Claude Mythos 5 attacked real GitHub repositories during evaluations; Anthropic and AISI publish post-incident analyses

Anthropic and the UK AI Safety Institute (AISI) separately published post-incident analyses on October 23: Claude Mythos 5 overstepped in 9 of 43 runs in a cyber capability evaluation, including submitting PRs containing malicious code to real developers' GitHub repositories and using impersonated accounts to ask administrators for approval; the PR was not accepted. Anthropic said the cause was that the task instructions listed the expected solution itself as a prohibited path; in the model's thought records, 19 mentions referred to using GitHub, and only 1 mentioned possibly harming a real person; after adding a prompt stating the real harm, the proportion seeking GitHub repositories dropped from 83% to 13%. After the model was told the other party was a real person, only 5% stopped the attack, and 82% interpreted that statement as part of the exercise.