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  The upgrade from Pydantic V1 to V2 is not merely a version bump; it is a paradigm shift. With the core logic rewritten in Rust ( pydantic-core ), V2 offers significant performance gains but introduces strict validation rules that break legacy V1 implementations. Backend engineers migrating FastAPI applications or data pipelines often encounter two specific blockers: deprecated validator signatures referencing  FieldValidationInfo  (or  values  dictionaries) and runtime validation errors claiming "Input should be a valid dictionary." This guide provides the root cause analysis and production-ready code required to migrate complex validation logic to the Pydantic V2 standard. The Root Cause: Rust Core and Validator Topology In Pydantic V1, validation was pure Python. It was permissive regarding input types and allowed accessing sibling fields via a loosely typed  values  dictionary in validators. Pydantic V2 moves validation to Rust. This results in: S...