Anthropic narrowed Fable 5's biology filter: the company says it redirects requests to its Opus 5 model about 85% less often

Anthropic narrowed Fable 5's biology filter: the company says it redirects requests to its Opus 5 model about 85% less often

On August 7, Anthropic announced that it had rewritten the rules for its biology filter and retrained it on new data. Before a response is generated, a separate classifier determines whether a biology request will be handled by Fable 5, the model available to users, or by Opus 5, another Anthropic model.

When Fable 5 launched in June, the broad filter sent even questions about cancer to the older model. In a new Anthropic post, the company explains how it changed this rule. It rewrote the classifier's rules, gathered feedback from internal and external experts, prepared new training data, and retrained the classifier.

The filter operates before the model generates a response. It reads the request and decides whether Fable 5 will handle it or the system will route it to Opus 5. In Anthropic's tests, the update reduced the number of these reroutes by about 85%.

According to Anthropic, the updated Fable 5 answers health questions more often, helps users study biology, and handles some clinical medicine tasks. Dual-use requests can support both medical treatment and the development of biological weapons. The company has retained a separate routing path for these requests:

“Today, Fable still routes requests that we consider dual-use, including those involving virology, toxicology, and molecular design, to Opus 5. It is therefore not yet suitable for professional biological research or drug development.”

The company says it will provide separate trusted access channels for professional biology work.