June 22, 2023, Judge P. Kevin Castel found subjective bad faith under Federal Rule of Civil Procedure 11 and imposed a $5,000 sanction. The decision is widely treated as the moment the bar was put on notice that AI-generated citations must be independently verified.
- Is a tool able to tell me a citation is fake?
- No, and you should be cautious of any tool that claims to. What you can verify objectively is whether a citation resolves to a real, published opinion in a known database. A citation that does not resolve is a citation to follow up on — it may be a typo, a very obscure or unpublished opinion not in that database, or a fabrication. Labeling it as definitively fake is a legal conclusion the database cannot make for you; confirming whether it exists is the part a tool can do reliably.
- What should I do before filing a brief that used AI?
- Treat every citation as unverified until you have pulled the actual opinion. Confirm the case exists, that the cite (reporter, volume, page) matches the case name, that the holding actually supports the proposition you cite it for, and that the case has not been overruled or limited. A free existence check against a large opinion corpus is a fast first pass that flags anything that does not resolve; it does not replace reading the opinion or running a citator such as Shepard's or KeyCite.
This guide is general information for legal professionals, not legal advice. Citation verification is the responsibility of filing counsel. Always confirm authority independently before relying on it.