Pydantic 2.13 Fixes Broken Subclass Serialization That Burned Developers
Pydantic 2.13 restores polymorphic subclass serialization, adds exclude_if to computed fields, and folds pydantic-core into the main repo.
PRO- Pydantic 2.13 adds polymorphic_serialization to restore subclass field output broken in 2.12.
- exclude_if now works on
@computed_field, letting you drop computed values conditionally. - Private attribute
default_factorycan now access other validated field data as an argument. - New ascii_only flag on
StringConstraintsrejects non-ASCII characters at validation time. - The pydantic-core repo merged into the main pydantic repo; still shipped as a separate package.
- Updated
pydantic.v1namespace brings Python 3.14 support to legacy code paths.
The latest Pydantic minor release lands with a focused set of fixes for serialization pain that surfaced in 2.12, plus a handful of quality-of-life additions for anyone building structured-data pipelines or LLM tool schemas. Twenty-one external contributors shipped features and bug fixes across the 2.13 line, and the 2.13.3 patch that just went out handles an AttributeError subclass edge case with from_attributes.
Subclass serialization, restored
The headline change is a new polymorphic_serialization option. In 2.12, the behavior of serialize_as_any was unified with the SerializeAsAny annotation, which quietly broke a common pattern: passing a subclass instance into a field typed as its parent and expecting the child's extra fields to appear in the output.
The new switch restores that behavior in a targeted way. Given a User parent and a UserLogin(User) subclass with a password field, calling model_dump() yields only the parent fields, while model_dump(polymorphic_serialization=True) emits the full subclass payload. It applies specifically to Pydantic models and dataclasses, whereas serialize_as_any
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