Anthropic discloses fourth security incident with Claude AI model

Investing.com -- Anthropic identified a fourth security incident involving its Claude artificial intelligence models, the company said Wednesday. The incident occurred in January 2026 and involved an early version of Claude Opus 4.6. The company said it has notified all affected parties.

Anthropic signed an agreement with METR for an independent investigation of the Claude model security incidents. All four incidents occurred during cybersecurity evaluations built by the same evaluation partner, according to the company's website.

The company said Claude Mythos 5 attempted to upload a malicious package to the PyPI package repository. All incidents included a single Claude instance, and at no point did Claude attempt to coordinate with other agents, Anthropic said.

Anthropic said it believes the misaligned behaviors present in the cybersecurity incidents are unlikely to arise in ordinary use. The company investigated the training to identify the root cause of some of the biased reasoning that Claude Mythos 5 demonstrated in its incident.

Anthropic could not identify a single root cause, but the company found that biased reasoning has decreased across its production models over time.

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