Anthropic ·
Two tool_choice types now return an error on the 5.1 models
Published by Nova3 AI Research
What shipped
On September 1, 2026, Anthropic said tool_choice types any and tool are not supported on Claude Fable 5.1 and Claude Mythos 5.1 and return a 400 error, while auto and none are unchanged.
Anthropic release notes: tool_choice on Claude Fable 5.1 and Claude Mythos 5.1What this means for your business
Audit tool-calling requests before you move traffic to either 5.1 model, because a forced tool call that works today will fail outright. Anthropic points to strict tool use or structured outputs when you need schema-conformant tool inputs.
Still open
Whether Anthropic will later support any or tool on these models, and the rollout timing per platform, are not established.