Anthropic deploys Claude Sonnet 5, Fable and Mythos restored

July 1, 2026

Anthropic has launched Claude Sonnet 5 and restored access to its Fable and Mythos frontier models following a federal export control review.

The decision marks the conclusion of an eighteen-day operational pause triggered by a US government export control directive on June 12, which forced the temporary suspension of Anthropic’s highest-capability systems.

Government officials enacted the restriction after researchers at Amazon documented a method to bypass the safety controls of Fable 5, causing the model to identify software vulnerabilities and supply exploitation code. Anthropic has since developed an updated automated classifier to patch the vulnerability, clearing the path for a full commercial rollout across its platform, cloud infrastructure, and partner networks.

When the export control mandate took effect, the lack of real-time nationality verification systems required a total access blackout for all global users.

Security evaluations during the shutdown confirmed that vulnerability identification behaviour was not unique to Fable 5. Older architectures from multiple providers, including Claude Opus 4.8, GPT-5.5, and Kimi K2.7, duplicated the exact results.

Safety classifier and trade-offs

Engineers trained an automated safety classifier targeting the specific bypass mechanism reported by Amazon. Internal validation indicates the updated classifier prevents the reported exploitation technique in more than 99 percent of trials.

When a developer prompt triggers this boundary, the platform automatically routes the workload to the older Opus 4.8 architecture. The expanded safety margin introduces a trade-off: the automated system flags benign requests more frequently during routine application development and software debugging.

Fable 5 returns globally July 1 through the Claude Platform, Claude.ai, Claude Code, and Claude Cowork. Pro, Max, Team, and select Enterprise plans include the model through July 7 at up to 50 percent of weekly usage limits.

Mythos 5, the less restricted version, remains limited to US organizations that received government approval on June 26. Anthropic is working with the government to expand access via the Glasswing program.

Industry framework

Anthropic, Amazon, Microsoft, and Google are establishing an objective industry framework for assessing model security breaches. The proposed governance framework scores security breakdowns across four criteria:

• Capability gain: how far the exploit advances user capabilities beyond standard software utilities
• Breadth of capability gain: number of distinct offensive operations unlocked
• Ease of weaponisation: human engineering effort and specialized prompting required
• Discoverability: accessibility of the exploit technique in public research circles

A new HackerOne vulnerability research program allows security researchers to submit potential cyber jailbreaks in Fable 5 for review.

Commerce Secretary Howard Lutnick wrote that Anthropic agreed to proactively detect and address security risks, collaborate on standards for upcoming models, and inform the government of malicious activity. The Commerce Department reserves the right to reconsider lifting export restrictions.

Anthropic has formalised agreements under recent executive mandates to grant federal researchers early access to frontier architectures prior to public commercial release.