Claude Fable 5 and Mythos 5: The System Card

First things first: Claude Fable 5 is the new best publicly available model. I have noticed a step change, where Fable can suddenly help me in ways that previous models were not worth bothering to query. Almost everything it has noticed in one of my drafts so far has been spot on and it is downright scary. Suddenly I am motivated to once again continue improving my Chrome extension. I only ask for things I actually want or am curious about, and it has nailed every question I have asked it. That does not mean it is the right tool for every job. There are four good reasons to often not use Fable. Speed and price. Fable is importantly slower and more expensive than Opus 4.8, and often you will not need to make this trade. After the 22nd, when Fable may no longer be included in subscription plans if demand is too high, we may have to all pay by the token outside our subscriptions (although I suspect subscribers will get at least some credits to help with this), which could add up fast. Relative strengths. Capabilities are jagged. There will still be some tasks in which GPT-5.5 or another model will turn out to be better, or you want to use a different harness that works better with another model, or similar. Restrictions. Anthropic does not want its rivals to use this for advanced machine learning that is plausibly relevant to frontier model training, and has implemented countermeasures. Also, Anthropic has sufficient worries about biological misuse risks that a (sometimes comically) broad range of biological questions will bust you down to Opus 4.8. Data retention. You need to allow 30 day retention of data in order to use Fable. Those considerations aside, yes, it seems very obviously to be the best model. Another Week Another Giant System Card This is the traditional first post on a new frontier model, where I read the system card. I’ll get to capabilities in earnest next week, and will also deal with model welfare. We’ve been through very similar documents for Mytho