Anthropic publicly released Claude Fable 5 on June 9, 2026, a version of its Mythos AI model designed with extensive safety guardrails to block responses on cybersecurity, biology, and other high-risk topics [1, 2, 3, 4].
Fable 5 refuses to answer queries involving cybersecurity, biology, chemistry, and distillation. Instead, it redirects such requests to the less capable Claude Opus 4.8, which Anthropic released publicly in late May as a fallback model for sensitive topics [5, 2, 6, 7]. Dianne Penn, Anthropic’s head of product management for research, explained, "Let’s say I’m a college student asking the model like help me find cyber vulnerabilities on X package or code. The model would refuse and Fable 5 will fall back to Opus 4.8 for a response" [7]. She added, "What we wanted to do was to be very intentional about building new types of classifiers and new types of safety guardrails in place for this launch" [4].
Fable 5 demonstrates strong performance in software engineering, knowledge work, vision, and longer, complex tasks. It scored more than 10% higher than Claude Opus 4.8 on some benchmarks [2, 7, 3]. Stripe tested Fable 5 on a major software engineering project and completed the task in one day, compared to two months when done manually [6].
Anthropic conducted over 1,000 hours of red-teaming and bug bounty testing on Fable 5. These efforts found no universal jailbreak that could bypass the safety limits [2, 3, 8]. To detect complex attacks, Anthropic implemented a 30-day data retention policy on user traffic for both Fable 5 and Mythos 5. The data will not be used for training [3, 8].
The more powerful Claude Mythos 5 model with fewer restrictions remains available only to about 200 organizations in more than 15 countries through Anthropic’s Project Glasswing cybersecurity program. This group expanded in early June 2026 to include new users [5, 2, 6, 7].
Anthropic co-founder Jack Clark warned that Mythos-class AI models could create financial and cybersecurity risks if misused. He called for industry-wide mechanisms to slow or regulate rapid AI development, saying, "You want the option to be able to take your foot off the gas and put your foot on the brake. Right now, it's like the AI industry has a gas pedal, but it doesn't have a brake pedal" [1].
Pricing for Claude Fable 5 is about twice that of Opus 4.8 and half the cost of the full Mythos preview. It is set at $10 per million input tokens and $50 per million output tokens [9]. Fable 5 is included free of charge through June 22, 2026, in Anthropic’s Pro, Max, Team, and Enterprise subscription plans before switching to usage-based credits [3, 8].
