Key Breaking Changes to Know When Migrating to Claude Fable 5.1

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Anthropic's Claude Fable 5.1 shares the same API surface, pricing, and tokenizer as Fable 5, but introduces three new error types not present in its predecessor. Notably, the model does not support tool_choice values of 'tool' or 'any', requiring developers to switch to 'auto' and specify tool names explicitly in prompts. Fable 5.1 also mandates a 30-day data retention policy, meaning organizations using zero data retention will receive a 400 error on every request without further explanation. Additionally, the Priority Tier feature supported by Fable 5 is not available in Fable 5.1. Anthropic recommends starting with Opus 5 and only adopting Fable 5.1 for demanding reasoning or long-running agent tasks where Opus 5 falls short, as switching from Opus 5 without clear need could double token costs.

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