After months of headlines that teased a large language model with extraordinary capabilities, Anthropic launched Claude Mythos 5, which can crack software previously believed to be secure, and Claude Fable 5, a version for general use that limits what users can do in an unprecedented way.
What’s new: Claude Mythos 5 and Claude Fable 5 update Claude Mythos Preview, which has received strictly limited distribution since its rollout in early April. The two new models are identical, except Claude Fable 5 doesn’t respond to prompts related to security, biology, chemistry, or distillation and degrades its responses to prompts about building cutting-edge AI. They set new states of the art in a variety of areas including software engineering and knowledge work, and they’re priced at around half the price of Claude Mythos 5 Preview and twice the price of Claude Opus 4.8, Anthropic’s previous flagship model.
- Input/output: text, image in (up to 1 million tokens), text out (up to 128,000 tokens, 108 seconds to first token)
- Features: Adaptive reasoning adjusts depth and duration of reasoning based on input prompts (always on), five levels of reasoning effort (low, medium, high, xhigh, max), tool use, parallel subagents, safety classifiers (Claude Fable 5 only)
- Performance: Tops Artificial Analysis Intelligence Index, Humanity’s Last Exam (without tools), and evaluations of skill in coding and agentic coding, knowledge work, tool use, cybersecurity, spatial reasoning, scientific research
- Availability: Claude Mythos 5 initially to selected partners via Project Glasswing; Claude Fable 5 available via consumption-based pro- and enterprise-grade subscription plans (usage credits may apply after June 23), API $10/$50 per 1 million input/output tokens
- Undisclosed: parameter count, architecture, training data and methods
How it works: Anthropic disclosed little information about how it built Claude Mythos 5 and Claude Fable 5. Claude Mythos 5 is fine-tuned for alignment but not designed to be “safe for general use.” On the other hand, Claude Fable 5 implements extra layers of precaution, according to a lengthy system card. Anthropic advises that these precautions are not perfect and could hinder performance inappropriately.
- Prompts to Claude Fable 5 pass through classifiers that flag requests related to cybersecurity, biology, chemistry, distillation, or building cutting-edge AI. Given a prompt like this, Claude Fable 5 can be set to either refuse to respond or hand them off to Claude Opus 4.8, which is less capable. Users receive a message informing them that the less powerful model is responding to their request.
- With Claude Mythos 5, Claude Fable 5, and future models of equal or greater capability, Anthropic will retain “business customer data” for 30 days. It will use this information to improve management of malicious activity, not to train new models.
Performance: Independent evaluations were not available for Claude Mythos 5 at the time of this publication. Anthropic says its capabilities match those of Claude Fable 5, which Artificial Analysis ranked at the top of its Intelligence Index as well as several of the index’s component evaluations.
- On the Artificial Analysis Intelligence Index, Claude Fable 5 set to max effort with fallback to Claude Opus 4.8 ranks four points ahead of the next-best model, Claude Opus 4.8 itself. It achieved state-of-the-art metrics on GDPval-AA (performance of agentic real-world tasks), Terminal-Bench Hard (agentic coding and terminal use), 𝜏²-Bench Telecom (telephone customer service with tool use), AA-Omniscience Accuracy (recall of facts), Humanity’s Last Exam (reasoning based on factual recall), SciCode (scientific coding), and CritPt (reasoning over physics problems).
- Claude Fable 5 topped the AA-Omniscience Index, which balances a model’s ability to recall facts against its tendency to invent incorrect facts. While the model outperformed all others tested in the AA-Omniscience Accuracy component, a test of factual recall, it ranked 15th on the AA-Omniscience Non-Hallucination Rate, which measures how often a model answers incorrectly rather than refusing or admitting ignorance.
- In Artificial Analysis’ assessments of knowledge in specific domains and specific programming languages, Claude Fable 5 showed the greatest breadth among all models tested.
Safety concerns: Anthropic rates Claude Mythos 5’s and Claude Fable 5’s propensity to take actions that betray the user’s intentions “very low.” Nonetheless, it expressed concerns over Claude Mythos 5’s potential to behave in undesirable ways or help malevolent users — concerns it has addressed in Claude Fable 5.
- Anthropic worries that Claude Mythos 5 poses a threat to systems that afford “extensive access to sensitive assets” and “moderate capacity for autonomous, goal-directed operation and subterfuge.”
- The company believes the model cannot substitute for human expertise in pursuits like developing chemical or biological weapons. However, it finds it “difficult to say” whether people who possess undergraduate-level technical knowledge could use Claude Mythos 5 for these purposes.
- It's not worried that the model might “dramatically accelerate” research in areas like energy, weapons, or robotics.
Controversy: At its debut, Claude Fable 5 had a further limitation that Anthropic since has modified.
- Initially, given a prompt that related to building highly capable AI — for example, designing pretraining pipelines, distributed training infrastructure, or machine-learning accelerators — the model would “limit effectiveness through methods such as prompt modification, steering vectors, or parameter-efficient fine-tuning,” according to the system card. Moreover, it did this without notifying users that the model had degraded its capabilities.
- This limitation sparked sharp criticism from developers and researchers. For instance, AI researcher and policy analyst Dean W. Ball wrote, ”Degrading performance on ML research *without telling the user* is shockingly hostile.” Long-time tech blogger Robert Scoble observed, “I've never seen the AI community so angry at a major new model release.”
- Anthropic rapidly changed course. It revised this limitation so that inputs related to building highly capable AI, like inputs related to biology, chemistry, and cybersecurity, cause the model either to refuse to respond or pass the prompt to a less capable model, and it will notify users in either case.
Why it matters: Since April, when Anthropic revealed Claude Mythos Preview, security personnel have been working to prepare their operations for an AI-assisted onslaught, while the public has wondered just what this new class of model can accomplish. Indeed Claude Mythos 5 and Claude Fable 5 represent a significant improvement, notably in AI-assisted coding. While some observers view Anthropic’s emphasis on Mythos-class safety skeptically, sensing an effort to persuade the market that it has the most powerful technology, the bifurcation of Mythos into a fully capable model that has limited distribution and a guardrailed version for general use is reasonable while security teams continue their work.
We’re thinking: These models are impressive! But Anthropic’s decision to degrade Claude Fable 5’s ability to help developers build technology that might compete with Anthropic’s raises concerns — even if users are notified when it happens. Users should be able to use products as they wish for any legitimate purpose. Imagine Microsoft telling developers they couldn’t use Windows to build applications that competed with its own applications, or Google saying you couldn’t use its web search to find information on how to build a company that would compete with it! A fair, level playing field, as well as openness in technology and research, will result in better outcomes in the long term.