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.