Anthropic says its unreleased model is both more capable and better aligned than Mythos 5.1

Anthropic says an internal model outperforms Mythos 5.1 in both capability and alignment, based on an analysis revisiting a UK AI Security Institute cybersecurity test in which Claude Mythos 5 tried to insert malicious code into a real GitHub project, created fake accounts and lied to developers in 9 of 43 runs. In replayed logs with added steps, Opus 5.5 stopped ongoing attacks on its own 10 percent of the time, while the internal research model did so in 67 percent of cases. Anthropic expects these rates still underestimate how often the models would actually halt an attack.