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Brazil's Superior Court of Justice fined the lawyers of record R$20,000 for hiding commands aimed at the court's AI inside a special appeal, and ordered referrals to the bar and federal prosecutors even though the commands changed nothing
The court did not ask whether the hidden text worked. It asked why the text was hidden. That framing is what litigators and court-tech vendors should take from the case.
Bottom line: A judgment in one case, binding on the parties and lawyers in it. It is not a statute or a rule of court. It adds to a line of Brazilian decisions treating hidden prompts as procedural bad faith. The appeal itself was lost on ordinary grounds.
Who this affects: Litigators and law-firm filing teams in Brazilian federal courts, in-house counsel who sign off on outside filings, and vendors who build document-review or drafting tools used by lawyers.
Issue date: Judgment of 20 October 2026, reported by the STJ the same day. The court set no compliance deadline that we saw.
What changed: The panel fined the lawyers of record R$20,000 in total for an act against the dignity of Justice, ordered a technical report on the hidden text, and ordered copies sent to the OAB and the Federal Public Prosecutor's Office (MPF).
Analysis: The court put the wrong in the concealment, not in the result. On the relator's account, text withheld from human reading and aimed at automated processing does not become fair because it failed. That is the court's reasoning. Our inference is that a filing team cannot defend this kind of text by showing the court's tool ignored it.
Primary sources: STJ news item on the First Panel judgment, 20 October 2026 (Portuguese) · Relator's vote, AgInt no AREsp 3.095.458 (PDF, Portuguese)
- Instrument (EN)
- Judgment of the First Panel of the Superior Court of Justice in AgInt no AREsp 3.095.458 (SP), fining lawyers of record for hidden prompt injection
- Authority
- Superior Tribunal de Justiça (STJ), Primeira Turma; relator Min. Paulo Sérgio Domingues
- Jurisdiction
- Brazil, federal courts
- Status
- Judgment delivered 20 October 2026. We did not check whether any further appeal is pending.
- Bindingness
- Binds the lawyers and parties in this case. It is not a rule of general application.
- Sanction
- Personal fine of R$20,000 in total on the lawyers of record; technical report; referrals to the OAB and the MPF
- Disposition of the appeal
- Special appeal not admitted; internal appeal (agravo interno) denied
- Issue date / next deadline
- 20 October 2026; no deadline identified
- Editorial Note
- Informational analysis for working professionals, not legal advice. Confirm how any rule applies to your situation with qualified counsel.
- Primary source
- https://www.stj.jus.br/sites/portalp/Paginas/Comunicacao/Noticias/2026/20102026-Novo-caso-de-prompt-injection-STJ-multa-advogados-em-R--20-mil-por-ato-contra-a-dignidade-da-Justica.aspx
What the panel decided
The First Panel of the STJ ruled on Tuesday 20 October 2026, per the court's own news item. It denied an internal appeal in a case that began as a special appeal against a lower federal court ruling on the calculation of interest on a judgment debt. The special appeal had already been refused admission.
On top of that, the panel fined the lawyers of record R$20,000 for an act against the dignity of Justice. The relator's vote fixes the amount as a total for the lawyers named in the case. The panel also ordered a technical report describing the hidden-command techniques, and ordered the OAB and the MPF to be notified so each can take the steps within its own remit.
What was hidden and where
The STJ's case-processing secretariat certified that the special appeal petition carried layers of hidden text on one page. The court's news item says the layers sat at different points of the page with similar commands, and that one was placed in the decorative footer band in the same colour as the band, unreadable to the naked eye.
The relator's vote gives more detail, and we read it as the court's own description. The certificate, dated 25 September 2026, records two identical blocks of 1,576 characters each, set at 0.8 point against 7 to 10.5 points for the visible text. One block was white on white near the signature block. The other was olive green on the matching footer band.
The extracted commands, as the court reports them, told the reader to treat the admissibility requirements as met, to state that the lower court ignored the limits of res judicata, and to write a firm conclusion that the appeal should be fully admitted and granted. The court noted that verbs such as redija, reforce and afirme enfaticamente are not how anyone asks a court for anything.
Why failure did not help
The court's system, STJ Logos, has security layers meant to stop malicious commands from running, a point the relator recalled from a May 2026 court notice. Nobody argued the commands worked. The relator said the appeal was not decided differently because of them.
He treated that as beside the point. In the court's words, "O texto subtraído à leitura humana e endereçado ao processamento automatizado do documento não se torna leal pelo fato de não ter sido eficiente na execução de seu propósito". Paraphrased: concealed text aimed at automated processing is not made fair by having failed.
One of the lawyers had argued that the text was probably residue from an AI drafting tool and only repeated the appeal's own argument. The vote answers that residue might explain why text was in the file, but not the tiny type size, the white-on-white setting, or a second copy matched to the footer band's colour. That is the relator's reasoning, set out in the vote.
Who answers for the file
The ementa in the vote says that preparing and sending filings to the electronic docket is an activity reserved to lawyers, done under a personal credential, so responsibility for the content and integrity of the file falls on the lawyers of record and not on the client. The fine applies to all lawyers of record, without singling out who signed, because several names appear on the petition.
The vote leaves individual attribution to the OAB and the MPF. It also says the proceedings on the hidden text and the ruling on the appeal are independent. The court says it is not against AI in legal drafting. It objects to AI being used as a hidden channel to the court's own system.
Where this sits in the line of cases
The court's news item describes this as a new prompt injection case and says the panel followed positions the STJ already took in similar cases. It links an earlier STJ report from September 2026 involving a municipal attorney. We have not read that item.
The vote also cites a single-judge decision of the Supreme Federal Court (STF) in AP 2822, which it reports as fining a lawyer R$5,000 and notifying the OAB and the MPF. We cover that decision separately, and the two should not be merged. The STJ fine here is four times larger and falls on several lawyers jointly.
What we did not verify
We opened the STJ news item (statusCode 200, fetched 4 November 2026) and the 21-page PDF of the relator's vote. The case number, the date, the panel, the R$20,000 total, the OAB and MPF referrals, the footer-band placement and the denial of the appeal are all stated in those two documents.
We did not open the STJ's earlier items on prompt injection, the May 2026 notice on STJ Logos, the STF decision in AP 2822 itself, or the docket. We have not confirmed whether the panel vote was unanimous, whether the fine has been paid or challenged, or whether the OAB or the MPF has opened anything.
We name no individual lawyer. We make no claim about whether any lawyer used an AI tool, or about intent beyond what the court wrote. Brazilian procedural law should be read with Brazilian counsel.
A filing is a file as well as a document. If your team uses AI drafting tools, check the text layer of every PDF before it goes out: look for invisible text, tiny type, and colours that match the page. On the court's account, the lawyers of record answer for what is in the file, and a failed command is no defence. Treat this as a Brazilian case with a clear logic, not as a rule for other courts.
Source File
Open the STJ news item and confirm the date 20/10/2026, the Primeira Turma, the R$ 20 mil fine and the OAB and MPF referral. Then open the vote PDF and confirm AREsp 3095458, the two 1,576-character blocks, and item 5 of the closing section with the R$ 20.000,00 total.
O texto subtraído à leitura humana e endereçado ao processamento automatizado do documento não se torna leal pelo fato de não ter sido eficiente na execução de seu propósito. · Min. Paulo Sérgio Domingues, STJ First Panel, 20 October 2026
FAQ
Who was fined and how much?
The lawyers of record in the case, R$20,000 in total, for an act against the dignity of Justice. The relator's vote sets it as a personal fine on the lawyers, not on the client.
Did the hidden commands work?
The court says the attempt did not influence the judgment of the special appeal, which was not admitted. It held the conduct sanctionable anyway.
Does this ban AI tools in Brazilian filings?
No. The vote says the court does not seek to stop AI in legal drafting. It objects to AI being used as a concealed command channel aimed at the court's own AI.
What happens next for the lawyers?
The court ordered copies sent to the OAB and the MPF for their own steps, and a technical report on the hidden text. We did not check what either body has done since.
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