High-end agent work is entering the routing phase
Anthropic has announced Claude Fable 5.1 and Claude Mythos 5.1 as advanced models for coding and knowledge work. The practical change is that agent work now needs routing, meaning each task gets assigned to the cheapest capable model rather than the newest model by default. A Japan report says Fable 5.1 may reduce some costs by up to about 45% through lower cache-read pricing, which matters for long sessions where the same project context is reused. Treat this as a test, not a blanket migration: define three lanes, quick edits, trusted production changes, and deep research, then measure which lane actually earns the premium model.