Anthropic announced Claude Fable 5 on June 9, 2026 (US time). The new model opens up the "Mythos class"—a capability tier above Opus that had previously been available only to a limited group of cyber defense organizations—to general users for the first time, with a new set of safeguards built in. The company also released Claude Mythos 5, a version with some of those safeguards lifted, on a restricted basis[1].

The Mythos Class, Above Opus, Opens to the Public for the First Time

The Mythos class is a model tier Anthropic established above Opus. The first model in the tier, Claude Mythos Preview, was released in April 2026 exclusively to a limited group of cyber defenders and critical infrastructure providers through Project Glasswing, a cyber defense program run in collaboration with the US government[1].

Fable 5 is more capable than any model the company has ever made generally available, achieving state-of-the-art results on nearly all tested benchmarks. Performance gains are especially pronounced in software engineering, knowledge work, vision, and scientific research, and the longer and more complex the task, the larger Fable 5's lead over existing models[1].

Fable and Mythos are the same underlying model; the only difference is the safeguards. Mythos 5, with the cybersecurity restrictions lifted, will initially be provided to Project Glasswing participants[1].

Performance Across Domains, From Software Engineering to Drug Design

Stripe, a payments company with early access, reported that Fable 5 completed a migration across a 50-million-line Ruby codebase in a single day—work that would otherwise have taken a whole team more than two months by hand. The model also scored highest among frontier models on Cognition's FrontierCode coding evaluation[1].

One example of its progress in vision is beating Pokémon FireRed. Previous Claude models needed complex harnesses with additional helper tools, but Fable 5 completed the game with a minimal setup that takes only raw game screenshots as input[1].

In scientific research, internal testing with Mythos 5 (the version with safeguards lifted) accelerated parts of the protein design process in drug development by around ten times. Nine of 14 protein targets yielded strong drug design candidates, and in blinded comparisons of molecular biology hypotheses, the company's scientists preferred Mythos's hypotheses over those of Opus-class models about 80 percent of the time[1].

New Safeguards: Some Queries Are Answered by Opus 4.8 Instead

Because releasing Mythos-class capabilities unfiltered carries risks of misuse for cyberattacks or dangerous biological research, Fable 5 ships with new classifiers. When they detect a request related to cybersecurity, biology and chemistry, or distillation (extracting the model's capabilities), the response is handled by the next-most-capable model, Claude Opus 4.8, instead of Fable 5 itself—and users are informed whenever this occurs[1].

Anthropic has deliberately tuned these safeguards conservatively, meaning harmless requests will sometimes be falsely flagged. However, the fallback occurs in less than 5 percent of sessions, and in the remaining sessions Fable 5's performance is effectively the same as Mythos 5's. An external bug bounty spanning more than 1,000 hours of testing produced no universal jailbreaks that disable the safeguards entirely[1].

For Mythos-class models, Anthropic will also require 30-day data retention on all traffic, including business customers, to help detect novel attacks and reduce false positives. The company states this data will not be used to train new models[1].

Pricing: $10 Input / $50 Output, Included in Subscriptions Until June 22

Both Fable 5 and Mythos 5 are priced at 10 USD (about 1,600 yen) per million input tokens and 50 USD (about 8,000 yen) per million output tokens—less than half the price of Mythos Preview. Developers can use the model via the Claude API with the model name claude-fable-5[1].

For subscription plans, the rollout is staged. From launch day through June 22, Fable 5 is included on Pro, Max, Team, and seat-based Enterprise plans at no extra cost; from June 23, using it will require usage credits. Anthropic says it aims to restore Fable 5 as a standard part of subscriptions once capacity allows[1].

※1 USD = 160 JPY (as of June 9, 2026)

Summary

Claude Fable 5 is the first model to open Mythos-class capabilities—previously limited to a select few—to general users, packaged with new safeguards. The decision to prioritize an early release even at the cost of false positives may divide opinion, but the window through June 22, when the top-tier model is available on monthly plans, is a good opportunity to judge its performance firsthand[1].

出典:https://www.anthropic.com/news/claude-fable-5-mythos-5