Microsoft Blocks Claude Fable 5 for Its Employees: Here's What Happens and Why

Anthropic released Claude Fable 5 on June 9, the first public model of the Mythos class. The launch is accompanied by a mandatory retention policy that prompted Microsoft to block the model for its employees in the hours following the release, pending a legal assessment.

The policy is the most significant novelty compared to the other models in the family. Fable 5 requires all operators integrating it, on any first or third-party surface, to retain prompts and outputs for 30 days. Data flagged as potential policy violations can be held for up to two years.

Anthropic clarifies that the collected material is not used for training new models or for purposes unrelated to security.

Opus, Sonnet, and Haiku continue to operate under Zero Data Retention (ZDR), which guarantees enterprise operators no retention of conversation data. Fable 5 breaks this uniformity.

Microsoft Blocks Fable 5 Internally

According to Reuters, Microsoft has already restricted access to the model for its employees: Fable 5 is not available in the selector used internally for enterprise versions of GitHub Copilot. Microsoft's legal team is assessing the compliance of Anthropic's new retention policy. Concerns particularly relate to customer data and confidential information, which, under the new rule, would remain within Anthropic’s infrastructure for at least 30 days. Other Claude models remain available internally, precisely because they are covered by ZDR.

Microsoft has not made any public statements. The news is based on anonymous internal sources; the company currently does not wish to comment.

Safeguards and Pricing

Fable 5 introduces an automatic mechanism for handling sensitive requests. When internal classifiers detect a question related to cybersecurity, biology, and chemistry, or model distillation, the response is automatically routed to Claude Opus 4.8. Anthropic states that this routing activates on average in less than 5% of sessions.

Pricing is positioned at the high end of frontier models: $10 per million tokens in input and $50 per million tokens in output. The mandatory retention policy addresses retrospective audit requirements on the behaviors of Mythos-class models but introduces a new constraint in relations with enterprise operators. Microsoft's reaction, currently lacking a public stance, seems to suggest that managing this issue will likely be at the center of commercial negotiations in the coming weeks.