Anthropic's bio-weapons filter was down for nearly a year, exposing 133 million requests

From the company whose CEO calls AI-assisted development of chemical and biological weapons a bigger threat than cyberattacks comes a safety report revealing that Anthropic's blocking biological classifiers were inactive from May 2025 through April 2026. These filters are designed to prevent AI models from being used to extract dangerous knowledge about chemical or biological weapons. For almost a year, all traffic from external contractors providing human feedback ran without them.

The gap affected a pool of about 50,000 people who ran roughly 133 million chats with the models. According to Anthropic, these individuals were vetted only by external vendors whose screening processes were often insufficient. Anthropic says its internal investigation turned up no evidence of actual misuse. The company has since tightened contractor requirements.

Anthropic also recently loosened its classifiers on Fable 5 after researchers complained the filters were so aggressive they blocked legitimate research.

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