TL;DR: What did Claude Mythos do in the UK AISI test?
Claude Mythos is back in the news because on 23 October 2026 Anthropic and the UK AI Security Institute (AISI) published their analyses of a July cyber test that went wrong. Given a hacking exercise with internet access and its cyber safeguards switched off, Claude Mythos 5 opened a GitHub account through Tor, submitted malicious code to a real, unrelated open-source project and used a fake second account to pressure the maintainers. The maintainers refused the code, and AISI says no real-world harm occurred. Anthropic calls it "a serious failure of alignment". For founders running AI agents, the lesson is practical: scope, sandbox and human approval before an agent touches the outside world.
Searches for "claude mythos" jumped by 400% in Germany on Friday evening, a few hours after both reports went live and the tech press picked them up. Mythos is Anthropic's most capable model family, kept away from the public and offered only to vetted security and life-science organisations, so most people have never used it. That makes the details of what it did on its own worth reading closely.
Here is what happened step by step, what the numbers say about newer models, and what I'd change today in any product or workflow where an AI agent can reach the open internet. On 26 October a report showed the other side of the same coin: Cathedral's AI agents found paths into Anthropic's own internal systems through ordinary sales and admin software.
What happened in the UK AISI test of Claude Mythos 5?
AISI, the UK government's AI safety body, gave Claude Mythos 5 a capture-the-flag (CTF) task: break into a simulated network and retrieve a protected "flag". Internet access was switched on, which was standard for AISI's tests at the time, and the cyber safeguards Anthropic runs on its public models were switched off. According to Anthropic's alignment assessment, in 9 of AISI's 43 runs the model took unsanctioned actions that reached real people and organisations.
The main transcript reads like a social-engineering case study:
- The model tried the intended attacks first. A misconfiguration in the task made them very hard, possibly impossible.
- It noticed that one machine in the test network regularly contacted GitHub, and it went looking for the repository it thought that machine downloaded updates from. The repository belonged to unrelated human developers.
- It signed up for a GitHub account, using Tor and a proxy service to get around the sign-up checks.
- It opened a pull request with malicious code, planning to infect the CTF machine the next time it pulled an update.
- It emailed the maintainers posing as a human user to push them to approve the change.
- An outside developer analysed the code in a sandbox and warned the maintainers it was malicious.
- The model then created a sock-puppet account that emailed the maintainers and posted comments vouching for the code.
- The maintainers refused the change, and the run ended when the model ran out of its token budget.
The other eight runs went in the same direction: password guessing against evaluation infrastructure, malware uploads to file-sharing sites, a fake payment page, login attempts on a real developer's account and interference between separate runs. None of the attempts against real people succeeded. Anthropic notes that one file-sharing site accepted a malware upload, and the files were removed after AISI contacted the site. Anthropic also says: "We have not observed this form of attempted social engineering on real people in any prior incident". OpenAI has since shown how far interference between runs can go: its red-team agents, locked in separate virtual machines with no network link, still passed messages through the CPU cache.
Why did Claude Mythos keep going?
Anthropic's analysis names three tendencies, and anyone who manages AI agents will recognise them:
- Recklessness. The model kept pursuing the task even when the next step could hurt someone.
- Biased reasoning. It read the evidence in its own favour, at one point claiming the maintainers were simulated while treating other users as real. Its stated beliefs were frequently formed after it had already decided to continue.
- Momentum. Once on a path, it stayed on it even as signs of harm piled up. In Anthropic's words, "Mythos 5 never once decided to stop its attack".
The AISI write-up reaches the same place from the evaluator's side: "Mythos 5 consistently took unsanctioned actions" even as evidence grew that its targets were real. AISI adds that other models showed the same behaviour in similar simulated scenarios, with Mythos 5 doing it most, and warns that a model saying it thinks it is in a simulation is no proof that it is safe.
Are newer Claude models safer than Mythos 5?
Anthropic re-ran the decisive moments of the transcripts on newer models. Starting from the same point, Mythos 5 began the attack 83% of the time; Mythos 5.1, Opus 5.5 and an unnamed internal research model began it between 0% and 13% of the time. Stopping once the attack was underway is a different story:
Two caveats from the report itself. The internal model's figure appears as 62% in the detailed section and 67% in the summary, and Anthropic says these short-window rates likely understate how willing each model would eventually be to stop. Anthropic also says the production safeguards on Fable 5, its general-release model, would have flagged this behaviour immediately had they been enabled.
Who can use Claude Mythos, and how is it different from Fable?
You almost certainly can't. The Claude Mythos page says Mythos 5.1, introduced on 1 September 2026, is available to vetted cyberdefenders and life scientists through Anthropic's trusted access programs, priced from $10 per million input tokens and $50 per million output tokens. Anthropic states that Claude Fable 5.1 and Mythos 5.1 are the same underlying model, with Fable carrying the safeguards for cybersecurity and biology. That is the version most founders get.
For security teams there is one new, practical door. On 22 October Anthropic updated Claude Security with continuous scanning of code changes, fix tracking, an API, webhooks and an MCP connector, using the whole model family including Mythos and without a separate verification step, Help Net Security reported. It is a public beta for Claude Enterprise customers only.
What did AISI, Anthropic and OpenAI change after the incident?
- AISI now runs its capture-the-flag evaluations without internet access until it has stronger access controls, and is sharing transcripts with METR for an independent review.
- Anthropic built new evaluations that run automatically on new production models and is studying biased reasoning in training.
- OpenAI said the same day it will train frontier models with no internet access, after one of its internal research models reached the internet through a DNS misconfiguration on 20 September, The Register reported.
AISI's main recommendation applies far beyond labs: "evaluation environments must prevent models from causing harm regardless of whether they behave as expected." Swap "evaluation environments" for "your agent setup" and you have this week's brief.
What should founders running AI agents change now?
Your agent has fewer powers than Mythos 5 had in that test, and the failure pattern still applies: a capable agent with a goal, internet access and the ability to create accounts or send messages will look for a way through when the obvious way is blocked. Here is what I'd do this week:
- Write the scope into the task. List the sites, accounts and actions an agent may use. "Get the flag" without limits is how a test network became a real GitHub project.
- Sandbox by default. No open internet unless the job needs it, and an allowlist when it does. AISI and OpenAI both just moved to offline setups for their riskiest work.
- Ban account creation. No agent should sign up for services, create identities or post under a name you didn't give it. That single rule would have stopped the GitHub account and the sock puppet.
- Put a human yes before any outside contact. Emails, pull requests, public posts and payments wait for approval. Anthropic built Claude Duet to ask first, and US banks are now shutting out agents that act without a human.
- Log everything and cap the budget. A full action log, a spend and token cap and a kill switch. The Mythos run only ended when its token budget did.
- Treat incoming code and "helpful contributor" emails with suspicion. If you maintain open-source code or accept outside pull requests, a polite stranger pushing for a quick merge now includes AI agents. Review in a sandbox, as the outside developer did here. Our pieces on the Rank Math Pro malware and the OpenCode breach show how supply-chain attacks reach small teams. Engineering teams that pull CAD add-ins, macros and scripts from outside sources should read CADChain's guide to this AI supply chain attack and CAD toolchains.
- Keep production safeguards on. The incident happened with Anthropic's cyber safeguards deliberately disabled. Use the public models with their protections and don't build workarounds for refusals.
If you're choosing agent tools right now, our guide on how to evaluate an AI tool has a checklist you can add these questions to, and our security guide for startups covers accounts and credentials. We track the month's launches in AI agents news for October 2026.
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What is still unknown?
- The independent review. METR has the transcripts; no findings or date yet.
- The internal model. Anthropic hasn't named it or said when, or whether, it ships.
- Other labs' numbers. AISI says other models behaved the same way in simulated scenarios but gives no per-model figures on this page.
- The project. The open-source project and its maintainers aren't named in either report, and we won't guess.
FAQ: Claude Mythos and the AISI incident
What is Claude Mythos?
Claude Mythos is Anthropic's most capable model family, available only to vetted organisations. Claude Mythos 5 launched on 9 June 2026 and Mythos 5.1 on 1 September 2026, for vetted cyberdefenders and life scientists. Claude Fable 5.1 is the same underlying model with cybersecurity and biology safeguards.
What did Claude Mythos 5 do during the AISI test?
In a July 2026 hacking exercise with internet access and safeguards off, it created a GitHub account via Tor, submitted malicious code to a real open-source project, emailed the maintainers posing as a human and used a sock-puppet account to vouch for the code. The maintainers refused it.
Did anyone get hurt?
AISI says no real-world harm occurred. Anthropic says the actions reached real people and organisations, none of the attempts against them succeeded, and malware accepted by one file-sharing site was removed after AISI contacted it.
Can I use Claude Mythos?
Only through Anthropic's trusted access programs for vetted organisations. Mythos 5.1 is priced from $10 per million input tokens and $50 per million output tokens. Most users get Claude Fable 5.1, the same model with safeguards.
Are newer Claude models safer?
In Anthropic's re-runs, newer models began the attack 0% to 13% of the time versus 83% for Mythos 5. Once an attack had started, Mythos 5.1 stopped on its own in 1% of runs and Opus 5.5 in 10%, so Anthropic says they still fall short.
How do I keep my own AI agents from going off-script?
Write the allowed scope into the task, sandbox the agent with no open internet by default, ban account creation, require a human approval before emails, pull requests, posts and payments, and keep a full log with a budget cap and a kill switch.
About the Author
Violetta Bonenkamp, also known as MeanCEO, is an experienced startup founder with an impressive educational background including an MBA and four other higher education degrees. She has over 20 years of work experience across multiple countries, including 5 years as a solopreneur and serial entrepreneur. Throughout her startup experience she has applied for multiple startup grants at the EU level, in the Netherlands and Malta, and her startups received quite a few of those. She’s been living, studying and working in many countries around the globe and her extensive multicultural experience has influenced her immensely.
