1.10.0

This release is based on Kotlin 2.3.0 and contains all of the changes from 1.10.0-RC.
The only additional change is a fix for ProtoBuf packing of Kotlin unsigned types (#3079).
Big thanks to KosmX for contributing the fix.

For your convenience, the changelog for 1.10.0-RC is duplicated below:

Stabilization of APIs

kotlinx-serialization 1.10 and subsequent releases will be focused on stabilization of existing APIs.
The following APIs and configuration options are no longer experimental because they're widely used without any known major issues:

• Json configuration options: decodeEnumsCaseInsensitive, allowTrailingComma, allowComments, and prettyPrintIndent. (#3100)
• @EncodeDefault annotation and its modes. (#3106)
• JsonUnquotedLiteral constructor function (#2900)
• JsonPrimitive constructor function overloads that accept unsigned types. (#3117)
• JSON DSL functions on JsonElement with Nothing? overloads. (#3117)

Readiness for return value checker

Kotlin 2.3.0 introduces a new feature aimed at helping you to catch bugs related to the accidentally ignored return value of the function.
kotlinx-serialization 1.10.0-RC code is fully marked for this feature, meaning that you can get warnings for unused function calls like Json.encodeToString(...). To get the warnings, the feature has to be enabled in your project as described here.

Polymorphism improvements

Polymorphic serialization received a couple of improvements in this release:

New subclassesOfSealed utility to automatically register sealed subclasses serializers in polymorphic modules (#2201).

Use it in your SerializersModule when configuring a polymorphic hierarchy which contains both abstract and sealed classes. For example, when root of your hierarchy is an interface, but most of your inheritors are sealed classes. The new function will register all known sealed subclasses for you, so you don’t need to list them one by one. This makes writing your SerializerModules much faster and simpler. Big thanks to Paul de Vrieze for contributing this feature.

Class discriminator conflict check rework (#3105).

If a payload already contains a property with the same name as the configured discriminator (for example, type),
it is called a class discriminator conflict. To produce a correct output and allow more inputs to be deserialized at the same time, the following changes were made:

• Conflicts introduced by JsonNamingStrategy transformations are now detected during serialization as well and will cause SerializationException.
  It also affects non-polymorphic classes.
• Conflicts from ClassDisciminatorMode.ALL_JSON_OBJECTS and SerializersModuleBuilder.polymorphicDefaultSerializer are also detected.
• It is allowed to deserialize such a conflicting key for both sealed and open polymorphic hierarchies.
  Previously, it was possible in the sealed hierarchies alone due to missing assertion. See #1664 for details.

General improvements

• Add .serialName to MissingFieldException for clearer diagnostics. (#3114)
• Generate unique Automatic-Module-Name entries for metadata JARs. (#3109)
• Revised ProGuard rules and added R8 tests. (#3041)
• CBOR: Improved error message when a byte string/array type mismatch is encountered. (#3052)

Bugfixes

• Fix the typo in the BIGNUM_NEGATIVE tag name. (#3090)
• CBOR: Fix various bugs in the decoder implementation to be more strict and consistent with the specification.

1.10.0-RC

This is a release candidate for 1.10.0 based on Kotlin 2.3.0. It stabilizes a set of frequently used JSON APIs and builder options,
adopts a new 'Return Value Checker' Kotlin feature, and provides a lot of improvements and bug fixes.

Stabilization of APIs

kotlinx-serialization 1.10 and subsequent releases will be focused on stabilization of existing APIs.
The following APIs and configuration options are no longer experimental because they're widely used without any known major issues:

• Json configuration options: decodeEnumsCaseInsensitive, allowTrailingComma, allowComments, and prettyPrintIndent. (#3100)
• @EncodeDefault annotation and its modes. (#3106)
• JsonUnquotedLiteral constructor function (#2900)
• JsonPrimitive constructor function overloads that accept unsigned types. (#3117)
• JSON DSL functions on JsonElement with Nothing? overloads. (#3117)

Readiness for return value checker

Kotlin 2.3.0 introduces a new feature aimed at helping you to catch bugs related to the accidentally ignored return value of the function. kotlinx-serialization 1.10.0-RC code is fully marked for this feature, meaning that you can get warnings for unused function calls like Json.encodeToString(...). To get the warnings, the feature has to be enabled in your project as described here.

Polymorphism improvements

Polymorphic serialization received a couple of improvements in this release:

New subclassesOfSealed utility to automatically register sealed subclasses serializers in polymorphic modules (#2201).
Use it in your SerializersModule when configuring a polymorphic hierarchy which contains both abstract and sealed classes.
For example, when root of your hierarchy is an inteface, but most of your inheritors are sealed classes. The new function will register all known sealed subclasses for you, so you don’t need to list them one by one. This makes writing your SerializerModules much faster and simpler. Big thanks to Paul de Vrieze for contributing this feature.

Class discriminator conflict check rework (#3105).
If a payload already contains a property with the same name as the configured discriminator (for example, type),
it is called a class discriminator conflict. To produce a correct output and allow more inputs to be deserialized at the same time, the following changes were made:

• Conflicts introduced by JsonNamingStrategy transformations are now detected during serialization as well and will cause SerializationException.
  It also affects non-polymorphic classes.
• Conflicts from ClassDisciminatorMode.ALL_JSON_OBJECTS and SerializersModuleBuilder.polymorphicDefaultSerializer are also detected.
• It is allowed to deserialize such a conflicting key for both sealed and open polymorphic hierarchies.
  Previously, it was possible in the sealed hierarchies alone due to missing assertion. See #1664 for details.

General improvements

• Add .serialName to MissingFieldException for clearer diagnostics. (#3114)
• Generate unique Automatic-Module-Name entries for metadata JARs. (#3109)
• Revised ProGuard rules and added R8 tests. (#3041)
• CBOR: Improved error message when a byte string/array type mismatch is encountered. (#3052)

Bugfixes

• Fix the type in the BIGNUM_NEGATIVE tag name. (#3090)
• CBOR: Fix various bugs in the decoder implementation to be more strict and consistent with the specification.

1.9.0

This release updates Kotlin version to 2.2.0, includes several bugfixes and provides serializers for kotlin.time.Instant.

Add kotlin.time.Instant serializers

Instant class was moved from kotlinx-datetime library to Kotlin standard library. As a result, kotlinx-datetime 0.7.0 no longer has serializers for the Instant class. To use the new kotlin.time.Instant class in your @Serializable classes, you can use this 1.9.0 kotlinx-serialization version (Kotlin 2.2 is required).
You can choose between the default InstantSerializer, which uses its string representation, or specify InstantComponentSerializer that represents instant as its components.
See details in the PR.

Other bugfixes

• Fix resize in JsonPath (#2995)
• Fixed proguard rules for obfuscation to work correctly (#2983)

1.8.1

This release updates Kotlin version to 2.1.20, while also providing several important improvements
and bugfixes.

Improvements

• Implemented encoding null in key and value of a map in Protobuf (#2910)
• Make type argument in JsonTransformingSerializer nullable (#2911)
• Use SPDX identifier in POMs (#2936) (thanks to Leon Linhart)
• Add watchosDeviceArm64 to Okio integration module (#2920) (thanks to Daniel Santiago)
• Update kotlinx-io version to 0.6.0 (#2933) (thanks to Piotr Krzemiński)

Bugfixes

• Fix incorrect enum coercion during deserialization from JsonElement (#2962)
• Supply proper equals(), hashCode(), and toString() for SerialDescriptor() wrapper (#2942)
• Do not encode empty packed collections in protobuf (#2907)

1.8.0

This release contains all of the changes from 1.8.0-RC. Kotlin 2.1.0 is used as a default, while upcoming 2.1.10 is also supported.
Also added small bugfixes, including speedup of ProtoWireType.from (#2879).

Changelog for 1.8.0-RC is presented below:

@JsonIgnoreUnknownKeys annotation

Previously, only global setting JsonBuilder.ignoreUnknownKeys controlled whether Json parser would throw exception if
input contained a property that was not declared in a @Serializable class.
There were a lot of complaints that this setting is not
flexible enough.
To address them, we added new @JsonIgnoreUnknownKeys annotation that can be applied on a per-class basis.
With this annotation, it is possible to allow unknown properties for annotated classes, while
general decoding methods (such as Json.decodeFromString and others) would still reject them for everything else.
See details in the corresponding PR.

Stabilization of SerialDescriptor API and @SealedSerializationApi annotation

SerialDescriptor, SerialKind, and related API has been around for a long time and has proven itself useful.
The main reason @ExperimentalSerializationApi was on SerialDescriptor's properties is that we wanted to discourage
people from subclassing it.
Fortunately, Kotlin 2.1 provides a special mechanism for such a
case — SubclassOptInRequired.
New kotlinx.serialization.SealedSerializationApi annotation designates APIs
as public for use, but closed for implementation — the case for SerialDescriptor, which is a non-sealed interface for
technical reasons.
Now you can use most of SerialDescriptor and its builders API without the need to opt-in into experimental
serialization API.
See the PR for more details.

Note: All SerialKinds are stable API now, except PolymorphicKind — we may want to expand it in the future.

Generate Java 8's default method implementations in interfaces

TL;DR This change ensures better binary compatibility in the future for library. You should not experience any
difference from it.

kotlinx.serialization library contains a lot of interfaces with default method implementations. Historically, Kotlin
compiled a synthetic DefaultImpls class for them.
Starting from Kotlin 1.4,
it was possible to compile them using as Java 8's default methods to ensure
that new methods can still be added to interfaces without the need for implementors to recompile.
To preserve binary compatibility with existing clients, a special all-compatbility mode is supported in compiler
to generate both default methods and synthetic DefaultImpls class.

Now, kotlinx.serialization finally makes use of this all-compatibility mode,
which potentially allows us to add new methods to interfaces such as SerialDescriptor, Encoder, Decoder, etc.,
without breaking existing clients. This change is expected to have no effect on existing clients, and no action from
your side is required.
Note that Kotlin 2.2 plans to enable all-compatibility
mode by default.

Other bugfixes and improvements

• Correctly skip structures with Cbor.ignoreUnknownKeys setting (#2873)
• Handle missing system property without NPE (#2867)
• Fixed keeping INSTANCE field and serializer function for serializable objects in R8 full mode (#2865)
• Correctly parse invalid numbers in JsonLiteral.long and other extensions (#2852)
• Correctly handle serial name conflict for different classes in SerializersModule.overwriteWith (#2856)
• Add inline reified version of encodeToString as a Json member to streamline the experience for newcomers. (#2853)
• Do not check kind or discriminator collisions for subclasses' polymorphic serializers if Json.classDiscriminatorMode
  is set to NONE (#2833)

v1.8.0-RC

This is a release candidate for the next version. It is based on Kotlin 2.1.0 and includes a few new features, as well
as bugfixes and improvements:

@JsonIgnoreUnknownKeys annotation

Previously, only global setting JsonBuilder.ignoreUnknownKeys controlled whether Json parser would throw exception if input contained a property that was not declared in a @Serializable class. There were a lot of complaints that this setting is not flexible enough. To address them, we added new @JsonIgnoreUnknownKeys annotation that can be applied on a per-class basis. With this annotation, it is possible to allow unknown properties for annotated classes, while general decoding methods (such as Json.decodeFromString and others) would still reject them for everything else. See details in the corresponding PR.

Stabilization of SerialDescriptor API and @SealedSerializationApi annotation

SerialDescriptor, SerialKind, and related API has been around for a long time and has proven itself useful. The main reason @ExperimentalSerializationApi was on SerialDescriptor's properties is that we wanted to discourage people from subclassing it. Fortunately, Kotlin 2.1 provides a special mechanism for such a case — SubclassOptInRequired. New kotlinx.serialization.SealedSerializationApi annotation designates APIs as public for use, but closed for implementation — the case for SerialDescriptor, which is a non-sealed interface for
technical reasons. Now you can use most of SerialDescriptor and its builders API without the need to opt-in into experimental
serialization API. See the PR for more details.

Note: All SerialKinds are stable API now, except PolymorphicKind — we may want to expand it in the future.

Generated Java 8's default method implementations in interfaces

TL;DR This change ensures better binary compatibility in the future for library. You should not experience any
difference from it.

kotlinx.serialization library contains a lot of interfaces with default method implementations. Historically, Kotlin compiled a synthetic DefaultImpls class for them. Starting from Kotlin 1.4, it was possible to compile them using as Java 8's default methods to ensure that new methods can still be added to interfaces without the need for implementors to recompile. To preserve binary compatibility with existing clients, a special all-compatbility mode is supported in compiler to generate both default methods and synthetic DefaultImpls class.

Now, kotlinx.serialization finally makes use of this all-compatibility mode, which potentially allows us to add new methods to interfaces such as SerialDescriptor, Encoder, Decoder, etc., without breaking existing clients. This change is expected to have no effect on existing clients, and no action from your side is required. Note that Kotlin 2.2 plans to enable all-compatibility mode by default.

Other bugfixes and improvements

• Correctly skip structures with Cbor.ignoreUnknownKeys setting (#2873) (thanks to ardune)
• Handle missing system property without NPE (#2867)
• Fixed keeping INSTANCE field and serializer function for serializable objects in R8 full mode (#2865)
• Correctly parse invalid numbers in JsonLiteral.long and other extensions (#2852)
• Correctly handle serial name conflict for different classes in SerializersModule.overwriteWith (#2856)
• Add inline reified version of encodeToString as a Json member to streamline the experience for newcomers. (#2853)
• Do not check kind or discriminator collisions for subclasses' polymorphic serializers if Json.classDiscriminatorMode
  is set to NONE (#2833)

1.7.3

This release aims to fix important issues that were discovered in the 1.7.2 release,
including the inability to sync certain projects into Android Studio/IntelliJ IDEA and exceptions from custom Uuid serializers.

It uses Kotlin 2.0.20 by default.

• Use explicit kotlin-stdlib and kotlin-test versions from version catalog (#2818)
• Drop usage of deprecated Any?.freeze() in K/N target (#2819)
• Check against serialName instead of simpleClassName (#2802)
• Ignore NoClassDefFoundError when initializing builtins map for serializer() function (#2803)
• Clarify example for SerializationException (#2806)

1.7.2

This release provides several new features, including a major Cbor configuration rework.
It uses Kotlin 2.0.20 by default.

Cbor feature set for COSE compliance

This change brings a lot of features to the CBOR format, namely:

• Serial Labels — see @CborLabel annotation and preferCborLabelsOverNames flag.
• Tagging of keys and values — see encode*Tags and verify*Tags set of flags
• Definite length encoding — see useDefiniteLengthEncoding. This flag affects object encoding, since decoding of arrays with definite lenghts is automatically supported.
• Option to globally prefer major type 2 for byte array encoding — see alwaysUseByteString flag.

Since there are quite a lot of flags now, they were restructured to a separate CborConfiguration class, similarly to JsonConfiguration. It is possible to retrieve this configuration from CborEncoder/CborDecoder interfaces in your custom serializers (see their documentation for details).

All of these features make it possible to serialize and parse COSE-compliant CBOR, for example, ISO/IEC 18013-5:2021-compliant mobile driving license data. In case you want to make use of them, there is a predefined Cbor.CoseCompliant instance.
However, some canonicalization steps (such as sorting keys) still need to be performed manually.

This functionality was contributed to us by Bernd Prünster and Christian.

Keeping generated serializers

One of the most requested features for serialization plugin was to continue to generate a serializer even if a custom one is specified for the class. It allows using a plugin-generated serializer in a fallback or delegate strategy, accessing type structure via descriptor, using default serialization behavior in inheritors that do not use custom serializers.

Starting with this release, you can specify the @KeepGeneratedSerializer annotation on the class declaration to instruct the plugin to continue generating the serializer. In this case, the serializer will be accessible using the .generatedSerializer() function on the class's companion object.

This annotation is currently experimental. Kotlin 2.0.20 or higher is required for this feature to work.

You can check out the examples in the documentation and in the PRs: #2758, #2669.

Serializer for kotlin.uuid.Uuid

Kotlin 2.0.20 added a common class to represent UUIDs in a multiplatform code. kotlinx.serialization 1.7.2 provides a corresponding Uuid.serializer() for it, making it possible to use it in @Serializable classes.

Note that for now, serializer should be provided manually with @Contextual annotation. Plugin will be able to automatically insert Uuid serializer in Kotlin 2.1.0.

See more details in the corresponding PR.

Other bugfixes and improvements

• Prohibited using of zero and negative field numbers in ProtoNumber (#2766)
• Improve readability of protobuf decoding exception messages (#2768) (thanks to xiaozhikang0916)
• docs(serializers): Fix grammatical errors (#2779) (thanks to jamhour1g)
• Fixed VerifyError after ProGuard optimization (#2728)
• Add wasm-wasi target to Okio integration (#2727)

1.7.1

This is a bugfix release that aims to fix missing kotlinx-serialization-hocon artifact.
It also contains experimental integration with kotlinx-io library.
Kotlin 2.0.0 is used by default.

Fixed HOCON publication

Sadly, 1.7.0 release was published incomplete: kotlinx-serialization-hocon artifact is missing from 1.7.0 and 1.7.0-RC releases.
This release fixes this problem and now kotlinx-serialization-hocon is available again with 1.7.1 version.
No other changes were made to this artifact. Related ticket: #2717.

Add integration with a kotlinx-io library

kotlinx-io is an official multiplatform library that provides basic IO primitives, similar to Okio.
kotlinx.serialization integration is now available in a separate artifact, located at the kotlinx-serialization-json-io coordinates. Integration artifact provides functions similar to existing Okio integration: encodeToSink, decodeFromSource, and decodeSourceToSequence. Check out the PR for more details.

Other bugfixes

• Prohibited use of elements other than JsonObject in JsonTransformingSerializer with polymorphic serialization (#2715)

1.7.0

This release contains all of the changes from 1.7.0-RC and is compatible with Kotlin 2.0.
Please note that for reasons explained in the 1.7.0-RC changelog, it may not be possible to use it with the Kotlin 1.9.x
compiler plugin. Yet, it is still fully backward compatible with previous versions.

The only difference with 1.7.0-RC is that the classDiscriminatorMode property in JsonBuilder is marked as experimental,
as it should have been when it was introduced (#2680).