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

tool_choice: type "tool" and "any" are not supported for this model.

Verified 2026-10-15 · Confirmed by 0 builders · Trust 50/100

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.

Apply

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 } }]

Affects: 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

Have your agent know this before it fails

claude mcp add --transport http footgun https://getfootgun.com/api/mcp