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msgspec

0.22.030M downloads/momsgspec/msgspec ↗New releases in your daily digest, changelog included.

A fast serialization and validation library, with builtin support for JSON, MessagePack, YAML, and TOML.

Last release 5 days ago
29 Sep 2026
Release timing varies
gaps range from 2 weeks to 11 months
Nearly every release is documented
notes for 39 of 40 stable releases
1 version withdrawn
withdrawn after publishing
6 years old
41 releases · first in 2021

Release timeline

41 releases since 2021

202120222023202420252026

20212026

One column per quarter.

Releases

0.22.029 Sept 2026
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 CPyth
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

Additional notes

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 ({pr}1207).
BREAKING: Encode an object as a dataclass only when its type defines __dataclass_fields__, as dataclasses.is_dataclass does. Objects that expose it only through instance attribute access, such as proxies wrapping a dataclass instance, are now passed to enc_hook, and encoding them raises TypeError when no enc_hook is given. An enc_hook can return the wrapped instance, which is then encoded as a dataclass. Objects with a Python-level __getattr__, such as pydantic models, now reach enc_hook without that __getattr__ being called ({pr}1196).
Add frozendict support on Python 3.15+ ({pr}1052, {pr}1105).
Support passing a callable as decimal_format to msgspec.json.Encoder and msgspec.msgpack.Encoder for custom Decimal encoding ({pr}978).
Support Literal[True] and Literal[False] types ({pr}1004).
Publish CPython 3.15 wheels, including freethreaded builds ({pr}1152).
Publish Linux riscv64 wheels ({pr}987).
Support the PyEmscripten (Pyodide) platform ({pr}1083).
Fix handling of PEP 695 type parameter syntax (class Foo[T]) and of types.GenericAlias instances in type annotations ({pr}962).
Fix NameError when creating a Struct with an unquoted forward reference on Python 3.14 ({issue}1165).
Fix a crash on incorrect typing.ClassVar annotations ({pr}1097).
Raise TypeError instead of crashing when a base class of a Struct type is a C extension type that has not been initialized with PyType_Ready yet ({pr}1199).
Fix an AttributeError when converting to a Struct type defined in a namespace without a __name__ ({pr}1072).
Fix overriding an inherited field alias back to the field's own name ({pr}1133).
Raise ValidationError instead of SystemError when convert receives an out-of-range int for a float target ({pr}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 ({pr}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 ({pr}1176).
Correct the error messages for out-of-range Meta length bounds ({pr}1172).
Fix a reference leak when decoding msgpack Ext payloads ({pr}1109).
Fix backing type declaration of Ext.code ({pr}1135).
Raise DecodeError instead of TypeError when a decoded msgpack map key is unhashable, such as a map or an Ext ({pr}1209).
Fix a reference leak in msgspec.to_builtins, and therefore in yaml.encode, for array_like=True structs ({pr}1177).
Fix reference leaks in typenode_collect_literal, Meta.__rich_repr__, ms_decode_bigint, and Encoder.__init__ ({pr}1021, {pr}1022, {pr}1023, {pr}1040).
Fix a reference leak of the annotations dict, and of the module namespace when a string ClassVar annotation is present, on every definition of a Struct type with annotations ({pr}1194).
Fix a reference leak of the type dict of a non-Struct base, including typing.Generic, on every definition of a Struct type with such a base on Python 3.12+. The leaked dict could also keep the base class and the objects in its namespace alive ({pr}1199).
Fix msgspec.inspect.type_info and msgspec.json.schema crashing on mixed-type Literals such as Literal[1, None] ({pr}1080).
Place null last in the anyOf generated for optional unions in JSON schemas ({pr}1028).
Fix minItems in the JSON schema for array_like=True structs whose fields are all optional ({pr}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 ({pr}1183).
Preserve JSON Schema metadata and string constraints in propertyNames for annotated dictionary key types ({issue}866).
Fix msgspec.json.encode raising TypeError for a dict keyed by a plain enum.Enum with str values ({pr}1118).
Fix missing GC traversal and clearing of some module state members ({pr}1060).
Ensure an exception is always set on allocation failures ({pr}1044).
Fix compilation warnings on Python 3.15 ({pr}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 ({pr}1116).
Correct the signatures of several public callables in inspect.signature and help() output, the rendered docs and the type stubs, including msgspec.Raw, msgspec.msgpack.Ext, several msgspec.structs functions and the encode and decode methods of encoders and decoders ({pr}1197).
Add overloads to the Meta type stub, so type checkers reject mixing gt with ge or lt with le ({pr}700).
Many type stub improvements and fixes ({pr}1014, {pr}1043, {pr}1053, {pr}1055, {pr}1057, {pr}1062, {pr}1063, {pr}1064, {pr}1065, {pr}1074, {pr}1093, {pr}1114, {pr}1116, {pr}1163, {pr}1173, {pr}1179).
Document the differences between msgspec.structs.asdict/astuple and msgspec.to_builtins ({pr}1025).
Document that omit_defaults ignores fields with a custom default_factory ({pr}1076).
Fix the msgspec.json.decode docstring to say dec_hook should raise NotImplementedError for unsupported types, not TypeError ({issue}774).
msgspec moved to the msgspec GitHub organization; documentation now lives at msgspec.dev (repository references updated in {pr}1045).

Open source →

0.21.112 Apr 20262 sources agree

Fix ValidationError and DecodeError raised in dec_hook being incorrectly wrapped in another ValidationError ( #1013 ).

Fix ValidationError and DecodeError raised in dec_hook being incorrectly wrapped in another ValidationError (#1013).
Fix a potential NULL dereference in structmeta_get_module_ns (#1016).
Fix a reference leak in ms_passes_big_int_constraints (#1017).
Fix missing ref_template parameter in msgspec.json.schema type stub (#1002).
Clarify order='deterministic' encoder docstrings (#1011).
Add a porting guide for users migrating from orjson (#1007).

Open source →

0.21.08 Apr 20262 sources agree

Fix a segfault on Python 3.13+ that could occur when creating structs that contained a __dict__, instead of only using __slots__ (#960).

Fix a segfault on Python 3.13+ that could occur when creating structs that contained a __dict__, instead of only using __slots__ (#960).
Improve type annotations for msgspec.UNSET (#975).
Fix hashing of decoded tuples on Python 3.14 (#980).
Add ref_template arg to msgspec.json.schema (#983).
Add missing libm linkage to some architectures (#993).
Fix memory leak when decoding sets (#991).
Add uniqueItems annotation to set/frozenset JSON schemas (#984).
BREAKING: Call __post_init__ from msgspec.structs.replace/copy.replace (#1000).

Open source →

0.20.024 Nov 20252 sources agree

Support Python 3.14, including freethreaded mode (#852, #877).

Support Python 3.14, including freethreaded mode (#852, #877).
Expose the StructMeta metaclass (#890, #927, #928, #940, #945).
Add msgspec.inspect.is_struct and msgspec.inspect.is_struct_type functions for checking whether an object is a msgspec.Struct-like instance or class (#950).
Support Windows arm64 builds (#943).
Enable ThinLTO on macOS aarch64 builds (#937).
Fix leaks of re.Pattern objects when using pattern constraints of msgspec.Meta (#899).
Fixed memoryview support for msgspec.msgpack (#836).
Fix crash when typing.TypedDict contains incorrect metadata (#853, #947).
Add setuptools-scm as a build dependency instead of vendoring versioneer (#905).
Remove support for legacy packaging builds (#907).
Improved the developer experience by adding the just command runner and Dev Container support (#918).

Open source →

0.19.027 Dec 2024Deprecated2 sources agree

Remove deprecated from_builtins ( #761 ).

Improve JSON encoding performance by up to 40% (#647).
Ensure tuple and frozenset default values are treated identically whether specified by value or default_factory (#653).
Fix memory leak of match_args in StructConfig object (#684).
Fix memory leak in Raw.copy() (#709).
Update decode signatures for PEP 688 (#740).
Generate __replace__ method on Struct types, for use with copy.replace (#747).
Fix incorrect decoding of certain > 64 bit integers (#751).
Call __post_init__ when converting from an object to a Struct (#752).
BREAKING: Expand buffer when encode_into is passed a buffer smaller than offset (#753).
Support Raw objects as inputs to convert (#754).
Error nicely when a dataclass type (instead of an instance) is passed to encode (#755).
Drop support for Python 3.8 (#756, #763).
Add support for Python 3.13 (#711).
Remove deprecated from_builtins (#761).
Support encoding any Enum type whose .value is a supported type (#757).
Don't fail eagerly when processing generic types with unsupported __parameters__ (#772).
Use eval_type_backport to backport type annotations to Python 3.9 (#773).

Open source →

0.18.622 Jan 20242 sources agree

Support coercing integral floats to ints when strict=False ( #619 ).

Support coercing integral floats to ints when strict=False (#619).
Preserve leading _ when renaming fields to camel or pascal case (#620).
Support zero-copy decoding binary fields to a memoryview (#624).
Fix a bug when inheriting from the same Generic base class multiple times (#626).
Add an order option to all encoders for enforcing deterministic/sorted ordering when encoding. This can help provide a more consistent or human readable output (#627).
Support inheriting from any slots-class when defining a new Struct type with gc=False (#635).
Automatically infer the input field naming convention when converting non-dict mappings or arbitrary objects to Struct types in msgspec.convert (#636).

Open source →

0.18.513 Dec 20232 sources agree

Support unhashable Annotated metadata in msgspec.inspect.type_info ( #566 ).

Support unhashable Annotated metadata in msgspec.inspect.type_info (#566).
Fix bug preventing decoding dataclasses/attrs types with default values and slots=True, frozen=True (#569).
Support passing parametrized generic struct types to msgspec.structs.fields (#571).
Validate str constraints on dict keys when decoding msgpack (#577).
Support UUID subclasses as inputs to msgspec.convert (#592).
Call __eq__ from generated __ne__ if user defines manual __eq__ method on a Struct type (#593).
Include the Struct type in the generated hash (#595).
Add a cache_hash struct option (#596).
Fix a bug around caching of dataclass type info when dealing with subclasses of dataclasses (#599).
Add msgspec.structs.force_setattr (#600).
Support custom dict key types in JSON encoder and decoder (#602).
Include dict key constraints in generated JSON schema via the propertyNames field (#604).
Add a schema_hook for generating JSON schemas for custom types (#605).
Add support for Python 3.12's type aliases (#606).

Open source →

0.18.45 Oct 20232 sources agree

Resolve an issue leading to periodic segfaults when importing msgspec on CPython 3.12

Resolve an issue leading to periodic segfaults when importing msgspec on CPython 3.12 (#561)

Open source →

0.18.33 Oct 20232 sources agree

Improve type annotation for Struct.__rich_repr__

Improve type annotation for Struct.__rich_repr__ (#557)
Add pre-built wheels for Python 3.12 (#558)

Open source →

0.18.226 Aug 20232 sources agree

Support Enum._missing_ hooks for handling unknown enum values ( #532 ).

Support Enum._missing_ hooks for handling unknown enum values (#532).
Fix JSON encoding of datetime.datetime objects with zoneinfo.ZoneInfo timezone components (#534).
Add support for attrs validators (#538).
Relax datetime/time parsing format to allow some RFC3339 extensions from ISO8601 (#539).

Open source →

0.18.116 Aug 20232 sources agree

Support custom builtin_types in msgspec.to_builtins ( #517 ).

Support custom builtin_types in msgspec.to_builtins (#517).
Try getattr before getitem when converting with from_attributes=True (#519).
More efficient module state access in top-level functions (#521).

Open source →

0.18.010 Aug 20232 sources agree

Add a new msgspec.json.Decoder.decode_lines method for decoding newline-delimited JSON into a list of values (#485).

Add a new msgspec.json.Decoder.decode_lines method for decoding newline-delimited JSON into a list of values (#485).
Support for decoding UUIDs from binary values (#499).
Support for encoding UUIDs in alternate formats (#499).
Overhaul how dataclasses are encoded to support more dataclass-like objects (#501).
Encode all declared fields on a dataclass (#501).
Support encoding edgedb.Object instances as dataclass-like objects (#501).
Improve performance when json decoding float values (#510).
Support for JSON encoding dicts with float keys (#510).
Support for JSON decoding dicts with float keys (#510).
Add float_hook to msgspec.json.Decoder to support changing the default for how JSON floats are decoded (#511).

Open source →

0.17.012 Jul 20232 sources agree

Ensure None may be explicitly passed to defstruct for module/namespace/bases (#445).

Ensure None may be explicitly passed to defstruct for module/namespace/bases (#445).
Support decoding datetime.datetime values from int/float values (interpreted as seconds since the Unix epoch) when strict=False (#452).
Support subclasses of collection types (list, dict, ...) as inputs to convert (#453).
Support str subclasses as keys in to_builtins and all protocol encode methods (#454).
Improved performance when JSON encoding decimal.Decimal values (#455).
Improved performance when JSON encoding int/float values (#458).
Improved performance when JSON encoding str values (#459).
Wrap errors in dec_hook with a ValidationError (#460).
Support decoding decimal.Decimal values from numeric values (#463)
Support encoding decimal.Decimal values as numeric values (#465).
Support converting decimal.Decimal values to float in convert (#466).
Preliminary support for CPython 3.12 beta releases (#467).
Support decoding integers that don't fit into an int64/uint64 (#469).
Add a new optional __post_init__ method for Struct types (#470).
Support decoding 0/1 into bool types when strict=False (#471).
Wrap errors raised in __post_init__/__attrs_post_init__ in a ValidationError when decoding (#472).
Add native support for encoding/decoding datetime.timedelta types (#475).
Add a new msgspec.json.Encoder.encode_lines method for encoding an iterable of values as newline-delimited JSON (#479).

Open source →

0.16.012 Jun 2023Deprecated2 sources agree

Deprecate msgspec.from_builtins in favor of `msgspec.convert`. The new convert function provides a superset of the functionality available in the old…

Deprecate msgspec.from_builtins in favor of msgspec.convert. The new convert function provides a superset of the functionality available in the old from_builtins function. See the converters docs for more information (#431).
Add a from_attributes argument to msgspec.convert for allowing conversion between object types with matching attribute names. One use case for this is converting ORM objects to Struct or dataclasses types (#419).
Support passing generic Mapping objects as inputs to msgspec.convert. These may be coerced to dict/Struct/dataclasses/attrs types (#427).
Add a new strict keyword argument to all decode functions, Decoder classes, as well as msgspec.convert. This defaults to True, setting it to false enables a wider set of coercion rules (e.g. coercing a str input to an int). See "Strict" vs "Lax" Mode for more information (#434).
Allow any of msgspec's supported types as inputs to msgspec.convert (#431, #418).
Passthrough input unchanged when coercing to typing.Any type in msgspec.convert (#435).
Support parametrizing Decoder types at runtime (#415).
Support encoding subclasses of UUID (#429).

Open source →

0.15.119 May 2023

Fix a reference counting bug introduced in 0.15.0 when decoding naive (no timezone) datetime/time objects in both the msgpack and json decoders (#409)

Fix a reference counting bug introduced in 0.15.0 when decoding naive (no timezone) datetime/time objects in both the msgpack and json decoders (#409).
Work around an upstream bug in CPython to properly support Required/NotRequired in TypedDict when __future__.annotations is enabled (#410).

Open source →

Additional notes

Fix a reference counting bug introduced in 0.15.0 when decoding naive (no timezone) datetime/time objects in both the msgpack and json decoders ({pr}409).
Work around an upstream bug in CPython to properly support typing.Required/typing.NotRequired in typing.TypedDict when __future__.annotations is enabled ({pr}410).

Open source →

0.15.010 May 20232 sources agree

Add support for Generic Struct types (#386, #393).

Add support for Generic Struct types (#386, #393).
Add support for Generic dataclasses and attrs types (#396).
Add support for Generic typing.TypedDict and typing.NamedTuple types (#398).
BREAKING: No longer normalize timezones to UTC when decoding datetime objects from JSON (#391).
Support decoding unhyphenated UUIDs (#392).
A few type annotation fixups (#383, #387).
Dedent docstrings for descriptions when generating JSON schemas (#397).
Use a variant of __qualname__ when auto-generating Struct tags rather than __name__ (#399).
Fix bug when handling typing.Literal types containing a literal None (#400).
Make all Encoder/Decoder methods threadsafe (#402).
BREAKING: Drop the write_buffer_size kwarg to Encoder (#402).

Open source →

0.14.220 Apr 20232 sources agree

Remove __del__ trashcan usage for structs with gc=False (#369).

Remove __del__ trashcan usage for structs with gc=False (#369).
Support overriding __setattr__ in Struct types (#376).
Support encoding large integers in JSON (#378).
Fix a memory leak when msgpack decoding variable length tuples with more than 16 elements (#380).
Remove JSON compatibility checks when constructing a msgspec.json.Decoder. Trying to decode messages into types that JSON doesn't support will now error at decode time, rather than when the decoder is constructed (#381).

Open source →

0.14.17 Apr 20232 sources agree

Further optimize decoding of JSON arrays into lists #363

Further optimize decoding of JSON arrays into lists #363
Fix a bug preventing using structs configured with dict=True on Python 3.11 #365
Avoid preallocating large lists/tuples in the msgpack decoder #367

Open source →

0.14.02 Apr 20232 sources agree

Support encoding and decoding attrs types (#323).

Support encoding and decoding attrs types (#323).
Add repr_omit_defaults configuration option for omitting struct default values in the repr (#322).
Expose a struct's configuration through a __struct_config__ attribute (#328).
Add msgspec.structs.fields utility function for inspecting the fields configured on a Struct (#330).
Add a dict configuration option for adding a __dict__ attribute to a Struct (#331).
Allow non-struct mixins to be used with struct types (#332).
Fix a bug when defining both lt and gt constraints on an integer type (#335).
Fix a bug supporting fields defined with msgspec.field() with no arguments (#343).
Allow arbitrary input types to msgspec.from_builtins (#346).
Support decoding into subclasses of int & bytes in msgspec.from_builtins (#346).
Add msgspec.UNSET and msgspec.UnsetType for tracking unset fields. See the docs for more information (#350).
BREAKING: In the unlikely event you were using the previous msgspec.UNSET singleton to explicitly indicate no default value on struct types, you should now make use of msgspec.NODEFAULT instead (#350).
Improve struct type annotations now that mypy supports typing.dataclass_transform (#352).
Support typing.Final annotations for indicating that an object field should be treated as immutable (#354).
Add a name keyword option to msgspec.field for renaming a single field (#356).
BREAKING: The rules around class inheritance and a struct's rename option have changed. See #356 for more information.

Open source →

0.13.110 Feb 20232 sources agree

Fix a memory leak in the JSON decoder introduced in 0.13.0, caused by a reference counting bug when decoding into Struct types (#312).

Fix a memory leak in the JSON decoder introduced in 0.13.0, caused by a reference counting bug when decoding into Struct types (#312).

Open source →

0.13.09 Feb 2023withdrawn: Memory leak in JSON decoder2 sources agree

Add to_builtins function for converting messages composed of any supported type to ones composed of only simple builtin types commonly supported by Py

Add to_builtins function for converting messages composed of any supported type to ones composed of only simple builtin types commonly supported by Python serialization libraries (#258).
Add from_builtins function for converting and validating messages composed of simple builtin types to ones composed of any type msgspec supports (#266, #302).
Add msgspec.yaml module for encoding/decoding YAML (#267).
Add msgspec.toml module for encoding/decoding TOML (#268).
Add msgspec.structs.replace function for creating a copy of an existing Struct with some changes applied (#262).
Add msgspec.structs.asdict and msgspec.structs.astuple functions for converting a struct instance to a dict or tuple respectively (#300).
Support arbitrarily nested typing.NewType/typing.Annotated types (#272).
Improve error message for invalid keyword arguments passed to Struct.__init__ (#273).
Support default_factory configuration for Struct fields (#274).
BREAKING: With the exception of empty builtin collections ([], {}, set(), bytearray()), mutable default values in Struct types are no longer deepcopied when used. If a different mutable default value is needed, please configure a default_factory instead (#274).
Improve performance of creating Structs with default parameters (#274).
Support typing.ClassVar annotations of Struct types (#281).
Support encoding/decoding decimal.Decimal types (#288).
Support "abstract" type annotations like collections.abc.MutableMapping/typing.MutableMapping in decoders (#290).
Support any string-like or int-like type as a dict key when encoding or decoding JSON (#292).
Improved performance encoding large collections in JSON and MessagePack encoders (#294, #298).

Open source →

0.12.05 Jan 20232 sources agree

Support encoding set and frozenset subclasses (#249).

Support encoding set and frozenset subclasses (#249).
Support encoding/decoding typing.NewType types (#251).
Allow creating a msgspec.Raw object from a str (#252).
Add new experimental msgspec.inspect module for inspecting type annotations. This is intended to be used for building downstream tooling based on msgspec-compatible types. See the docs for more information (#253).
Add new extra field to msgspec.Meta, for storing arbitrary user-defined metadata (#255).
Improved performance for JSON encoding strings (#256).

Open source →

0.11.019 Dec 20222 sources agree

Improve performance of constructors for Struct types when using keyword arguments (#237).

Improve performance of constructors for Struct types when using keyword arguments (#237).
Support constraints on dict keys for JSON (#239).
Add support for keyword-only arguments in Struct types, matching the behavior of kw_only for dataclasses (#242).
BREAKING: Change the parameter ordering rules used by Struct types to match the behavior of dataclasses. For most users this change shouldn't break anything. However, if your struct definitions have required fields after optional fields, you'll now get an error on import. This error can be fixed by either:
- Reordering your fields so all required fields are before all optional fields
- Using keyword-only parameters (by passing the kw_only=True option).
See Field Ordering for more information (#242).
Support encoding/decoding dictionaries with integer keys for JSON (#243).

Open source →

0.10.18 Dec 20222 sources agree

Ignore attributes with leading underscores ("_") when encoding dataclasses

Ignore attributes with leading underscores ("_") when encoding dataclasses (#234)

Open source →

0.10.08 Dec 20222 sources agree

Add forbid_unknown_fields configuration option to Struct types

Add forbid_unknown_fields configuration option to Struct types (#210)
BREAKING: Encode all enum types by value, rather than name (#211)
Fix a bug in the JSON encoder when base64 encoding binary objects (#217)
Add support for encoding/decoding dataclasses (#218)
Add support for encoding/decoding datetime.date objects (#221)
Add support for encoding/decoding uuid.UUID objects (#222)
BREAKING: support encoding/decoding datetime.datetime values without timezones by default (#224).
Add a tz constraint to require aware or naive datetime/time objects when decoding (#224).
Add support for encoding/decoding datetime.time objects (#225)
Add a msgspec.json.format utility for efficiently pretty-printing already encoded JSON documents (#226).
Support decoding JSON from strings instead of just bytes-like objects (#229)

Open source →

0.9.128 Oct 20222 sources agree

Support greater than microsecond resolution when parsing JSON timestamps

Enable Python 3.11 builds (#205)
Support greater than microsecond resolution when parsing JSON timestamps (#201)
Work around a limitation in mypy for typed decoders (#191)

Open source →

0.9.014 Sept 20222 sources agree

Support for constraints during validation. For example, this allows ensuring a field is an integer >= 0.

Support for constraints during validation. For example, this allows ensuring a field is an integer >= 0. (#176)
New utilities for generating JSON Schema from type definitions (#181)
Support for pretty printing using rich (#183)
Improve integer encoding performance (#170)
Builtin support for renaming fields using kebab-case (#175)
Support for passing a mapping when renaming fields (#185)

Open source →

0.8.02 Aug 20222 sources agree

Support integer tag values when using tagged unions (#135).

Support integer tag values when using tagged unions (#135).
Support decoding into typing.TypedDict types (#142).
Support encoding/decoding typing.NamedTuple types (#161).
Test against CPython 3.11 prelease builds (#146).
Add ValidationError (a subclass of DecodeError) to allow differentiating between errors due to a message not matching the schema from those due to the message being invalid JSON (#155).
Support encoding subclasses of list/dict (#160).
Fix a bug preventing decoding custom types wrapped in a typing.Optional (#162).

Open source →

0.7.128 Jun 20222 sources agree

Further reduce the size of packaged wheels (#130).

Further reduce the size of packaged wheels (#130).
Add weakref support for Struct types through a new weakref configuration option (#131).
Fix a couple unlikely (but possible) bugs in the deallocation routine for Struct types (#131).

Open source →

0.7.020 Jun 2022Breaking2 sources agree

Rename nogc struct option to gc (a breaking change). To disable GC on a Struct instance you now want to specify gc=False instead of nogc=True (#124).

Dramatically speedup JSON string decoding, up to 2x speedup in some cases (#118).
Adds a cache for decoding short (< 32 character) ASCII dict keys. This results in up to a 40% speedup when decoding many dicts with common keys using an untyped decoder. It's still recommended to define Struct types when your messages have a common structure, but in cases where no type is provided decoding is now much more performant (#120, #121).
Adds order and eq configuration options for Struct types, mirroring the dataclasses options of the same name. Order comparisons for Struct types are very performant, roughly 10x to 70x faster than alternative libraries (#122).
Speedup Struct decoding for both JSON and MessagePack, on average 20% faster (#119).
Various additional performance improvements, mostly to the JSON implementation (#100, #101, #102).
Add defstruct method for dynamically defining new Struct types at runtime (#105).
Fix ARM support and publish ARM wheels for Linux and Mac (#104).
Reduce published wheel sizes by stripping debug symbols (#113).
Fix a memory leak in Struct.__reduce__ (#117).
Rename nogc struct option to gc (a breaking change). To disable GC on a Struct instance you now want to specify gc=False instead of nogc=True (#124).

Open source →

0.6.06 Apr 2022

Add a new `msgspec.Raw` type for delayed decoding of message fields / serializing already encoded fields.

Add a new msgspec.Raw type for delayed decoding of message fields / serializing already encoded fields.
Add omit_defaults option to Struct types (docs). If enabled, fields containing their respective default value will be omitted from serialized message. This improves both encode and decode performance.
Add rename option to Struct types (docs) for altering the field names used for encoding. A major use of this is supporting camelCase JSON field names, while letting Python code use the more standard snake_case field names.
Improve performance of nogc=True structs. GC is now avoided in more cases, and nogc=True structs use 16 fewer bytes per instance. Also added a benchmark for how msgspec can interact with application GC usage.
Cache creation of tagged union lookup tables, reducing memory usage for applications making heavy use of tagged unions.
Support encoding and decoding frozenset instances
A smattering of other performance improvements.

Open source →

Additional notes

Add a new msgspec.Raw type for delayed decoding of message fields / serializing already encoded fields ({pr}92).
Add omit_defaults option to Struct types. If enabled, fields containing their respective default value will be omitted from serialized message. This improves both encode and decode performance ({pr}94).
Add rename option to Struct types for altering the field names used for encoding. A major use of this is supporting camelCase JSON field names, while letting Python code use the more standard snake_case field names ({pr}98).
Improve performance of nogc=True structs. GC is now avoided in more cases, and nogc=True structs use 16 fewer bytes per instance. Also added a benchmark for how msgspec can interact with application GC usage ({pr}93).
Cache creation of tagged union lookup tables, reducing memory usage for applications making heavy use of tagged unions ({pr}91).
Support encoding and decoding frozenset instances ({pr}95).
A smattering of other performance improvements.

Open source →

0.5.09 Mar 2022Breaking

Rename the Struct option asarray to array_like (a breaking change)

Support tagged unions for encoding/decoding a Union of msgspec.Struct types.
Further improve encoding performance of enum.Enum by 20-30%
Reduce overhead of calling msgspec.json.decode/msgspec.msgpack.decode with type=SomeStructType. It's still faster to create a Decoder once and call decoder.decode multiple times, but for struct types the overhead of calling the top-level function is decreased significantly.
Rename the Struct option asarray to array_like (a breaking change)

Open source →

Additional notes

Support tagged unions for encoding/decoding a Union of msgspec.Struct types ({pr}83).
Further improve encoding performance of enum.Enum instances by 20-30% ({pr}84).
Reduce overhead of calling msgspec.json.decode/msgspec.msgpack.decode with type=SomeStructType. It's still faster to create a Decoder once and call decoder.decode multiple times, but for struct types the overhead of calling the top-level function is decreased significantly ({pr}77, {pr}88).
BREAKING: Rename the Struct option asarray to array_like ({pr}85).

Open source →

0.4.228 Feb 2022

Support Literal string types as dict keys in JSON

Support Literal string types as dict keys in JSON
Support Python 3.10 style unions (e.g. int | float | None)
Publish Python 3.10 wheels

Open source →

Additional notes

Support typing.Literal string types as dict keys in JSON ({pr}78).
Support Python 3.10 style unions (for example, int | float | None) ({pr}75).
Publish Python 3.10 wheels ({pr}80).

Open source →

0.4.123 Feb 2022

Optimize decoding of Enum types, ~10x faster

Optimize decoding of Enum types, ~10x faster
Optimize decoding of IntEnum types, ~12 faster
Support decoding typing.Literal types
Add nogc option for Struct types, disabling the cyclic garbage collector for their instances

Open source →

Additional notes

Optimize decoding of Enum types, on average ~10x faster ({pr}69).
Optimize decoding of IntEnum types, on average ~12x faster ({pr}68).
Support decoding typing.Literal types ({pr}71).
Add nogc option for Struct types, disabling the cyclic garbage collector for their instances ({pr}72).

Open source →

0.4.08 Feb 2022

This is a major release with several large changes:

Moved MessagePack support to the msgspec.msgpack submodule
New JSON support available in msgspec.json
Improved error message generation to provide full path to the mistyped values
Renamed the immutable kwarg in msgspec.Struct to frozen to better match python conventions
Renamed EncodingError to EncodeError/DecodingError to DecodeError to better match python conventions
Improved pyright support, allowing more errors to be statically caught by their type checker
Adds support for Python 3.10 pattern matching on msgspec.Struct types
Adds support for decoding into typing.Union types (with a few restrictions)
General performance improvements across all encoders/decoders

Open source →

Additional notes

Moved MessagePack support to the msgspec.msgpack submodule ({pr}56).
New JSON support available in msgspec.json ({pr}56).
Improved error message generation to provide full path to the mistyped values ({pr}56).
Renamed the immutable kwarg in msgspec.Struct to frozen to better match python conventions ({pr}60).
Renamed EncodingError to EncodeError/DecodingError to DecodeError to better match python conventions ({pr}61).
Improved pyright support, allowing more errors to be statically caught by their type checker ({pr}60).
Adds support for Python 3.10 pattern matching on msgspec.Struct types ({pr}53).
Adds support for decoding into typing.Union types (with a few restrictions) ({pr}54).
General performance improvements across all encoders/decoders.

Open source →

0.3.223 Jul 2021

Faster float encoding and decoding

Faster float encoding and decoding
General micro-optimizations for write/read hot path. Most visible for large messages.

Open source →

Additional notes

Faster float encoding and decoding ({pr}47).
General micro-optimizations for MessagePack encode/decode. This is most visible for large messages ({pr}48, {pr}50).

Open source →

0.3.113 Jul 2021

Use a freelist for small structs to improve struct allocation time.

Use a freelist for small structs to improve struct allocation time.
Small perf improvement for struct serialization

Open source →

Additional notes

Use a freelist for small structs to improve struct allocation time ({pr}44).
Small performance improvements for struct serialization ({pr}45).

Open source →

0.3.07 Jul 2021

Add Encoder.encode_into api, for encoding into an existing buffer without copying

Add Encoder.encode_into api, for encoding into an existing buffer without copying
Add support for encoding/decoding MessagePack extensions
Add support for encoding/decoding datetime objects
Add support for encoding/decoding custom objects without relying on MessagePack extensions
Add support for marking Struct types as hashable
Add support for serializing Struct types as MessagePack array objects rather than map objects.
Several performance improvements. On average 50% faster encoding and 30% faster decoding.

Open source →

Additional notes

Add Encoder.encode_into api, for encoding into an existing buffer without copying ({pr}34).
Add support for encoding/decoding MessagePack extensions ({pr}31).
Add support for encoding/decoding datetime objects ({pr}36).
Add support for encoding/decoding custom objects without relying on MessagePack extensions ({pr}32, {pr}33).
Add support for marking Struct types as hashable ({pr}39).
Add support for serializing Struct types as MessagePack array objects rather than map objects ({pr}39).
Several performance improvements. On average 50% faster encoding and 30% faster decoding.

Open source →

0.2.025 Feb 2021

Add default callback to encode/Encoder ({pr}21).

Add default callback to encode/Encoder ({pr}21).
Fix bug in Encoder dealloc ({pr}21).

Open source →

0.1.023 Feb 2021

- Initial Release

Initial Release

Open source →

0.0.123 Feb 2021

Nothing published for this version

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Version 0.22.0 (2026-09-29)

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