Fable 5.2 Migration Brings Four Breaking API Changes and Two Silent Default Shifts

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Fable 5.2 has been available for two days, topping benchmark rankings and costing 20% less than its predecessor, with Copilot adopting it immediately. Anthropic's migration guide reportedly omits four breaking changes and two silent default changes that can cause production requests to fail with 400 errors. The most critical shift is that thinking blocks are now strictly bound to their original system prompt, tools, and history, so rewriting earlier conversation turns triggers failures that retries cannot fix. Other breaking changes include the retirement of the computer_20251124 tool version, a requirement that advisor models match the main model's family, and a silent switch of tool_choice from auto to required. The default effort level also rose from low to medium, which can erase the price savings for simpler tasks unless developers pin effort explicitly.

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