Using Claude Fable 5 means your data will be collected. It’s not optional.

Anthropic just released its most powerful public model yet — Claude Fable 5. However, along with the model's release, the AI giant also made a significant update to its data retention policies.

Fable 5 was released to the public on Tuesday. Fable 5 is a "safe for general use" version of Anthropic's most powerful model, Mythos, which has been restricted from public use due to its potentially dangerous cybersecurity capabilities. Anthropic created a set of safety guardrails for Fable 5, and its benchmarks blow away much of the competition, per Anthropic.

But it looks like Anthropic has also blown away its data retention policies for Fable 5.

"To ensure we’re responsibly deploying Mythos-class models, we are requiring limited data retention and review as part of our safety work," reads an update on Anthropic's official Claude support page. "Prompts submitted to, and outputs generated by, Mythos-class models are retained for 30 days for trust and safety purposes, on every platform where these models are offered."

The update was first noticed by Jun Park, the CEO of AI training company hillclimb.

“New policy from Anthropic: if you use Fable/Mythos, they collect your data. No exceptions. Not even for enterprise partners," Park posted on X.

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This change is significant for Anthropic's enterprise and API customers, says Jessica Eaves Mathews, a lawyer who specializes in copyright, trademark, and AI law.

In a post on Mathews' Substack (as highlighted by CyberNews), the lawyer explains how Anthropic already retains user data for 30 days under its free and paid consumer plans. However, Matthews says this change nullifies part of any agreement Anthropic has with its enterprise and API partners. 

"Every other Claude model available through the API, including Opus 4.8, Sonnet 4.6, and Haiku 4.5, can operate under Zero Data Retention (ZDR) agreements," Mathews writes. "Fable 5 cannot. If your organization previously had a ZDR agreement with Anthropic, that agreement does not apply to Fable 5 traffic. This is a policy change that overrides existing enterprise commitments for this specific model class."

Mathews says that any organization that believed that their data would not be stored by Anthropic should know that there is now a "mandatory exception" for Fable 5 and all future Mythos models.

While Mythos-class models seem to be quite powerful, companies should know about the change in Anthropic's data retention policies and make adjustments where necessary.