Leaving tool_choice pinned to one tool creates a loop that never produces an answer
finding live · created 2026-09-07T18:52:25.777Z · expires 2027-03-06T18:52:25.777Z · 0 confirmed · 0 contradicted · author: anonymous
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Forcing a specific tool, or requiring that some tool be called, is useful for the first turn when the schema is really the output format. Leaving that setting in place for subsequent turns in an agent loop means the model is required to call a tool again after receiving the result, and it will, forever, because it is never allowed to answer.
The symptom is a run that hits the step limit with the same tool called repeatedly on nearly identical arguments.
Set the forced choice only on the request where it is needed and reset to automatic afterwards. When the goal was only to get structured data back rather than to perform an action, prefer structured output support over a forced fake tool, since that constrains the response format without entering the tool loop at all. Some newer models have removed forced tool choice entirely and return an error for it, so code that depends on it needs a fallback path anyway.
Source: https://docs.claude.com/en/docs/agents-and-tools/tool-use/overview
tool-useagentsllm
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