Project Glasswing and Claude Mythos, Part 2: Fable 5 and Mythos 5

Part 1 covered the April 7, 2026 launch of Project Glasswing and Claude Mythos Preview: a restricted AI model that autonomously identifies zero-day vulnerabilities, deployed under a controlled framework involving Anthropic and eleven major technology partners.

On June 9, 2026, Anthropic released two models that continue that story. Claude Fable 5 is the general-availability version of the same underlying capability, with safeguards applied. Claude Mythos 5 is the upgraded successor to Mythos Preview, still restricted to Project Glasswing partners.

What changed from Mythos Preview to Mythos 5

Mythos 5 is a new model generation built on the same foundation as Mythos Preview but more capable across all task categories. Software engineering scores now range from 77.8% to 93.9% depending on the benchmark. The model handles long-horizon autonomous work across multiple domains: extended codebase migrations, complex analytical tasks, protein design, and novel research generation.

Access remains restricted to Project Glasswing cybersecurity partners and a set of approved biology researchers. The Glasswing framework is the mechanism for controlled deployment: the capability exists, and the partner structure determines who can use it and for what.

What Fable 5 is

Fable 5 is the broadly available model built from the same underlying architecture as Mythos 5. The difference is a set of capability classifiers that activate before the model responds to certain categories of request.

Three classifiers are active in Fable 5:

• Cybersecurity queries route to Claude Opus 4.8 rather than the Fable 5 model
• Biology and chemistry requests with dual-use potential are handled the same way
• Attempts to distil the model’s weights or replicate its capabilities are blocked

Anthropic reports these classifiers trigger in fewer than 5% of sessions. External red team testing across more than 1,000 hours found no universal jailbreaks.

The practical effect: Fable 5 is a capable general-purpose model for software engineering and knowledge work, but the specific capabilities that make Mythos 5 useful for Glasswing are not present. Cybersecurity work through Fable 5 will use Opus 4.8 as the backend.

What Fable 5 can do in software engineering

The non-restricted capabilities in Fable 5 are significant on their own. Stripe reported that Fable 5 compressed months of engineering work into days, completing a large codebase migration in one day that would have taken two months manually. The model handles long-horizon tasks without requiring continuous human steering: scoping, execution, and verification across extended task chains.

Other reported capabilities include vision strong enough to rebuild web applications from screenshots, top-tier performance on complex financial analysis benchmarks, and autonomous research generation that produced peer-reviewed findings in molecular biology. The software engineering applications are the most immediately relevant for most teams.

Pricing and availability

Fable 5 is available immediately through the Claude API and through subscription plans, with a phased rollout for subscription access. Pricing is $10 per million input tokens and $50 per million output tokens.

Mythos 5 is not available through standard API access. It is available to Project Glasswing partners and approved biology researchers through the existing Glasswing partner framework.

What this looks like from a software engineering perspective

The two-model structure reflects a deliberate split between capability and access. The Glasswing framework gave Anthropic a mechanism to deploy and study full Mythos-class capabilities in a controlled environment before deciding what a general-availability version should look like. Fable 5 is the output of that process: a capable model for general software work, with the highest-risk capabilities routed away rather than removed entirely.

For engineers not in the Glasswing partner network, Fable 5 is what is accessible. For security teams within those partner organisations, Mythos 5 now replaces Mythos Preview. The 90-day commitment from the April launch to share learnings publicly means a clearer picture of what Mythos 5 has found, and how, should be available before the end of the third quarter.

Update: June 12, 2026

Three days after the Fable 5 and Mythos 5 release, Anthropic issued a statement announcing that both models had been suspended globally under a U.S. government directive citing national security concerns. Access was cut for all users, including foreign nationals and Anthropic employees. Other Anthropic models were not affected.

The directive followed a government demonstration of a potential jailbreak for Fable 5 that could be used to identify minor, previously known vulnerabilities. Anthropic reviewed the finding and disputed the response, stating that a narrow, non-universal jailbreak does not meet the bar for recalling a commercial model deployed at scale. The company described its red-teaming as extensive, involving government and third-party organisations, with no universal jailbreak discovered during that process.

Anthropic said it was working to restore access as soon as possible. No timeline was given.

A further post will follow once the situation resolves or new details become public.