Claude Fable 5 and Mythos 5 ship with major coding and reasoning gains
Tim Zinin describes the June 9 release of Claude Fable 5 (public) and Mythos 5 (closed, for cybersecurity defense partners), noting benchmark leadership, dramatic real-world acceleration in coding and pharma, and a free access window through June 22.
Score breakdown
The post reports that Fable 5 tops coding and reasoning benchmarks and delivered immediate, measurable acceleration on large-scale real-world tasks, marking a notable step-change in agentic coding capability.
- 01Claude Fable 5 is a public Mythos-class model priced at $10/M input and $50/M output.
- 02Mythos 5 is a closed release under Project Glasswing for cybersecurity defense partners, with loosened operational constraints for specialized threat research.
- 03Stripe ran a 50M-line Ruby migration through Fable 5 and compressed the timeline from months to days.
Tim Zinin's post describes the June 9 release of two Anthropic models and his firsthand experience rebuilding his agent stack overnight. Claude Fable 5 is the public release — Mythos-class, priced at $10/M input and $50/M output — while Mythos 5 is a closed version released under Project Glasswing specifically for cybersecurity defense partners, with loosened operational constraints for specialized threat research. Zinin reports that Fable 5 sits at the top of nearly all benchmarks used to evaluate serious coding and reasoning work.
Pricing-wise, Fable 5 runs at twice the cost of Opus 4.8 but below half the price of Mythos Preview.
Real-world results cited in the post are striking: Stripe ran a 50M-line Ruby migration through Fable 5 and compressed the timeline from months to days, pharma teams reported up to 10x acceleration on drug development stages, and in a blind evaluation scientists chose the model's output 80% of the time. On the safety side, dangerous queries trigger an automatic fallback to Opus 4.8 in under 5% of sessions, and Anthropic conducted over 1,000 hours of adversarial testing without finding a universal jailbreak at release.
Pricing-wise, Fable 5 runs at twice the cost of Opus 4.8 but below half the price of Mythos Preview. Free access is available through June 22 on Pro, Max, Team, and Enterprise tiers. Zinin describes the improvement in agentic task completion as immediately perceptible — noticeable within the first hour of use.
Key facts
- 01Claude Fable 5 is a public Mythos-class model priced at $10/M input and $50/M output.
- 02Mythos 5 is a closed release under Project Glasswing for cybersecurity defense partners, with loosened operational constraints for specialized threat research.
- 03Stripe ran a 50M-line Ruby migration through Fable 5 and compressed the timeline from months to days.
- 04Pharma teams reported up to 10x acceleration on drug development stages.
- 05Scientists chose the model's hypotheses 80% of the time in blind evaluations.
- 06Dangerous queries trigger a fallback to Opus 4.8 in under 5% of sessions; Anthropic ran over 1,000 hours of adversarial testing with no universal jailbreak found at release.
- 07Free access runs through June 22 on Pro, Max, Team, and Enterprise tiers; Fable 5 costs twice Opus 4.8 but less than half the price of Mythos Preview.
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