msgspec 0.22.0

Version 0.22.0

on Python PyPI

Version 0.22.0 (2026-09-29)

• BREAKING: Setting gc=False on a struct type that has a weakref slot,
  whether from weakref=True or from a base class, now raises ValueError.
  On CPython 3.12 and later the weakref slot is stored in a pre-header in
  front of the instance, so releasing an instance of a gc=False type with
  that slot corrupted memory, in most builds crashing the interpreter
  outright. The combination worked on 3.10 and 3.11, and the restriction
  applies there as well, so that the same class definition behaves the same
  way on every supported version (#1207).
• Add frozendict support on Python 3.15+ (#1052, #1105).
• Support passing a callable as decimal_format to msgspec.json.Encoder and
  msgspec.msgpack.Encoder for custom Decimal encoding (#978).
• Support Literal[True] and Literal[False] types (#1004).
• Publish CPython 3.15 wheels, including freethreaded builds (#1152).
• Publish Linux riscv64 wheels (#987).
• Support the PyEmscripten (Pyodide) platform (#1083).
• Fix handling of PEP 695 type parameter syntax (class Foo[T]) and of
  types.GenericAlias instances in type annotations (#962).
• Fix NameError when creating a Struct with an unquoted forward
  reference on Python 3.14 (#1165).
• Fix a crash on incorrect typing.ClassVar annotations (#1097).
• Fix an AttributeError when converting to a Struct type defined in a
  namespace without a __name__ (#1072).
• Fix overriding an inherited field alias back to the field's own name
  (#1133).
• Raise ValidationError instead of SystemError when convert receives an
  out-of-range int for a float target (#1162).
• Report raw rather than the misleading any as the expected type when a
  non-Raw value is given for a Raw field in convert, and therefore in
  yaml.decode and toml.decode (#1169).
• Fix an empty expected-type name in validation errors for a required Raw
  field in a TypedDict, or a Raw field with a default_factory on a
  dataclass or an attrs class (#1176).
• Correct the error messages for out-of-range Meta length bounds
  (#1172).
• Fix a reference leak when decoding msgpack Ext payloads (#1109).
• Fix backing type declaration of Ext.code (#1135).
• Fix a reference leak in msgspec.to_builtins, and therefore in
  yaml.encode, for array_like=True structs (#1177).
• Fix reference leaks in typenode_collect_literal, Meta.__rich_repr__,
  ms_decode_bigint, and Encoder.__init__ (#1021, #1022,
  #1023, #1040).
• Fix msgspec.inspect.type_info and msgspec.json.schema crashing on mixed-type Literals such as Literal[1, None] (#1080).
• Place null last in the anyOf generated for optional unions in JSON
  schemas (#1028).
• Fix minItems in the JSON schema for array_like=True structs whose fields
  are all optional (#1124).
• Use [] or "" rather than a Python set or bytearray object as the
  JSON schema default for Struct fields with a set or bytearray
default_factory (#1183).
• Preserve JSON Schema metadata and string constraints in propertyNames for
  annotated dictionary key types (#866).
• Fix msgspec.json.encode raising TypeError for a dict keyed by a plain
  enum.Enum with str values (#1118).
• Fix missing GC traversal and clearing of some module state members (#1060).
• Ensure an exception is always set on allocation failures (#1044).
• Fix compilation warnings on Python 3.15 (#1077).
• Report the leading argument of the encode and decode functions and
  methods as positional-only in inspect.signature and the rendered docs,
  matching runtime behavior (#1116).
• Add overloads to the Meta type stub, so type checkers reject mixing gt
  with ge or lt with le (#700).
• Many type stub improvements and fixes (#1014, #1043, #1053,
  #1055, #1057, #1062, #1063, #1064, #1065,
  #1074, #1093, #1114, #1116, #1163, #1173,
  #1179).
• Document the differences between msgspec.structs.asdict/astuple and
  msgspec.to_builtins (#1025).
• Document that omit_defaults ignores fields with a custom
  default_factory (#1076).
• Fix the msgspec.json.decode docstring to say dec_hook should raise
  NotImplementedError for unsupported types, not TypeError (#774).
• msgspec moved to the msgspec GitHub organization;
  documentation now lives at msgspec.dev (repository
  references updated in #1045).

New Contributors

• @ypnos made their first contribution in #1014
• @sobolevn made their first contribution in #1063
• @gounthar made their first contribution in #987
• @notpushkin made their first contribution in #1103
• @dependabot[bot] made their first contribution in #1106
• @NyanFisher made their first contribution in #978
• @ncoghlan made their first contribution in #813
• @binggao1230 made their first contribution in #1080
• @SarthakB11 made their first contribution in #1028
• @adriangb made their first contribution in #700
• @rvaccone made their first contribution in #1127
• @OPNA2608 made their first contribution in #1135
• @vyrnsynx made their first contribution in #1168
• @STiFLeR7 made their first contribution in #1162
• @Ananthr16 made their first contribution in #1169
• @vzer200 made their first contribution in #1173
• @Yasser-Ameur made their first contribution in #1174
• @fallenmi made their first contribution in #1159
• @dylanpulver made their first contribution in #1163
• @vmphase made their first contribution in #1179
• @Kumzy made their first contribution in #1196

Full Changelog: 0.21.1...0.22.0