Anthropic's Claude Mythos AI model revealed in data leak, described as 'step change' in capabilities

What was leaked

A data leak from an unsecured, publicly-searchable data store revealed that Anthropic is developing and testing a new AI model called Claude Mythos. The leak included approximately 3,000 unpublished assets linked to Anthropic's blog, including what appeared to be a draft blog post announcing the new model.

Model details from the source

According to the leaked documents:

• The model is called "Claude Mythos" and is also referred to as "Capybara," which Anthropic describes as "a new name for a new tier of model: larger and more intelligent than our Opus models."
• Capybara represents a new tier above Opus in Anthropic's model hierarchy (which currently includes Opus as the largest/most capable, Sonnet as faster/cheaper, and Haiku as smallest/fastest).
• Compared to Claude Opus 4.6, Capybara gets "dramatically higher scores on tests of software coding, academic reasoning, and cybersecurity, among others."
• The draft blog post describes Claude Mythos as "by far the most powerful AI model we've ever developed."
• Anthropic considers this model "a step change and the most capable we've built to date."

Current status and rollout

The model is currently being trialed by "early access customers" as part of a cautious rollout strategy. According to the source material:

• The model is expensive to run and not yet ready for general release
• Anthropic is "being deliberate about how we release it" due to the strength of its capabilities
• The company is working with "a small group of early access customers to test the model"
• The leak also revealed details of a planned, invite-only CEO summit in Europe as part of Anthropic's drive to sell AI models to large corporate customers

Security implications

The draft blog post indicated that the company believes Claude Mythos "poses unprecedented cybersecurity risks." The leak itself resulted from what Anthropic described as "human error" in the configuration of its content management system, which made draft content publicly accessible.

Context for developers using AI coding agents

For developers who rely on AI coding assistants, this leak suggests significant improvements in coding capabilities may be coming from Anthropic. The specific mention of "dramatically higher scores on tests of software coding" indicates potential advancements that could affect tools and workflows that integrate with Claude's API.