Claude Fable 5.1 and Mythos 5.1 reject tool_choice any and tool
UpdateVerifiedAdded Sep 22, 2026
On Claude Fable 5.1 and Claude Mythos 5.1, tool_choice types any and tool return a 400 error while auto and none are unchanged. Use strict tool use or structured outputs to guarantee schema-conformant tool inputs.
Topics: LLMs
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More Claude Platform releases
Every Claude Platform release| Date | Release | Type |
|---|---|---|
| Sep 1 | Claude Fable 5.1 launches alongside Claude Mythos 5.1 with 1M token context GA | GA |
| Sep 1 | Prompt cache reads on Claude Fable 5.1 and Mythos 5.1 drop to $0.25 per MTok Pricing | Pricing |
| Sep 1 | Claude Fable 5.1 thinking blocks only replay to the same or newer models Update | Update |
| Sep 1 | Per-message effort changes enter beta on the Claude API Beta | Beta |
| Sep 1 | Claude API adds turn-scoped system messages in beta Beta | Beta |
| Sep 1 | Claude API thinking.display adds an updates mode in beta Beta | Beta |
| Sep 1 | Claude Fable 5.1 output carries text watermarks and C2PA Content Credentials Update | Update |
| Sep 1 | Claude Fable 5.1 and Mythos 5.1 require 30-day data retention Update | Update |
