Anthropic's Most Powerful AI Model Leaked: "Claude Mythos" Is a Tier Above Opus — and Too Dangerous to Release
March 31, 2026 · 7 min read · Happycapy Guide
How 3,000 Secret Documents Ended Up Public
On March 27, 2026, cybersecurity researchers Alexandre Pauwels and Roy Paz discovered that Anthropic's content management system had been misconfigured — leaving a cache of roughly 3,000 unpublished assets accessible without authentication. The trove included draft blog posts, internal memos, and product roadmap documents.
The leak was reported exclusively by Fortune. Anthropic confirmed the incident as "human error" and secured the data within hours, but the contents had already been widely documented by the security research community and reported across major tech outlets.
The most significant document was a draft blog post announcing a new AI model called Claude Mythos — and with it, an entirely new tier in Anthropic's product lineup.
What Is "Capybara"? Anthropic's New Tier Above Opus
Until March 27, Anthropic's model hierarchy had three consumer tiers: Haiku (fast and lightweight), Sonnet (balanced), and Opus (most capable). Claude Mythos changes that.
The leaked draft blog post states: "'Capybara' is a new name for a new tier of model: larger and more intelligent than our Opus models — which were, until now, our most powerful." The full new hierarchy is:
- Haiku — fast, low-cost tasks
- Sonnet — balanced performance and speed
- Opus — previously the most capable
- Capybara (Mythos) — new flagship, a "step change" above Opus
According to the leaked documents, Mythos achieves "dramatically higher" scores than Claude Opus 4.6 in three domains: software coding, academic reasoning, and cybersecurity. SiliconAngle and CoinDesk both confirmed that Anthropic described it as "by far its most powerful system to date."
- Software coding — writes, debugs, and refactors complex code at a qualitatively higher level
- Academic reasoning — dramatically improved on multi-step logic, math, and research synthesis tasks
- Cybersecurity — described as "far ahead of any other AI model" in identifying and exploiting vulnerabilities
- Multi-step agentic tasks — implied by internal documents referencing autonomous workflow execution at new accuracy levels
The Cybersecurity Warning That Spooked the Stock Market
The most consequential detail in the leaked documents was Anthropic's own assessment of risk. The draft blog post described Mythos's cybersecurity capabilities as "far ahead of any other AI model," raising concerns about "unprecedented cybersecurity risks" — specifically, the potential to enable automated exploitation at a scale that human defenders cannot match.
Axios reported that Anthropic is privately warning top US government officials that its own model "makes large-scale cyberattacks much more likely in 2026." That is a striking statement for a company to make about its own product.
Financial markets reacted immediately. Shares of CrowdStrike and Palo Alto Networks — two of the largest enterprise cybersecurity vendors — each dropped more than 5% on the day of the leak. Investors interpreted a powerful AI hacking assistant as a direct threat to the traditional cyber defense business model.
Mythos is currently in early access testing only with cyber defense organizations. Anthropic has not announced a public release date. The company stated the model requires efficiency improvements before general availability — both to reduce serving costs and, implicitly, to implement additional safeguards before it reaches the broader market.
Claude Mythos, GPT-5.4, Gemini 3, Mistral Large — the frontier is moving fast. Happycapy Pro gives you every major model in one workspace today, so when the next breakthrough drops, you're already set up to use it. No new subscriptions. No API key juggling.
Try Happycapy Free →Where Mythos Fits vs. the Current AI Frontier
Based on leaked benchmarks and Anthropic's own characterization, here is how Mythos compares to the current publicly available frontier.
| Model | Tier | Status | Key Strength | On Happycapy |
|---|---|---|---|---|
| Claude Mythos (Capybara) | Above Opus | Early access — cyber defense orgs only | Coding, reasoning, cybersecurity | When released |
| Claude Opus 4.6 | Opus | Generally available | Complex reasoning, long context | Yes |
| GPT-5.4 | OpenAI flagship | Generally available | Broad tasks, OSWorld-V leader | Yes |
| Gemini 3 Pro | Google flagship | Generally available | Multimodal, Google ecosystem | Yes |
| Mistral Large 3 | Mistral flagship | Generally available | EU-sovereign, open weights | Yes |
| Claude Sonnet 4.6 | Sonnet | Generally available | Speed + quality balance | Yes |
What This Means for AI Users
The Mythos leak tells a story about where AI capability is heading: the gap between frontier models and the previous generation is widening, not narrowing. Claude Opus 4.6 — released just months ago — is already described as a tier below Mythos in Anthropic's own internal documents.
For users, this reinforces the case for multi-model access over single-model lock-in. The best model for any task changes. When Mythos eventually reaches general availability, users who are already running Claude alongside GPT, Gemini, and Mistral in one workspace will be the first to benefit — without any additional setup.
For the cybersecurity industry, the market reaction was a preview of a coming structural shift. Automated vulnerability discovery and exploitation at the scale Mythos apparently enables will change what "enterprise security" means in practice.
Happycapy Pro gives you Claude Opus 4.6, GPT-5.4, Gemini 3, Mistral, and 150+ more models at $17/month — plus persistent memory and multi-agent workflows. When Mythos ships, it will be right there alongside everything else.
Start Free on Happycapy →Frequently Asked Questions
Claude Mythos is Anthropic's next-generation AI model, accidentally revealed via a CMS misconfiguration on March 27, 2026. It belongs to a new product tier called "Capybara," which sits above the existing Haiku, Sonnet, and Opus tiers. Anthropic confirmed it as a "step change" in capabilities — dramatically higher than Opus 4.6 in software coding, academic reasoning, and cybersecurity.
Anthropic is withholding Claude Mythos because it presents "unprecedented cybersecurity risks." Internal documents describe its cyber capabilities as "far ahead of any other AI model," raising concerns about enabling automated exploitation at a scale that defenders cannot match. The model is also currently too expensive to serve at scale.
Capybara is a new fourth tier in Anthropic's product hierarchy, positioned above Claude Opus. The full lineup from least to most powerful is: Haiku → Sonnet → Opus → Capybara (Mythos). While many call it "Opus 5," leaked internal documents confirm Capybara is a distinct tier representing a significant leap beyond Opus.
No public release date has been announced as of March 31, 2026. Claude Mythos is currently in early access testing with cyber defense organizations only. When it does become available, it will likely appear in multi-model AI platforms like Happycapy alongside GPT-5, Gemini, and Mistral.