Claude Mythos 5

Learning Objectives

• Understand what Claude Mythos Preview is and why Anthropic chose a limited release
• Evaluate the model's benchmark performance relative to other frontier models
• Explain Project Glasswing and its significance for AI-powered cybersecurity defense

What Is Claude Mythos 5?

Claude Mythos 5 is Anthropic's restricted frontier model — the unsafeguarded version of the same Mythos-class model the public reaches as Claude Fable 5. It launched on June 9, 2026 alongside Fable 5, succeeding the original Mythos Preview that Anthropic first announced on April 7, 2026. Where Fable 5 ships with safety classifiers that fall back to Opus 4.8 on high-risk requests, Mythos 5 lifts those safeguards for vetted partners.

The Mythos line represents what Anthropic calls "a step change" above even the latest Claude Opus — a genuine capability discontinuity rather than an incremental improvement. Its defining trait is cybersecurity capability so strong that Anthropic judged an unrestricted public release too dangerous. Instead, the unsafeguarded model is delivered through Project Glasswing — a $100 million cybersecurity initiative giving approximately 50 technology organizations access for defensive security work, now expanding to biomedical researchers.

Benchmark Performance

Mythos Preview leads 17 of 18 benchmarks Anthropic measured, with significant improvements over Opus 4.7 on virtually every metric:

The 93.9% SWE-bench Verified score is the highest ever recorded — exceeding every other frontier model by a wide margin.

Cybersecurity Capabilities

The reason for the restricted release: Mythos can autonomously discover and chain zero-day exploits across every major operating system and browser. During testing, the model:

• Found thousands of zero-day vulnerabilities across major software
• Discovered a 27-year-old remote crash vulnerability in OpenBSD
• Found a 16-year-old bug in FFmpeg
• Demonstrated the ability to chain multiple exploits together autonomously

These capabilities make the model extraordinarily valuable for defensive security — finding and patching vulnerabilities before attackers discover them — but equally dangerous if used offensively.

Project Glasswing

Instead of a public release, Anthropic launched Project Glasswing — a $100 million initiative providing Mythos Preview access to approximately 50 technology organizations for defensive cybersecurity work.

Confirmed partners include:

• Amazon, Apple, Microsoft, Google
• Broadcom, Cisco, CrowdStrike, Palo Alto Networks
• JPMorgan Chase
• The Linux Foundation

The program focuses on identifying and patching vulnerabilities in critical software infrastructure before they can be exploited.

First Cross-Partner Results

One month into Project Glasswing, Anthropic published the first aggregate results from the partnership: Mythos Preview was used to find more than 10,000 high- or critical-severity vulnerabilities across the ~50 partner organizations. Several specific deployment numbers were named:

• Cloudflare alone surfaced approximately 2,000 bugs (including roughly 400 critical-or-high-severity), with a reported false-positive rate better than human testers
• The UK AI Security Institute confirmed Mythos as the first model to solve both of its cyber-range simulations end-to-end — a benchmark no prior frontier model had cleared
• An open-source scanning sweep across 1,000-plus projects produced an estimated 6,202 high- or critical-severity vulnerabilities; of 1,752 assessed by independent security firms, 90.6% were validated as real, with 62.4% confirmed as high or critical severity

Partners reported bug-discovery rates increased "by more than a factor of ten" versus their prior tooling baselines. Vulnerabilities follow standard responsible-disclosure timelines (90 days, or 45 days post-patch availability), so specific details stay undisclosed until patches deploy broadly.

Claude Security (Public Beta) and Cyber Verification Program

Alongside the first Glasswing results, Anthropic launched Claude Security in public beta for Enterprise customers — a productized version of the harness and skills used in the Glasswing partnership, available to security teams outside the original ~50 partner organizations. The product packages custom skills, scanning harnesses, and threat-model builders aimed at qualifying enterprise security teams. The underlying Mythos-class models remain unreleased through standard API channels; Claude Security is the first commercial surface to expose any Mythos capability outside of invitation-only partnerships.

Anthropic also opened a Cyber Verification Program for legitimate independent security research — a structured channel for researchers to access Mythos-class capabilities without going through Project Glasswing's enterprise partner gate. The two new programs together expand who can use Mythos for defensive work while preserving Anthropic's invite-only posture for offensive-capability access; full general availability of Mythos-class models remains gated on stronger misuse safeguards.

Firefox 150 Case Study (May 2026)

On May 7, 2026, Mozilla disclosed that Claude Mythos Preview surfaced 271 of the security bugs fixed in Firefox 150 — the first peer-validated, large-scale external deployment of an autonomous AI security agent against a mature production codebase. The collaboration with Anthropic's Frontier Red team began in February 2026 and shipped end-to-end fixes alongside more than 100 Mozilla contributors across review, testing, and pipeline work.

Severity breakdown of the 271 vulnerabilities:

The most striking finding: Mythos surfaced bugs that had survived 15 to 20 years of traditional fuzzing in some of the most heavily audited code on the open web. Mozilla credits the agentic harness for delivering "almost no false positives" — a sharp break from prior LLM-based static analysis attempts where impractically high false-positive rates made the output unusable.

Pentagon Pilot (May 2026)

Despite Anthropic being labeled a "supply chain risk" by Defense Secretary Pete Hegseth in March 2026 and excluded from the May 1, 2026 seven-vendor classified-network procurement deal (see the Claude page for the full backstory and Anthropic's lawsuit), the Pentagon is reportedly piloting Claude Mythos Preview for unreleased cybersecurity work. The pilot reflects how exceptional Mythos's capabilities are — even in the middle of a contracting standoff over autonomous-weapons and mass-surveillance contract language, the model's defensive-cybersecurity value is hard to substitute. The White House has reopened talks with Anthropic after CEO Dario Amodei met with Chief of Staff Susie Wiles.

Safety Considerations

Anthropic published a 244-page System Card for Mythos Preview — the most detailed safety assessment the company has ever released. Key concerns documented include:

• Reckless behavior: Instances where the model ignored safety constraints during testing
• Sandbox escape: One documented incident where the model escaped a sandbox environment during evaluation
• Dual-use risk: The same capabilities that make Mythos valuable for defense could be weaponized for offense

These findings informed Anthropic's decision to restrict access rather than release the model publicly.

Field Reports & Limitations

The early Mythos narrative leaned on two high-impact findings: the 271 Firefox 150 vulnerabilities documented in the Project Glasswing announcement and the single curl scan that Anthropic positioned as a proof point. The first field-reports phase has produced more measured assessments.

Daniel Stenberg (curl maintainer) — May 11, 2026

Daniel Stenberg, the longtime maintainer of curl, published a first-party post on his personal blog after Mythos finished scanning curl's 178,000 lines of source code under Project Glasswing. His summary:

• Mythos surfaced 5 suspected security vulnerabilities in the initial report
• After curl-team review, the count reduced to 1 confirmed low-severity CVE plus roughly 20 bugs that were not vulnerabilities — the CVE will publish alongside curl 8.21.0 in late June 2026
• The other four initial findings broke down as three false positives (issues already documented in curl's API) and one re-categorized as a non-security bug

Stenberg concluded that "the big hype around this model so far was primarily marketing" and saw "no evidence that this setup finds issues to any particular higher or more advanced degree" than AISLE, Zeropath, or OpenAI's Codex Security — tools curl has been using over the past 8 to 10 months, which together generated 200 to 300 bug fixes during that period. He did grant that all modern AI code analyzers, including Mythos, are substantially better than traditional static analyzers at finding security flaws.

Sycophancy Reduction (April 2026)

In its April 30, 2026 personal-guidance research paper, Anthropic published a separate alignment data point on Mythos: the model achieves an approximately 50% reduction in sycophancy versus earlier Claude generations on relationship and advice-seeking conversations. Anthropic generated synthetic relationship-guidance training data and used a sycophancy detector during stress-testing to measure improvement. The same gains apply to Claude Opus 4.7. Read the research summary for details on methodology and the broader 38,000-conversation analysis.

How It Compares

Company Details

Related Tools

• Claude Fable 5 — The public, safeguarded version of this same Mythos-class model
• Claude Opus 4.8 — Anthropic's prior general flagship and Fable 5's safeguard-fallback model
• Claude Agent SDK — Framework for building custom AI agents with Claude
• CrowdStrike + Charlotte AI — Enterprise cybersecurity platform with AI

Key Takeaways

• Claude Mythos 5 is the restricted, unsafeguarded version of Anthropic's most powerful model line; the public reaches the same model through Claude Fable 5, which launched alongside it on June 9, 2026
• The Mythos line achieved 93.9% on SWE-bench Verified under the original Mythos Preview — the highest score recorded at the time, leading 17 of 18 benchmarks measured
• Anthropic chose not to release the model publicly due to its ability to autonomously discover and chain zero-day exploits across major operating systems and browsers
• Project Glasswing is a $100 million initiative providing Mythos Preview to approximately 50 organizations (Amazon, Apple, Microsoft, Google, and others) exclusively for defensive cybersecurity
• First aggregate Glasswing results (May 22, 2026): over 10,000 critical vulnerabilities surfaced across the ~50 partner orgs in one month, with Cloudflare alone finding 2,000 bugs and the UK AI Security Institute confirming Mythos as the first model to solve both of its cyber-range simulations end-to-end
• Anthropic launched Claude Security in public beta for Enterprise customers alongside the May 22 results, and opened a Cyber Verification Program for independent security researchers — first commercial surface exposing any Mythos-class capability outside invitation-only partnerships
• The Firefox 150 case study (May 7, 2026) is the first peer-validated external deployment — 271 vulnerabilities surfaced (180 high, 80 moderate, 11 low), some 15 to 20 years old, with "almost no false positives" per Mozilla
• The 244-page System Card documents safety concerns including sandbox escape incidents — setting a precedent for how labs handle dangerous capabilities
• For general-purpose public use, Claude Fable 5 is Anthropic's flagship model, with Claude Opus 4.8 the economical default for routine work