MythosLM

Independent reference. Not affiliated with, endorsed by, or sponsored by Anthropic PBC. Details.

Glossary term

Classifier fallback

Also: classifier reroute, Fable 5 fallback

Glossary informational Last reviewed Updated

Classifier fallback is the mechanism by which Fable 5 reroutes a request flagged by its safety classifiers to Claude Opus 4.8, notifying the user when it happens.

How it works

Fable 5 runs classifiers covering cyber, bio/chem, and distillation risk domains. When a request trips one, Fable 5 hands it off to Claude Opus 4.8 instead of answering with the full Mythos-class model — and the user is told this occurred.

Scale

The reroute affects fewer than 5% of sessions. Mythos 5, by contrast, has these classifiers lifted and is available only through Project Glasswing.

Sources & further reading

Facts on this page link to their source. Quotes are kept under 15 words and attributed; figures labelled unofficial are third-party until Anthropic publishes system-card numbers.