Claude Mythos 5

Claude Mythos 5 is a large language model released by Anthropic on June 9, 2026 as the restricted top tier of its Claude family. It belongs to a new "Mythos-class," which Anthropic describes as "a tier of Claude models that sit above our Opus class in capability." ^[1] Mythos 5 shares the same underlying model as its generally available sibling, Claude Fable 5; the two versions differ only in their safety measures. Mythos 5 runs with certain dual-use safeguards lifted and is available only to a small set of approved organizations, while Fable 5 carries additional safeguards and is open to the public. ^[1][2]

What is Claude Mythos 5?

Claude Mythos 5 is the higher-access version of Anthropic's Mythos-class model line, announced alongside Claude Fable 5 on June 9, 2026. ^[1] It succeeds Claude Mythos Preview, a restricted model that Anthropic began deploying in April 2026 through a cybersecurity effort called Project Glasswing. ^[1][2] Anthropic positions Mythos-class models above the Opus class, which had been represented by Claude Opus 4.8. The company reports strong results in software engineering, knowledge work, vision, and scientific research, and says the models can work autonomously for longer than any previous Claude release. ^[1]

Because Mythos 5 is restricted, Anthropic has not published a separate benchmark suite for it. Its capability is instead defined by the shared model. Anthropic states that for the more than 95% of Fable 5 sessions that involve no safety fallback, "Fable 5's performance is effectively the same as that of Mythos 5." ^[1] Independent evaluators reported Fable 5 at 95.0% on SWE-bench Verified, a leading score that reflects Mythos 5's underlying capability as well, since the two share the same model. ^[3]

How is Mythos 5 different from Claude Fable 5?

Fable 5 and Mythos 5 are the same underlying model. As Anthropic puts it, "the safeguards are what distinguish the two models." ^[1] The names reflect this relationship: Fable comes from the Latin fabula ("that which is told"), which is akin to the Greek mythos. ^[1]

Claude Fable 5 is the Mythos-class model that Anthropic has "made safe for general use." It runs safety classifiers that detect high-risk requests in specific domains and, when triggered, route the response to Claude Opus 4.8 instead of answering with the full Mythos-class model. Anthropic tuned these classifiers conservatively so they activate, on average, in fewer than 5% of sessions, and users are told whenever a fallback occurs. ^[1] Claude Mythos 5 is the same model with those safeguards "lifted in some areas," giving approved users direct access to the frontier capabilities that Fable 5 holds back. ^[1]

Both versions carry the same price, $10 per million input tokens and $50 per million output tokens, which Anthropic says is less than half the price of Claude Mythos Preview. ^[1][2]

Who can access Claude Mythos 5?

Access to Mythos 5 is limited to vetted organizations rather than the general public. ^[2] At launch it was available to a small group of cyberdefenders and critical infrastructure providers through Project Glasswing, which Anthropic runs in collaboration with the US government; organizations that already had Claude Mythos Preview could upgrade to Mythos 5. ^[1][2] Anthropic also described trusted access programs that let cybersecurity organizations apply for access and that enroll biology researchers from a range of life science organizations, with different safeguards lifted depending on the program. ^[1]

Use of Mythos 5 is governed by a 30-day data retention policy for safety monitoring. Anthropic says it will not use this traffic to train new Claude models "or for any non-safety-related purpose." ^[1] Anthropic briefly suspended access to both Fable 5 and Mythos 5 on June 12, 2026, then redeployed them on July 1, 2026. ^[1]

Why is access to Mythos 5 restricted?

The restriction reflects the dual-use nature of the model's strongest capabilities. Anthropic notes that advanced use of frontier models is often dual-use: the same cybersecurity and biology queries that help defenders and researchers could help malicious actors cause serious harm that they could not obtain elsewhere. ^[1] To manage this, Fable 5's classifiers cover three areas: cybersecurity (such as vulnerability exploitation and agentic hacking), biology and chemistry (dual-use bioweapons research), and model distillation. Mythos 5 removes some of these safeguards only for approved users whose work depends on that access. ^[1] This restricted-access model reflects Anthropic's broader approach to AI safety and responsible scaling.

Anthropic reported supporting evidence for the deployment. An external bug bounty found no universal jailbreaks in more than 1,000 hours of testing, though the UK AI Safety Institute made progress toward one in a brief initial window. In automated alignment assessments, Mythos 5 showed low levels of misaligned behavior, similar to Claude Opus 4.8. ^[1]

How does Mythos 5 relate to Claude Opus 4.8?

Mythos-class models sit one tier above the Opus class, so Mythos 5 represents a capability step beyond Claude Opus 4.8. ^[1] Opus 4.8 also plays a direct role in the Mythos-class deployment: it is the fallback model that answers Fable 5 queries flagged by the safety classifiers. For the small share of sessions where that happens, users receive an Opus 4.8 response rather than the full Mythos-class output, and Anthropic says the two are otherwise the same underlying system for unflagged work. ^[1] Opus 4.8 therefore serves as both the capability floor beneath the Mythos class and the safety backstop for its public version.

References

1. Anthropic. "Claude Fable 5 and Claude Mythos 5." June 9, 2026 (updated July 1, 2026). https://www.anthropic.com/news/claude-fable-5-mythos-5 ↩
2. Anthropic. "Claude Mythos." Product page. https://www.anthropic.com/claude/mythos ↩
3. llm-stats. "Claude Fable 5: Review, Benchmarks and Pricing." June 2026. https://llm-stats.com/blog/research/claude-fable-5-review ↩