- Mythos-based Claude Fable 5 is so powerful that Anthropic requires data retention for security monitoring
- Microsoft Concerned About Insiders Sharing Sensitive Information With Anthropic Model
- Workers told to use Microsoft alternatives, but third-party partner models still available
Microsoft has reportedly restricted employees from using Claude’s recently released Fable 5 model while legal and compliance teams review data handling.
According to Anthropic’s latest retention policy for the Mythos class of models, which includes Fable 5, the company requires fast and exit retention for security monitoring, which goes against enterprise-level policies that customers like Microsoft have in place.
Anthropic emphasized that it needs to retain indications and results for 30 days, but content flagged by its security systems can be retained for up to two years if necessary for an investigation or law enforcement.
Microsoft stops use of Claude Fable 5 due to data retention issues
The current restriction, which applies to Microsoft workers, is in place due to concerns that insiders could share sensitive information such as customer information, corporate details and other sensitive business information.
Fable 5 was released following the huge success of Mythos, which was praised for its strong cybersecurity and vulnerability discovery capabilities.
However, that model was only released to a select group of partners for fear that attackers could misuse it, hence the subsequent release of the tamed Fable variant that the company describes as “safe for general use.”
Although probably unrelated, Microsoft’s push to halt Fable 5’s rollout follows recent news that the company was canceling internal Claude Code licenses, moving its developers to its own GitHub Copilot CLI tools.
Together, the two changes reflect Microsoft’s desire to prioritize in-house tools over third-party alternatives, likely due to cost benefits, but a clear willingness to use Anthropic systems also suggests the company is willing to lean on partners when necessary.
“It is not yet clear whether Microsoft’s legal teams will clear Claude Fable for internal use.” The edge aggregate.
Although there is a temporary ban, domestic demand could force Microsoft to change its tune. Mythos 5 and Fable 5 outperform key frontier models like GPT-5.5, Gemini 3.1 Pro, and Opus 4.8 in 13 test categories, including agent coding, knowledge work, and cybersecurity.
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