Fable 5 Classifiers: The Three Domains and How Reroute Works

What the classifiers are

A classifier here is a safety check that inspects an incoming request and decides whether it falls into a gated high-risk domain. Fable 5 runs three of them:

• Cybersecurity — offensive/exploitation-adjacent requests with meaningful dual-use uplift.
• Biology / chemistry — requests touching dangerous biological or chemical capability.
• Distillation — attempts to use Mythos-class outputs to train or extract a smaller model.

How flagging and reroute work

If none of the classifiers fire, the request is answered with the full Mythos-class model. If a classifier flags the request, Fable 5 invokes the classifier fallback: rather than answering at full capability, it reroutes to Claude Opus 4.8 and answers there. Crucially, the user is informed that the reroute happened — it is not silent degradation.

How often this happens

The classifiers are tuned to be narrow. Fewer than 5% of sessions hit a reroute, so the overwhelming majority of Fable 5 usage runs on the full model. The goal is to gate genuinely high-risk content without taxing ordinary work.

Fable 5 vs Mythos 5

These classifiers are the only difference between Fable 5 and Mythos 5. Mythos 5 is the identical underlying model with the classifiers lifted, available solely through Project Glasswing to vetted organizations.