Anthropic’s Claude Fable 5, Mythos 5: What you need to know

Anthropic launched Claude Fable 5 and Mythos 5 with the former being a Mythos-class model with safeguards for cybersecurity. Based on the benchmarks, Claude Fable 5 is a nice leap in capability, but it'll cost you since it's double the price of Claude Opus 4.8.

Here's a look at what you need to know.

The difference between Claude Fable 5 and Mythos 5. Fable 5 has wide distribution and safeguards for cybersecurity. Queries on sensitive topics about cybersecurity are routed to Claude Opus 4.8. Claud Mythos 5 is the same model without as many safeguards. Mythos 5 will be deployed through Project Glasswing as an upgrade to Mythos Preview. See System Card for the safeguards.

• Claude Mythos and the New Cybersecurity Operating Model

Anthropic said:

"Fable 5 comes with a new set of classifiers: separate AI systems that detect potential misuse, including jailbreak attempts, and prevent the main model (in this case Fable 5) from responding. We’ve been running classifiers on our models for some time, and Fable 5’s classifiers are an extension of this previous work with extra coverage."

Fable 5 is a strong leap for knowledge work, software engineering, vision and scientific research. Fable 5 appears to widen its lead over other models the longer and more complex the task. Here's the benchmark chart. Early reviews on Substack and X have been promising.

See: My internal monologue reading Anthropic's big idea posts | Anthropic confidentially files for IPO

Distribution. Fable 5 is available on AWS, Google Cloud and Microsoft Azure.

What will Fable 5 and Mythos 5 cost you? Fable 5 and Mythos 5 will cost $10 per million input tokens and $50 per million output tokens. Anthropic cheerfully says that price is "less than half the price of Claude Mythos Preview," but neglects to mention Fable 5 and Mythos 5 is twice the cost of Claude Opus 4.8.

Capacity constraints likely mean usage credits for Fable 5 after a few weeks. Anthropic said Fable 5 will be included on Pro, Max, Team and enterprise plans through June 22 at no extra cost. On June 23, Anthropic will require usage credits. If Anthropic has capacity it'll extend the window. Ultimately, Fable 5 will be included into subscription plans when capacity is available.

The Fable 5 model launch will have the largest impact on software engineering, which is the most mature AI use case. Fable 5 is available on Cursor and will be the lead model on Claude Code. Fable 5 is more token efficient too, said Anthropic, who said Stripe found Fable 5 compressed months of engineering into days.

Use cases. Given the costs, enterprises will need to make sure they are using Fable 5 and Mythos 5 for the right use cases. Anthropic cited a drug design use case, which is interesting but likely more power than you'll need for most workflows.

Data retention policy changes. Anthropic said it is requiring a 30-day data retention policy for all traffic on Mythos-class models on first-party and third-party surfaces. Anthropic won't use the data to train new Claude models, but wants to log all human access to the data and ensure its deletion after 30 days.

Anthropic said:

"This change only applies to organizations that have set up workspaces with zero data retention (ZDR) in Claude Console, use Claude Code with ZDR in Claude Enterprise, or access Claude through AWS Bedrock, Google Cloud Agent Platform, or Microsoft Foundry with ZDR. For all other organizations, there is no change and there's nothing to configure. The rest of this section is for organizations that access Claude without data retention today and need to set up data retention in order to use designated models when they become available."

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