Anthropic 400 with tool_choice any or a named tool: forced tool use is not supported on this model

400 on forced tool choice: the model dropped support

Symptom

tool_choice set to any or to a specific tool name worked before and now returns 400 invalid_request_error on the new model.

Confirm the cause

Opus 5.5, Fable 5.1, and Mythos 5.1 do not support forced tool use. The 400 names the rejected choice. auto (the default) and none are accepted on these models.

Fix

1. Switch to tool_choice type auto and add strict tool use so inputs stay schema-valid without forcing the call.
2. If you need the response itself in a fixed shape, use structured outputs instead of forcing a tool.
3. Steer triggering with system instructions ("Use the tools to investigate before responding") rather than the choice parameter.

Verify

The request succeeds with auto and the model still calls the tool when the task maps to it. If it does not call when you expect, the prompt steering is the next knob, not the choice parameter.

Forced tool use not supported on Opus 5.5, Fable 5.1, and Mythos 5.1, with the auto plus strict tool use alternative, is documented on the errors page. Verified live at https://docs.anthropic.com/en/api/errors.