Fable 5.1 Breaks Two Operator Assumptions: `tool_choice` Force-Calls and Thinking Block Reuse
Fable 5.1 breaks two patterns operators rely on: `tool_choice: any` and `tool_choice: tool` now return 400, and thinking blocks are version-gated with prefix-mismatch enforcement for new accounts. Migrate both before routing production traffic.
via Anthropic