Pydantic AI v2.64.0 Adds Sticky Model Fallback for Conversation Continuity

The release of Pydantic AI v2.64.0 on October 30, 2026, introduces an opt-in sticky setting for ModelFallback, as detailed in the official release notes and the associated pull request #11330 . This feature allows a conversation to remain on a fallback model that has successfully answered a request, rather than defaulting back to the primary model at the start of every subsequent request step. By enabling sticky=True, builders can alter the default model selection logic to respect the recent failure history of a specific conversation. This configuration is disabled by default, ensuring that existing agent behavior remains unchanged unless explicitly opted into. The scope of this stickiness is strictly limited to an individual conversation.

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