tool_choice type "tool" or "any" returns 400 on Claude Opus 5.5, Sonnet 5.5, and Fable 5.1

Symptom

tool_choice: type "tool" and "any" are not supported for this model.count_tokens fails with the same errorAn extraction pipeline that forced one tool breaks right after the model id change

Cause

Opus 5.5, Sonnet 5.5, Fable 5.1, and Mythos 5.1 do not allow forcing a tool call, on the Messages endpoint or on token counting. Code that forced a single tool to get structured data breaks when the model is upgraded.

Fix

Send tool_choice auto, set strict true on the tool, and say in the prompt when it must be called. For pure extraction, switch to structured outputs. On Amazon Bedrock, where strict tools are not offered for Opus 5.5, use auto and validate the input in code.

ApplyCopy

tool_choice: { type: "auto" }, tools: [{ name: "record_result", strict: true, description: "Always call this to return the result.", input_schema: { type: "object", properties: { label: { type: "string" } }, required: ["label"], additionalProperties: false } }]

Affected

Claude Opus 5.5, Sonnet 5.5, Fable 5.1, Mythos 5.1, as of 2026-10; earlier models still accept forced tool use unless manual extended thinking is on

Sources

• https://platform.claude.com/docs/en/agents-and-tools/tool-use/define-tools
• https://platform.claude.com/docs/en/models/opus-5-5/migration-guide

Last verified 2026-10-15 · Confirmed by 0 builders