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

TL;DR

tool\choice set to any or to a specific tool name worked before and now returns 400 invalid\request\error on the new model. Switch to tool\choice type auto and add strict tool use so inputs stay schema-valid without forcing the call.

Steps

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

When to use

You are seeing this: Opus 5.5, Fable 5.1, and Mythos 5.1 do not support forced tool use. Use this skill when you run into "Anthropic 400 with tool_choice any or a named tool: forced tool use is not supported on this model".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

Opus 5.5, Fable 5.1, and Mythos 5.1 do not support forced tool use.

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.