Fable 5 is here——
the Mythos class opens to the API
Anthropic has released Fable 5, the first publicly available Mythos-class model sitting above Opus 4.8. It scores 80.3% on SWE-Bench Pro and is now live via Bedrock, Vertex AI, and Foundry.
For six months, Opus 4.8
was the practical ceiling
Anthropic's model lineup had a three-tier structure — Haiku, Sonnet, and Opus — with Opus 4.8 as the top performer available through the public API. The Mythos class had been announced but remained out of reach for general use.
Fable 5 is the first publicly released Mythos-class model. It achieves 80.3% on SWE-Bench Pro and is now available via Amazon Bedrock, Google Vertex AI, and Azure Foundry. In high-risk domains it falls back to Opus 4.8 automatically.
In high-risk domains,
Opus 4.8 stays on duty
Fable 5 does not replace Opus 4.8 in every use case. Requests flagged as high-risk are automatically routed to Opus 4.8 instead.
This fallback design requires no configuration changes from users — routing is managed internally by Anthropic. The exact criteria for high-risk classification are not public, but domains such as healthcare, legal, and safety-critical systems are the expected targets.
Start where performance
has been the bottleneck
Fable 5 is priced at twice Opus 4.8 (input $10, output $50 per 1M tokens), so a full migration needs a cost analysis first. The practical approach: identify tasks where quality limitations were a genuine obstacle, evaluate Fable 5 there, and expand once the ROI is clear.