Anthropic has released Claude Fable 5, a Mythos-class general-purpose model, alongside new safety guardrails. The company says Fable 5 handles longer and more complex tasks than any prior Claude model, working autonomously for extended periods while maintaining focus across millions of tokens.
Claude Fable 5 Goes Live
Fable 5 is available through Claude products and the API via the claude-fable-5 identifier. Anthropic reports strong performance in software engineering, knowledge work, vision, and scientific research, with the biggest lead on longer, more intricate tasks. In a test, Stripe completed a large Ruby migration in a single day using Fable 5; the same effort would have taken a team over two months by hand. The company also highlighted gains in finance, vision, memory, and research. However, Anthropic noted the extra controls were necessary given the model's advanced capabilities.
Fallback Mechanism Routes Some Queries to Opus 4.8
Certain Fable 5 requests will fall back to Claude Opus 4.8. This applies to selected cybersecurity, biology, chemistry, and distillation-related queries. Anthropic says the fallback triggers in less than 5% of sessions, with over 95% of sessions unaffected. The safeguards are tuned conservatively and may catch harmless requests; the company plans to reduce false positives post-launch. New classifiers detect potential misuse and jailbreak attempts, blocking direct responses from Fable 5. Cybersecurity controls cover exploitation and offensive cyber tasks, while biology and chemistry safeguards address dual-use risks widely.
Claude Mythos 5 Opens to Trusted Groups
Anthropic also introduced Claude Mythos 5 for a narrower set of approved users. Mythos 5 uses the same underlying model as Fable 5 and launches via Project Glasswing in coordination with the U.S. government. Initial access goes to cyberdefenders and critical software infrastructure providers. The company says Mythos 5 lifts some safeguards for approved cybersecurity users and plans a trusted access program for selected biology researchers and firms. Business customer traffic on Mythos-class models will be retained for 30 days but not used to train new Claude models.
Pricing and Availability
Fable 5 and Mythos 5 both price at $10 per million input tokens and $50 per million output tokens. Fable 5 is included in Pro, Max, Team, and seat-based enterprise plans through June 22. Starting June 23, additional usage will require credits unless capacity permits an extension.