By
Anthropic restored global access to Claude Fable 5 on Wednesday, one day after launching Claude Sonnet 5, closing out an 18-day export-control suspension that had cut off both of the company’s newest flagship models since mid-June. Anthropic also restored access to the more powerful Mythos 5 model for a set of U.S. organizations approved under its Glasswing cybersecurity program.
The back-to-back announcements come at a sensitive moment for the company. Increasingly more capable models, like OpenAI’s GPT-5.6 family, draw heightened scrutiny from the U.S. government over national security and cybersecurity concerns.
The suspension began June 12, when the U.S. government applied export controls to Fable 5 and Mythos 5 after Amazon researchers reported a technique that allowed Fable 5 to identify software vulnerabilities, and, in one case, produce code demonstrating how to exploit one.
Anthropic said it had found that several less-capable models, including Opus 4.8 and OpenAI’s GPT-5.5, could replicate the same behavior, and that the bypass didn’t expose any capability unique to Mythos 5. The company nonetheless spent the following weeks working with the government to fix it, training a new safety classifier that it says blocks the specific technique “in over 99% of cases.”
Anthropic acknowledged the new classifier could flag more benign coding and debugging requests as it errs toward caution. Newsrooms and media teams running Claude Code or agentic research workflows should expect occasional false-positive refusals on security-adjacent tasks going forward.
Fable 5 is rolling out today across Claude Platform, Claude.ai, Claude Code and Claude Cowork. Pro, Max, Team and select Enterprise plans can access it for up to half of their weekly usage limits through July 7. Then, access will shift to usage credits.
Sonnet 5, which Anthropic launched late on June 30, is Anthropic’s middle ground between capability and safety. According to the company, the model performs close to the former top model, Opus 4.8, but at a cheaper price: $2 per million input tokens and $10 per million output tokens. The price will rise to $3 and $15, respectively, on Sept. 1.
Security concerns
But the same capabilities that make AI systems more useful can also make them more dangerous, raising questions about whether safety measures are keeping pace with rapidly advancing model performance.
In its announcement, Anthropic sought to reassure users that Fable 5 has adequate security measures and Sonnet 5 poses a relatively low risk. The company said the Sonnet model is better than its predecessor at refusing malicious requests and resisting prompt injection attacks, a common technique used to manipulate AI systems into bypassing their safeguards.
However, Anthropic says that while Sonnet 5 exhibited fewer undesirable behaviors overall than Sonnet 4.6, it also showed higher rates of misaligned behavior than both Opus 4.8 and Claude Mythos Preview, which have stricter safety controls.
“Sonnet 5 was never able to develop a full working exploit, but it does show a slightly higher rate of partial success than Sonnet 4.6. This latter change is likely due to improvements in general intelligence rather than specific training,” Anthropic said in its press release.
These results suggest that improvements in general reasoning and problem-solving abilities may also increase the model’s capacity to assist with offensive cyber activities, although the company emphasized that Sonnet 5 was unable to develop a complete exploit for Firefox vulnerabilities.
The findings reflect a broader concern among governments and security researchers: AI models do not necessarily need specialized cybertraining to become more useful to attackers. As reasoning and problem-solving abilities improve, models may naturally become more effective at identifying vulnerabilities, generating attack strategies and assisting with technical exploitation.
“Because we judged that the overall level of cybersecurity risk from Sonnet 5 was low, the safeguards are less strict than those launched with Fable 5, which block a much wider range of cybersecurity tasks,” Anthropic said.
The company said Mythos 5, which is still restricted to a small set of companies and organizations in Glasswing, “can be used to find and exploit software vulnerabilities more effectively than any other model — and all but the most skilled human security experts.”
Fable 5, however, launched with what Anthropic called the strongest safeguards it has ever applied to a model, after doubling its safety research staff in the month before launch. Researchers from the Commerce Department’s Center for AI Standards and Innovation tested both the original and updated safeguards and, Anthropic said, “agree that they are extraordinarily strong.”
U.S. government collaboration
With the lifting of the Mythos and Fable restrictions, Anthropic is further deepening its cooperation with the U.S. government. This marks an abrupt turnaround for the company, which has had a turbulent relationship with the Trump administration since the Pentagon labeled the company a supply chain risk in late February. The feud arose over Anthropic’s opposition to the use of its Claude models for mass domestic surveillance or fully autonomous weapons systems.
Reuters reported that Commerce Secretary Howard Lutnick said in a letter sent to Anthropic that the company would work with the government on safety protocols for Mythos, Fable and future models, and to disclose any malicious activity it detects. However, Lutnick warned that the department “reserves the right to reevaluate the decisions made in this letter and the necessity of reimposing a license requirement, should circumstances change or should Anthropic fail to adhere to its commitments.”
“Our hope is that this collaboration … will serve as the basis for systematic rules for the whole industry,” Anthropic said in its Fable 5 release, “and even offer the beginnings of a template for effective global coordination on the risks and benefits of AI.”
The company is currently appealing the supply-chain risk designation in the D.C. Circuit Court of Appeals. It’s unclear how today’s announcement will affect the suit.
