Claude Fable 5 and Mythos 5: Capabilities

Only three days after the release of Claude Fable 5, Anthropic was forced by the United States Government to make it unavailable, when a jailbreak was brought to its attention, rather than the previous situation of ‘yes obviously experts can jailbreak anything if they care enough’ and ‘yes obviously you can ask Fable to fix your code.’ Three days was enough time for many of us to learn to love Fable, and for us to dearly miss it now that it is gone. The world was briefly smarter, and now it is again stupider. At some point it will get smarter again, which will likely be within two weeks. This post is written as if Fable 5 is again available for public use, rather than trying to include a lot of qualifying clauses. It remains to be seen how this will play out, and this post does not attempt to cover that question. My previous release coverage of Fable covered the model card and then model welfare. Coverage of the government takedown of Fable starts here, and continues here and here. The Official Pitch The pitch is that Fable 5 is the best model and can solve your hardest problems. Anthropic: Today we’re launching Claude Fable 5: a Mythos-class1 model that we’ve made safe for general use. Fable 5’s capabilities exceed those of any model we’ve ever made generally available. It is state-of-the-art on nearly all tested benchmarks of AI capability, showing exceptional performance in software engineering, knowledge work, vision, scientific research, and many other areas. The longer and more complex the task, the larger Fable 5’s lead over our other models. ClaudeDevs: Start at the top of your difficulty range: something harder than you’d assume previous versions of Claude can accomplish. Pick a backlog item you’d scope at a week, let Fable 5 interview you for the spec, turn on auto mode, and check back in the morning. You may notice that Fable feels different. Thinking is always on, and responses can take longer. Effort controls how much it thinks. We recommend high as the