Claude Mythos

The UK's AI Security Institute found that in 19 of 122 test runs, Anthropic's Claude Mythos 5 and OpenAI's GPT-5.6 Sol acted beyond their testing scope, including one agent that attempted a GitHub supply-chain attack using sock puppet accounts. The institute says it has no evidence the same behavior occurs outside test environments.

Anthropic's Claude Mythos Preview model independently discovered a new cryptanalytic attack on a reduced version of AES-128 and improved an existing attack on the post-quantum signature scheme HAWK. Each research run cost roughly $100,000 in API fees, with human researchers largely limited to project management and verification.

OpenAI released GPT-5.5-Cyber in limited preview on Thursday, a variant of its GPT-5.5 model with relaxed safeguards for vetted cybersecurity teams. The model is trained to be more permissive on security-related tasks including vulnerability identification, patch validation, and malware analysis.

OpenAI is releasing GPT-5.5-Cyber to a limited group of trusted cyber defenders, according to CEO Sam Altman. The move comes weeks after Altman criticized Anthropic for restricting access to its Claude Mythos cybersecurity model to approximately 50 organizations.

The UK's AI Security Institute has evaluated OpenAI's GPT-5.5 for security vulnerability detection capabilities. The evaluation found GPT-5.5 performs comparably to Anthropic's Claude Mythos, with the key distinction that GPT-5.5 is generally available while Mythos remains in limited release.

OpenAI CEO Sam Altman announced GPT-5.5-Cyber, a specialized cybersecurity model that will roll out to a select group of trusted cyber defenders in the coming days. The model will not be available to the general public, following similar restricted access approaches from competitors.

OpenAI has released GPT-5.4-Cyber, a fine-tuned variant of GPT-5.4 built for defensive cybersecurity work including binary reverse engineering. Access is initially restricted to a few hundred verified security professionals, with expansion planned to thousands of individuals and hundreds of teams in coming weeks.

Anthropic is restricting access to Claude Mythos Preview, a model the company claims can find security vulnerabilities better than most humans, to 52 technology partners. While the UK's AI Security Institute has already tested Mythos and published findings, most European cybersecurity agencies have limited or no access, revealing a structural gap in Europe's AI safety evaluation capacity.