Claude Mythos: leak reveals Anthropic’s most powerful AI model – ‘step change’ with serious cybersecurity risks

On March 27-28, 2026, a sensational leak from Anthropic, the company that created the famous Claude, spread in the network. In the open access were drafts of the blog post about the new flagship model Claude Mythos (internal name - Capybara). This is not just another update, but a brand new tier model that Anthropic calls “by far the most powerful AI model we’ve ever developed.”.

## What exactly leaked and how it happened

The leak occurred due to a human error in setting up the Anthropic CMS (Content Management System). The public data lake contained nearly 3,000 unpublished assets: drafts of posts, images, PDFs and even details of a closed CEO summit in Europe. One of the main documents is the two drafts of the blog post (Mythos v1 and Capybara v2). It’s called “larger and more intelligent than our Opus models.”.

Anthropic quickly confirmed the authenticity of the documents. A spokesperson for the company told Fortune:

“We are developing a general purpose model with meaningful advances in reasoning, coding, and cybersecurity. We consider this model a step change and the most capable we've built to date

## Key characteristics of Claude Mythos (Capybara)

• ** New tier above Opus**. So far, Anthropic has had three levels: Opus (the most powerful), Sonnet and Haiku. Mythos/Capybara is the fourth, fundamentally larger and smarter level.
• Significant productivity gains. Compared to the Claude Opus 4.6, the model shows “dramatically higher scores” in:
  • software coding,
  • academic thinking,
  • cyber security tasks.
• Special power in cybersecurity. Anthropic explicitly writes that Mythos “currently far ahead of any other AI model in cyber capabilities” and heralds a wave of models that will be able to find and exploit vulnerabilities faster than defenders have time to respond.

Model training is complete. It is currently in the closed testing phase for selected early access customers.

## Why Anthropic is in no rush to release

Due to unprecedented cyber risks, rollout will be extremely cautious. The draft clearly states that Mythos will first give cyber defenders time to strengthen their codebases before the “wave of AI-driven exploits”.

This is not a panic, but a conscious attitude. Similar concerns have been raised by OpenAI and other labs about frontier models.

## Market reaction

The news immediately hit the stocks of cyber companies: Palo Alto Networks, CrowdStrike, Zscaler and Tenable fell by 4-7%. Bitcoin also briefly dropped below $66,000. Investors fear that a powerful AI-attacking tool could crush demand for traditional defenses.

## Sources and degree of confirmation

Confirmed data (Anthropic official statement):

• The model is called Claude Mythos/Capybara.
• It is a “step change” and the company’s most powerful model to date.
• Training is completed, testing with early access.
• The main improvements are in reasoning, coding and cybersecurity.
• The release will be cautious due to cyber risks.

Secondary sources (all refer to Fortune and drafts):

• Mashable, CoinDesk, The Decoder, Economic Times, Techzine, etc.
• Discussions on X/Twitter, Reddit (r/singularity, r/ClaudeCode, etc.).
• YouTube channels and blogs that recount the leak.

Plums: Drafts of the blog (two variants Mythos and Capybara) are now available in the archives. They are consistent with what the company has confirmed.

There is no exact public release date yet. Anthropic clearly wants to give the defenders a head start, not the hackers.

Claude Mythos is not just the next Claude. It is the first in a new generation of models that could radically alter the balance of power in cyberspace.