# What changed in msgspec > 36 changes in msgspec 0.21 to 0.22 from its official release notes, upgrade guides and advisories: 5 breaking, 5 new APIs and options. Each one cited. Sources checked 2026-10-29. HTML: https://unstale.sh/pypi/msgspec/ To see which of these apply to a repository, run `npx @unstale/cli init` in it. ## msgspec 0.22 ### Breaking changes - Out-of-range integers converted to a float target now raise ValidationError instead of SystemError. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - JSON schema defaults for Struct fields with set or bytearray default factories use [] or "" rather than Python set or bytearray objects. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - Meta type stubs add overloads so type checkers reject mixing gt with ge or lt with le. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - Setting gc=False on a struct type with a weakref slot now raises ValueError, consistently on all supported Python versions. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) ### New APIs and options - Encoder accepts a callable as decimal_format for custom Decimal encoding in JSON and MessagePack. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - frozendict is supported on Python 3.15 and later. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - Literal[True] and Literal[False] are now supported types. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - The PyEmscripten (Pyodide) platform is now supported. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) ### Tooling - Linux riscv64 wheels are now published. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - Type stubs receive multiple improvements and fixes. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) ### Fixes and other changes - JSON schema minItems is corrected for array_like structs whose fields are all optional. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - When a non-Raw value is given for a Raw field, the expected type is reported as raw rather than any. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - Wheels are published for CPython 3.15, including freethreaded builds. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - inspect.signature and rendered docs now report the leading argument of encode and decode functions and methods as positional-only, matching runtime behavior. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - inspect.signature and rendered docs now report the leading argument of encode and decode functions and methods as positional-only, matching runtime behavior. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - The backing type declaration of Ext.code is fixed. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - Error messages for out-of-range Meta length bounds are corrected. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - type_info no longer crashes on mixed-type Literals such as Literal[1, None]. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - Encoding a dict keyed by a plain enum.Enum with str values no longer raises TypeError. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - JSON schema generation no longer crashes on mixed-type Literals such as Literal[1, None]. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - Generated JSON schemas place null last in the anyOf for optional unions. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - Handling of PEP 695 type parameter syntax, such as class Foo[T], is fixed. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - JSON Schema metadata and string constraints are preserved in propertyNames for annotated dictionary key types. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - Creating a Struct with an unquoted forward reference no longer raises NameError on Python 3.14. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - Handling of types.GenericAlias instances in type annotations is fixed. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - Incorrect typing.ClassVar annotations no longer cause a crash. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) - Validation errors no longer show an empty expected-type name for required Raw TypedDict fields or Raw fields with default_factory on dataclass or attrs classes. (0.22.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.22.0) ## msgspec 0.21 ### Breaking changes - Calls __post_init__ from msgspec.structs.replace and copy.replace. (0.21.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.21.0) ### New APIs and options - Adds the ref_template argument to msgspec.json.schema. (0.21.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.21.0) ### Tooling - Improves type annotations for msgspec.UNSET. (0.21.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.21.0) ### Fixes and other changes - ValidationError and DecodeError raised in dec_hook are no longer incorrectly wrapped in another ValidationError. (0.21.1) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.21.1) - The type stub now includes the ref_template parameter, so the declared signature matches the API. (0.21.1) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.21.1) - Fixes a segfault on Python 3.13+ when creating structs that contain a __dict__. (0.21.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.21.0) - Fixes a memory leak when decoding sets. (0.21.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.21.0) - JSON schemas for set and frozenset now include a uniqueItems annotation. (0.21.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.21.0) - Fixes hashing of decoded tuples on Python 3.14. (0.21.0) — [release notes](https://github.com/msgspec/msgspec/releases/tag/0.21.0)