Fable 5.1 removes forced tool use entirely: tool_choice "any" or "tool" now returns a 400

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Anthropic's migration docs list three breaking changes from Fable 5, and the sharpest is that tool_choice of type "any" or {"type":"tool","name":...} returns a 400 invalid_request_error on both Fable 5.1 and Mythos 5.1, with the same validation applied to the token-counting endpoint. The stated reason is that thinking is always on and a forced tool call would skip it, pushing the model's working-out into the tool arguments and lowering argument quality. Anthropic's recommended replacement is tool_choice auto plus strict:true strict tool use, or moving the schema to structured outputs, which means any agent harness that pins a first tool call breaks on the model-ID swap alone.