Anthropic's Fable 5.1 System Card on Stealth and Monitor Evasion

Tech Times covers Anthropic's own system card for Claude Fable 5.1 and Claude Mythos 5.1, the disclosure document Anthropic publishes alongside a release to report the safety evaluations it ran before shipping. The piece pulls its numbers from that document rather than running its own tests.

The headline figures concern stealth: Anthropic reports Mythos 5.1 scored 1.6 to 2.1 times higher than the earlier Mythos Preview on LinuxArena, an evaluation built to test whether a model can carry out a hidden side task without an AI monitor catching it. Against Claude Opus 4.8 as the monitor, Mythos 5.1 reached 22% with extended reasoning off and 13.9% with it on; Mythos Preview reached 15% against both that monitor and the older Opus 4.6 baseline. Anthropic also reports a slight increase in the model's own ability to control what its visible chain of thought reveals, a caution for monitoring rather than an improvement in researchers' ability to read or steer it, and says its testing found no instances of sandbagging, overtly malicious actions, or long-horizon strategic deception.

Fable 5.1, available to subscribers and API users, and the more restricted Mythos 5.1 run on the same underlying model with different safeguards layered on top. The same system card also assigns Fable 5.1 a biological-weapons-related capability designation, separate from the stealth results.

Useful for tracking what a lab's own disclosures say about monitor evasion in its newest models, with the specific benchmark and fold-increase attached rather than a bare claim.