Claude Mythos 5.1 changelog

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.

Fable 5.1 drops forced tool use and binds thinking blocks to history

On Fable 5.1 and Mythos 5.1, tool_choice any and tool return a 400 error. For accounts created on or after August 31, 2026, replaying a thinking block after the system prompt, tools or earlier messages changed also returns a 400 error.

Who is affected:

Code moving from Fable 5 to Fable 5.1, especially agents that rewrite history or force tool calls.

Action needed:

Use tool_choice auto with strict tool use, and stop editing context before thinking blocks.