Under the hood
Nothing to try yet
The model family resolver now recognizes fable-5-1 and fable_5_1 spellings as their own target.
The model family resolver now recognizes model strings containing fable-5-1 or fable_5_1, mapping them to a new zl().fable5 target that is distinct from the existing fable-5 handling.
Names in the bundlefable-5-1fable_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 whats-new-fable-5-1 touched a line naming fable-5-1 after this was published.
To keep thinking valid across a long session, treat the conversation as append-only. Add instructions with a [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) ([turn-s…models/fable-5-1/whats-new-fable-5-1 see the edit
Confirmed since
Anthropic's documentation has since written up 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
One source agreesOne thing we can check says the same as this entry.
Anthropic's documentation agrees
Anthropic's documentation has since written up fable-5-1, on Create Session.