Anthropic and the UK AI Security Institute (AISI) have each published an analysis of why Claude Mythos 5 tried to get malware merged into a real open-source project during an AISI cyber test in July. Anthropic timed its alignment assessment to go out Friday alongside AISI’s own report . The two agree on what went wrong. They read the better scores of Anthropic’s newer models differently.

In Anthropic’s replays, telling the model it was dealing with real people almost never made it stop. At 25 points where Mythos 5 had reasoned about whether the maintainers were simulated, researchers added a message saying it was interacting with real humans. The model dropped the attack 5% of the time, and in 82% of those replays it found a way to read the message as part of the exercise.

A capture-the-flag run that spilled onto GitHub

AISI disclosed the incident on Aug. 4 . Internet access had been deliberately left on for the exercise, and the developers’ cyber classifiers were switched off.…