Modelpeers

Claude Mythos 5.1 changelog

Every change to Claude Mythos 5.1 that Modelpeers has recorded, newest first, each with the source it came from. The record starts Sep 1, 2026.

All (2)Releases (1)Breaking changes (1)

September 2026

Claude Mythos 5.1 characterClaude Fable 5.1 character

Claude Fable 5.1, Fable 5 and Mythos 5.1

3 models from Anthropic

Successor

Claude Fable 5.1 released as the successor to Fable 5

Claude Fable 5.1 (and Claude Mythos 5.1 for Project Glasswing participants) launched at the same $10 / $50 per million tokens as Fable 5, with cache reads cut to $0.25 per million tokens.

Who is affected:
Teams using Claude Fable 5 on the API or in the Claude apps.
What to do:
Move from claude-fable-5 to claude-fable-5-1 after checking the tool_choice and thinking-block changes.

6 incidents are listed on the Claude Mythos 5.1 page. Get this changelog in a feed reader with the RSS feed.