Pydantic AI v2.64.0 adds ModelFallback options, artifact scanning and compatibility changes

Pydantic AI v2.64.0 was released on October 30, 2026, and includes several compatibility changes. An error status that a retrying transport gives up on is now raised as ModelHTTPError, so fallback moves on to the next model. Stored response, item, conversation and container IDs are now sent back only to the endpoint that created them, through Model.accepts_server_state.

The release extends the ModelFallback capability, which can now be built from agent specs and has a sticky option that keeps a conversation on the fallback model that answered. A first_event_timeout model setting lets a stream that never starts fall back. The new BoxdSandbox runs Coder, Shell, FileSystem and other harness capabilities in a boxd machine, and ArtifactScanner adds security scanning for Memory and Skills.

json-repair is now an optional dependency, needed only by Coder and RepairToolArguments. Coder's tools no longer use strict tool schemas by default, and Coder(strict=True) turns them on. The release also includes bug fixes across Bedrock, Anthropic token counting, realtime reconnects and Temporal sub-agents, and bumps joserfc from 1.7.1 to 1.7.6.

Code that uses Coder should install json-repair explicitly if it needs Coder or RepairToolArguments, and should pass strict=True if it depends on strict tool schemas. Fallback chains can now rely on retrying transport errors reaching them as ModelHTTPError. Teams that relied on server-side state IDs being sent to any endpoint should check their setup against the new Model.accepts_server_state behavior.