Claude Fable 5 ships: Mythos-class model now in the API
Fable 5: first public Mythos-class model above Opus 4.8. 80.3% SWE-Bench Pro. Falls back to Opus 4.8 in high-risk domains. Live on Bedrock, Vertex AI, Foundry.
For the past six months, Opus 4.8 was the practical ceiling — no generally available model sat above it.
The ceiling just rose. Worth testing Fable 5 where performance has been the bottleneck; stick with Opus 4.8 for cost-sensitive workloads.