It took Anthropic’s most advanced artificial-intelligence model about 20 minutes to find its first Firefox browser bug during an internal test of its hacking prowess.

The Anthropic team submitted it, and Firefox’s developers quickly wrote back: This bug was serious. Could they get on a call?

“What else do you have? Send us more,” said Brian Grinstead, an engineer with Mozilla, Firefox’s parent organization.

Anthropic did. Over a two-week period in January, Claude Opus 4.6 found more hig