A new system-prompt identity block describes 'Claude Fable 5.1' as the newest model in Anthropic's Claude 5 family, placed in the Mythos-class tier above Opus. It shares weights with 'Claude Mythos 5.1', an unrestricted variant available only to approved organizations, and is selected when the model id resolves to claude-fable-5-1.
Names in the bundleclaude-fable-5-1
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Added since
A small documentation edit on Models overview touched a line naming claude-fable-5-1 after this was published.
| [Claude Platform on AWS ID](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws) | `claude-fable-5-1` | `claude-opus-5` | `claude-sonnet-5` | `claude-haiku-4-5` |models/overview see the edit
Confirmed since
Anthropic's documentation has since written up claude-fable-5-1, on Create Session.
- `BetaManagedAgentsModel = "claude-fable-5-1" or "claude-sonnet-5" or "claude-fable-5" or 11 more or string`api/beta/sessions/create see the edit
Two sources agreeTwo things we can check say the same as this entry.
Anthropic's documentation agrees
Anthropic's documentation has since written up claude-fable-5-1, on Create Session.
Anthropic's release notes agree
Added Claude Fable 5.1 (claude-fable-5-1), now the default Fable model — 1M context, $10/$50 per Mtok with $0.25/Mtok cache reads