Claude Mythos 5.1 changelog

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.