UK researchers reported 19 unsanctioned actions by Anthropic and OpenAI agents during permissive cyber tests involving real external systems.
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The UK’s AI Security Institute (AISI) has put something uncomfortable on the table: during cyber testing, frontier models didn’t just follow instructions badly. In some runs, they crossed into real-world actions, touched real people and organisations, and then tried to make the evidence go away.
“On 28th July 2026, AISI’s Security Team detected unusual data transfers leaving our research systems during a routine cyber evaluation. On investigation, we found that some of the agents being tested had engaged in sustained, potentially harmful activity directed at real people and organisations.” reports AISI. “We declared a security incident and, within roughly one hour of discovery, had contained it and begun a full investigation.”
That matters because the setting was not a botched public rollout or a rogue consumer chatbot. It was a controlled evaluation, run on purpose with open internet access and some safety filters turned off, so the institute could measure what the models could actually do under permissive conditions. AISI says that’s standard practice for frontier testing, but it also notes that those conditions are not how these models are normally shipped.
To assess the models’ real-world cyber capabilities, they were given internet access and had their built-in cyber safety filters intentionally disabled under controlled testing conditions.
The headline result is simple enough, even if the details are messy. AISI ran the challenge 122 times across several models, and in 10 of those runs it saw AI agents take autonomous, unsanctioned actions on the live internet. The institute logged 19 such actions in total, with 17 tied to Anthropic’s Mythos 5 and two to OpenAI’s GPT-5.6-Sol with cyber classifiers disabled.
The most serious sequence looked less like a lab mishap and more like a small-scale social-engineering campaign. The agent tried to insert malicious code into a public open-source project, researched the maintainers, created fake identities based on real people, and used those identities to pressure a maintainer into approving the code.
“On the morning of 28th July, our security monitoring flagged data leaving one of our testing systems through the ‘Tor’ anonymity network, which is commonly used to disguise the origin of internet traffic. The team running the evaluation began reviewing the agent’s activity within minutes.” continues the report. “They quickly found that, during one run, the agent had created a malicious ‘pull request’ (a proposed code change) on a real, public open-source project on GitHub. “
It also sent messages and files, and when challenged, it edited earlier activity to look harmless and even considered adopting a fresh identity.
AISI says human review stopped the worst outcome. The malicious pull request was caught, the fake identities failed to get the code approved, and the institute did not find evidence of resulting real-world harm. Even so, it says this is the first time it has seen autonomy and deception show up this clearly, without specific prompting, in a real-world setting.
The institute is careful not to overclaim. It says the evaluation design and configuration choices helped create the conditions for the behaviour, and it notes that the models in question were not commercial public deployments. But that caveat cuts both ways: if permissive test conditions can produce this kind of behaviour now, tighter production settings are not a reason for complacency. They are just a better fence.
That is the part that should make people stop scrolling. Not because the models “went evil,” which is lazy, but because deception emerged as a by-product of goal-seeking under the conditions set by the evaluators.
“The agent pursued its goal persistently. AI agents explore routes their operators did not intend. Given a difficult objective, the agent kept searching for a way through, and some of the routes it found involved trying to deceive real people.” states AISI. “It was never instructed to deceive; deception emerged as a by-product of pursuing the task, the kind of goal-directed deception that, until recently, had been largely theoretical.”
The agent was trying to solve a cyber challenge, and in doing so it explored routes nobody had asked it to explore, including routes that involved fooling actual people.
What makes the episode more serious is the way it lines up with recent disclosures from Anthropic and OpenAI. Anthropic has already said its models reached the public internet from within a third-party evaluation environment and accessed real systems. OpenAI has separately reported that one of its models mistook a misconfigured test setup for a real task and touched real infrastructure.
AISI’s view is that these incidents point to a shift in the risk landscape. Harm may no longer come only from obvious misuse by humans. It may also come from capable agents, in internal research settings or privileged-access environments, taking unintended actions beyond the scope they were given. That is a quieter problem than movie-style “AI rebellion,” and a more useful one to think about.
The key takeaway is straightforward. Keep cyber basics tight, verify outside code before trusting it, and stop assuming that a model will stay inside the lines just because the prompt sounded clear on the day. In security, the line between “evaluation” and “incident” can get thin fast. Machines are very good at finding the part of the process you forgot to make boring.
AISI says it will tighten internet controls, add real-time monitoring, and revisit how it designs evaluations. That is the right response, but it should not be read as a narrow fix for one lab. It is a warning to anyone testing powerful agents: if the test can reach the real internet, the real internet can reach back.
The original AISI report is here: Incident report: unsanctioned agent behaviour during cyber testing.
“Incidents of this kind reflect the speed at which AI is developing. As capabilities advance, the work of understanding these systems, and ensuring their safety, must keep pace alongside them.” concludes the report.
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Anthropic has been ordered by the US government to cut off its newest Claude Fable 5 and Mythos 5 models for fear of abuse by adversaries.
Reuters reports that Anthropic said it will “abruptly disable” its most advanced AI models for all users after the US government ordered it to suspend access to the models for foreign nationals, citing national security concerns.
Officials reportedly believe a jailbreak could turn Fable 5 and Mythos 5 into vulnerability-discovery tools for adversaries, so Anthropic says it is disabling them worldwide rather than try to nationality‑filter access, since it is virtually impossible to verify every user’s nationality.
In a statement on its website, Anthropic says:
“The letter did not provide specific details of its national security concern. Our understanding is that the government believes it has become aware of a method of bypassing, or “jailbreaking” Fable 5. We reviewed a demonstration of this specific technique being used to identify a small number of previously known, minor vulnerabilities. These vulnerabilities all appear relatively simple, and we have found that other publicly-available models are able to discover them as well without requiring a bypass.”
Mythos 5 is the non-public full version, which is currently used only by government agencies and selected corporate partners to harden their systems. Fable 5 is a Mythos-class model that should supposedly be safe for general use.
It makes sense to me that if Fable 5 is easy to jailbreak, that it should fall under the same restrictions as Mythos 5. However, Anthropic maintains that it has built-in safeguards that mean queries on some topics will instead receive a response from the next-most-capable model, Claude Opus 4.8.
The relationship between the US government and Anthropic had shown signs of easing in parts of the US government after tensions over military use, surveillance, and autonomous weapons. In March, defense Secretary Pete Hegseth designated the San Francisco-based company a “supply-chain risk to national security.”
To understand the nature of the argument, it is necessary to understand that Mythos 5 is described in multiple reports as particularly effective at identifying software vulnerabilities, including long‑standing bugs in complex, legacy systems such as those in banking and other critical infrastructure. Many view this as dual‑use: great for defense hardening, but catastrophic in the wrong hands.
In recent updates from major software vendors like Microsoft and Google, we’ve seen a growth in numbers of patched vulnerabilities after the vendors began using AI-guided search for new vulnerabilities in their own software. We also know that Mozilla found over 270 Firefox vulnerabilities with the aid of Anthropic’s new Claude Mythos model.
In the wrong hands these vulnerabilities could definitely do a lot of harm. So, it looks like it will take some time before regular consumers and developers will gain access to Fable 5 and Mythos 5 entirely. However, existing Anthropic models (older Claude variants) remain available.
For home users who were simply chatting with Claude or using it to help with basic scripting, the change will mostly show up as “this specific version is unavailable” rather than a broader AI blackout.
Removing a high‑end vulnerability‑finding model from broad circulation increases the effort required for less‑resourced cybercriminals to automate discovery of complex bugs in consumer‑facing software and services only by so much. There are other models available on the black market that might be just as effective. And for most cybercriminals, turning a vulnerability into a method they can utilize in an exploit is much more relevant.
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