diff --git a/build.gradle.kts b/build.gradle.kts
index 87fa032d5..8d71233d7 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -94,11 +94,7 @@ tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinNativeCompile>().configur
 
 subprojects {
     tasks.withType<org.jetbrains.kotlin.gradle.tasks.AbstractKotlinCompile<*>>().configureEach {
-        if (name.contains("Test") || name.contains("Jmh")) {
-            compilerOptions.freeCompilerArgs.addAll(experimentalsInTestEnabled)
-        } else {
-            compilerOptions.freeCompilerArgs.addAll(experimentalsEnabled)
-        }
+        compilerOptions.freeCompilerArgs.addAll(globalCompilerArgs)
     }
 }
 
@@ -172,18 +168,8 @@ gradle.taskGraph.whenReady {
 // getters are required because of variable lazy initialization in Gradle
 val unpublishedProjects get() = setOf("benchmark", "guide", "kotlinx-serialization-json-tests")
 val excludedFromBomProjects get() = unpublishedProjects + "kotlinx-serialization-bom"
-val experimentalsEnabled get() = listOf(
-    "-progressive",
-    "-opt-in=kotlin.ExperimentalMultiplatform",
-    "-opt-in=kotlinx.serialization.InternalSerializationApi",
-    "-P", "plugin:org.jetbrains.kotlinx.serialization:disableIntrinsic=false"
-)
-
-val experimentalsInTestEnabled get() = listOf(
-    "-progressive",
-    "-opt-in=kotlin.ExperimentalMultiplatform",
-    "-opt-in=kotlinx.serialization.ExperimentalSerializationApi",
-    "-opt-in=kotlinx.serialization.InternalSerializationApi",
+val globalCompilerArgs
+    get() = listOf(
     "-P", "plugin:org.jetbrains.kotlinx.serialization:disableIntrinsic=false"
 )
 
diff --git a/core/commonMain/src/kotlinx/serialization/Annotations.kt b/core/commonMain/src/kotlinx/serialization/Annotations.kt
index 132b64414..c1a0835c6 100644
--- a/core/commonMain/src/kotlinx/serialization/Annotations.kt
+++ b/core/commonMain/src/kotlinx/serialization/Annotations.kt
@@ -341,8 +341,9 @@ public annotation class Polymorphic
  * Annotation is not allowed on classes involved in polymorphic serialization:
  * interfaces, sealed classes, abstract classes, classes marked by [Polymorphic].
  *
- * A compiler version `2.0.20` and higher is required.
+ * A compiler version `2.0.20` or higher is required.
  */
+@ExperimentalSerializationApi
 @Target(AnnotationTarget.CLASS)
 @Retention(AnnotationRetention.RUNTIME)
 public annotation class KeepGeneratedSerializer
diff --git a/docs/basic-serialization.md b/docs/basic-serialization.md
index 96e709817..ce98f4934 100644
--- a/docs/basic-serialization.md
+++ b/docs/basic-serialization.md
@@ -450,6 +450,7 @@ For that purposes, [EncodeDefault] annotation can be used:
 
 ```kotlin
 @Serializable
+@OptIn(ExperimentalSerializationApi::class) // EncodeDefault is an experimental annotation for now
 data class Project(
     val name: String,
     @EncodeDefault val language: String = "Kotlin"
@@ -462,6 +463,7 @@ It's also possible to tweak it into the opposite behavior using [EncodeDefault.M
 ```kotlin
 
 @Serializable
+@OptIn(ExperimentalSerializationApi::class) // EncodeDefault is an experimental annotation for now
 data class User(
     val name: String,
     @EncodeDefault(EncodeDefault.Mode.NEVER) val projects: List<Project> = emptyList()
diff --git a/docs/formats.md b/docs/formats.md
index a6537e3bc..307601234 100644
--- a/docs/formats.md
+++ b/docs/formats.md
@@ -65,6 +65,7 @@ fun ByteArray.toAsciiHexString() = joinToString("") {
 @Serializable
 data class Project(val name: String, val language: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization", "Kotlin") 
     val bytes = Cbor.encodeToByteArray(data)   
@@ -116,13 +117,14 @@ import kotlinx.serialization.cbor.*
 -->
 
 ```kotlin
-val format = Cbor { ignoreUnknownKeys = true }
-
 @Serializable
 data class Project(val name: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
-    val data = format.decodeFromHexString<Project>(
+  val format = Cbor { ignoreUnknownKeys = true }
+  
+  val data = format.decodeFromHexString<Project>(
         "bf646e616d65756b6f746c696e782e73657269616c697a6174696f6e686c616e6775616765664b6f746c696eff"
     )
     println(data)
@@ -183,12 +185,14 @@ fun ByteArray.toAsciiHexString() = joinToString("") {
 
 ```kotlin
 @Serializable
+@OptIn(ExperimentalSerializationApi::class)
 data class Data(
     @ByteString
     val type2: ByteArray, // CBOR Major type 2
     val type4: ByteArray  // CBOR Major type 4
-)        
+)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Data(byteArrayOf(1, 2, 3, 4), byteArrayOf(5, 6, 7, 8)) 
     val bytes = Cbor.encodeToByteArray(data)   
@@ -338,6 +342,7 @@ fun ByteArray.toAsciiHexString() = joinToString("") {
 @Serializable
 data class Project(val name: String, val language: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization", "Kotlin") 
     val bytes = ProtoBuf.encodeToByteArray(data)   
@@ -380,6 +385,7 @@ fun ByteArray.toAsciiHexString() = joinToString("") {
 -->
 
 ```kotlin    
+@OptIn(ExperimentalSerializationApi::class)
 @Serializable
 data class Project(
     @ProtoNumber(1)
@@ -388,6 +394,7 @@ data class Project(
     val language: String
 )
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization", "Kotlin") 
     val bytes = ProtoBuf.encodeToByteArray(data)   
@@ -432,6 +439,7 @@ fun ByteArray.toAsciiHexString() = joinToString("") {
 -->
 
 ```kotlin    
+@OptIn(ExperimentalSerializationApi::class)
 @Serializable
 class Data(
     @ProtoType(ProtoIntegerType.DEFAULT)
@@ -442,6 +450,7 @@ class Data(
     val c: Int
 )
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Data(1, -2, 3) 
     println(ProtoBuf.encodeToByteArray(data).toAsciiHexString())
@@ -497,6 +506,7 @@ data class Data(
     val b: List<Int> = emptyList()
 )
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Data(listOf(1, 2, 3), listOf())
     val bytes = ProtoBuf.encodeToByteArray(data)
@@ -561,6 +571,7 @@ import kotlinx.serialization.protobuf.*
 
 ```kotlin
 // The outer class
+@OptIn(ExperimentalSerializationApi::class)
 @Serializable
 data class Data(
     @ProtoNumber(1) val name: String,
@@ -571,11 +582,14 @@ data class Data(
 @Serializable sealed interface IPhoneType
 
 // Message holder for home_phone
+@OptIn(ExperimentalSerializationApi::class)
 @Serializable @JvmInline value class HomePhone(@ProtoNumber(2) val number: String): IPhoneType
 
 // Message holder for work_phone. Can also be a value class, but we leave it as `data` to demonstrate that both variants can be used.
+@OptIn(ExperimentalSerializationApi::class)
 @Serializable data class WorkPhone(@ProtoNumber(3) val number: String): IPhoneType
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
   val dataTom = Data("Tom", HomePhone("123"))
   val stringTom = ProtoBuf.encodeToHexString(dataTom)
@@ -654,6 +668,8 @@ data class SampleData(
     val description: String?,
     val department: String = "QA"
 )
+
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
   val descriptors = listOf(SampleData.serializer().descriptor)
   val schemas = ProtoBufSchemaGenerator.generateSchemaText(descriptors)
@@ -705,6 +721,7 @@ class Project(val name: String, val owner: User)
 @Serializable
 class User(val name: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization",  User("kotlin"))
     val map = Properties.encodeToMap(data)
@@ -737,7 +754,8 @@ implemented to get a basic working format.
 
 Let us start with a trivial format implementation that encodes the data into a single list of primitive
 constituent objects in the order they were written in the source code. To start, we implement a simple [Encoder] by
-overriding `encodeValue` in [AbstractEncoder].
+overriding `encodeValue` in [AbstractEncoder]. Since encoders are intended to be consumed by other parts of application,
+it is recommended to propagate the `@ExperimentalSerializationApi` annotation instead of opting-in.
 
 <!--- INCLUDE
 import kotlinx.serialization.*
@@ -747,6 +765,7 @@ import kotlinx.serialization.modules.*
 -->
 
 ```kotlin
+@ExperimentalSerializationApi
 class ListEncoder : AbstractEncoder() {
     val list = mutableListOf<Any>()
 
@@ -762,6 +781,7 @@ Now we write a convenience top-level function that creates an encoder that encod
 and returns a list.
 
 ```kotlin
+@ExperimentalSerializationApi
 fun <T> encodeToList(serializer: SerializationStrategy<T>, value: T): List<Any> {
     val encoder = ListEncoder()
     encoder.encodeSerializableValue(serializer, value)
@@ -774,6 +794,7 @@ the `encodeToList` function with a `reified` type parameter using the [serialize
 the appropriate [KSerializer] instance for the actual type.
 
 ```kotlin 
+@ExperimentalSerializationApi
 inline fun <reified T> encodeToList(value: T) = encodeToList(serializer(), value)
 ```                  
 
@@ -786,6 +807,7 @@ data class Project(val name: String, val owner: User, val votes: Int)
 @Serializable
 data class User(val name: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization",  User("kotlin"), 9000)
     println(encodeToList(data))
@@ -814,6 +836,7 @@ import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
 import kotlinx.serialization.modules.*
 
+@ExperimentalSerializationApi
 class ListEncoder : AbstractEncoder() {
     val list = mutableListOf<Any>()
 
@@ -824,12 +847,14 @@ class ListEncoder : AbstractEncoder() {
     }
 }
 
+@ExperimentalSerializationApi
 fun <T> encodeToList(serializer: SerializationStrategy<T>, value: T): List<Any> {
     val encoder = ListEncoder()
     encoder.encodeSerializableValue(serializer, value)
     return encoder.list
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> encodeToList(value: T) = encodeToList(serializer(), value)
 -->
 
@@ -845,6 +870,7 @@ A decoder needs to implement more substance.
   each structure that is being recursively decoded keeps track of its own `elementIndex` state separately.  
 
 ```kotlin
+@ExperimentalSerializationApi
 class ListDecoder(val list: ArrayDeque<Any>) : AbstractDecoder() {
     private var elementIndex = 0
 
@@ -865,11 +891,13 @@ class ListDecoder(val list: ArrayDeque<Any>) : AbstractDecoder() {
 A couple of convenience functions for decoding.
 
 ```kotlin
+@ExperimentalSerializationApi
 fun <T> decodeFromList(list: List<Any>, deserializer: DeserializationStrategy<T>): T {
     val decoder = ListDecoder(ArrayDeque(list))
     return decoder.decodeSerializableValue(deserializer)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> decodeFromList(list: List<Any>): T = decodeFromList(list, serializer())
 ```
 
@@ -885,6 +913,7 @@ data class User(val name: String)
 -->
 
 ```kotlin    
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization",  User("kotlin"), 9000)
     val list = encodeToList(data)
@@ -922,6 +951,7 @@ import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
 import kotlinx.serialization.modules.*
 
+@ExperimentalSerializationApi
 class ListEncoder : AbstractEncoder() {
     val list = mutableListOf<Any>()
 
@@ -932,16 +962,19 @@ class ListEncoder : AbstractEncoder() {
     }
 }
 
+@ExperimentalSerializationApi
 fun <T> encodeToList(serializer: SerializationStrategy<T>, value: T): List<Any> {
     val encoder = ListEncoder()
     encoder.encodeSerializableValue(serializer, value)
     return encoder.list
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> encodeToList(value: T) = encodeToList(serializer(), value)
 -->
 
 ```kotlin
+@ExperimentalSerializationApi
 class ListDecoder(val list: ArrayDeque<Any>) : AbstractDecoder() {
     private var elementIndex = 0
 
@@ -963,11 +996,13 @@ class ListDecoder(val list: ArrayDeque<Any>) : AbstractDecoder() {
 
 <!--- INCLUDE
 
+@ExperimentalSerializationApi
 fun <T> decodeFromList(list: List<Any>, deserializer: DeserializationStrategy<T>): T {
     val decoder = ListDecoder(ArrayDeque(list))
     return decoder.decodeSerializableValue(deserializer)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> decodeFromList(list: List<Any>): T = decodeFromList(list, serializer())
 
 @Serializable
@@ -976,6 +1011,7 @@ data class Project(val name: String, val owner: User, val votes: Int)
 @Serializable
 data class User(val name: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization",  User("kotlin"), 9000)
     val list = encodeToList(data)
@@ -1009,6 +1045,7 @@ import kotlinx.serialization.modules.*
 -->
 
 ```kotlin
+@ExperimentalSerializationApi
 class ListEncoder : AbstractEncoder() {
     val list = mutableListOf<Any>()
 
@@ -1027,12 +1064,14 @@ class ListEncoder : AbstractEncoder() {
 
 <!--- INCLUDE
 
+@ExperimentalSerializationApi
 fun <T> encodeToList(serializer: SerializationStrategy<T>, value: T): List<Any> {
     val encoder = ListEncoder()
     encoder.encodeSerializableValue(serializer, value)
     return encoder.list
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> encodeToList(value: T) = encodeToList(serializer(), value)
 -->
 
@@ -1042,6 +1081,7 @@ in addition to the previous code.
 > The formats that store collection size in advance have to return `true` from `decodeSequentially`.
 
 ```kotlin
+@ExperimentalSerializationApi
 class ListDecoder(val list: ArrayDeque<Any>, var elementsCount: Int = 0) : AbstractDecoder() {
     private var elementIndex = 0
 
@@ -1066,11 +1106,13 @@ class ListDecoder(val list: ArrayDeque<Any>, var elementsCount: Int = 0) : Abstr
 
 <!--- INCLUDE
 
+@ExperimentalSerializationApi
 fun <T> decodeFromList(list: List<Any>, deserializer: DeserializationStrategy<T>): T {
     val decoder = ListDecoder(ArrayDeque(list))
     return decoder.decodeSerializableValue(deserializer)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> decodeFromList(list: List<Any>): T = decodeFromList(list, serializer())
 -->
 
@@ -1083,6 +1125,7 @@ data class Project(val name: String, val owners: List<User>, val votes: Int)
 @Serializable
 data class User(val name: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization",  listOf(User("kotlin"), User("jetbrains")), 9000)
     val list = encodeToList(data)
@@ -1114,6 +1157,7 @@ import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
 import kotlinx.serialization.modules.*
 
+@ExperimentalSerializationApi
 class ListEncoder : AbstractEncoder() {
     val list = mutableListOf<Any>()
 
@@ -1139,14 +1183,17 @@ In the encoder implementation we override [Encoder.encodeNull] and [Encoder.enco
 <!--- INCLUDE
 }
 
+@ExperimentalSerializationApi
 fun <T> encodeToList(serializer: SerializationStrategy<T>, value: T): List<Any> {
     val encoder = ListEncoder()
     encoder.encodeSerializableValue(serializer, value)
     return encoder.list
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> encodeToList(value: T) = encodeToList(serializer(), value)
 
+@ExperimentalSerializationApi
 class ListDecoder(val list: ArrayDeque<Any>, var elementsCount: Int = 0) : AbstractDecoder() {
     private var elementIndex = 0
     
@@ -1177,11 +1224,13 @@ In the decoder implementation we override [Decoder.decodeNotNullMark].
 <!--- INCLUDE
 }
 
+@ExperimentalSerializationApi
 fun <T> decodeFromList(list: List<Any>, deserializer: DeserializationStrategy<T>): T {
     val decoder = ListDecoder(ArrayDeque(list))
     return decoder.decodeSerializableValue(deserializer)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> decodeFromList(list: List<Any>): T = decodeFromList(list, serializer())
 -->
 
@@ -1194,6 +1243,7 @@ data class Project(val name: String, val owner: User?, val votes: Int?)
 @Serializable
 data class User(val name: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization",  User("kotlin") , null)
     val list = encodeToList(data)
@@ -1230,7 +1280,8 @@ import kotlinx.serialization.modules.*
 import java.io.*
 -->
 
-```kotlin            
+```kotlin     
+@ExperimentalSerializationApi
 class DataOutputEncoder(val output: DataOutput) : AbstractEncoder() {
     override val serializersModule: SerializersModule = EmptySerializersModule()
     override fun encodeBoolean(value: Boolean) = output.writeByte(if (value) 1 else 0)
@@ -1256,17 +1307,20 @@ class DataOutputEncoder(val output: DataOutput) : AbstractEncoder() {
 
 <!--- INCLUDE
 
+@ExperimentalSerializationApi
 fun <T> encodeTo(output: DataOutput, serializer: SerializationStrategy<T>, value: T) {
     val encoder = DataOutputEncoder(output)
     encoder.encodeSerializableValue(serializer, value)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> encodeTo(output: DataOutput, value: T) = encodeTo(output, serializer(), value)
 -->
 
 The decoder implementation mirrors encoder's implementation overriding all the primitive `decodeXxx` functions.
 
 ```kotlin 
+@ExperimentalSerializationApi
 class DataInputDecoder(val input: DataInput, var elementsCount: Int = 0) : AbstractDecoder() {
     private var elementIndex = 0
     override val serializersModule: SerializersModule = EmptySerializersModule()
@@ -1300,11 +1354,13 @@ class DataInputDecoder(val input: DataInput, var elementsCount: Int = 0) : Abstr
 
 <!--- INCLUDE
 
+@ExperimentalSerializationApi
 fun <T> decodeFrom(input: DataInput, deserializer: DeserializationStrategy<T>): T {
     val decoder = DataInputDecoder(input)
     return decoder.decodeSerializableValue(deserializer)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> decodeFrom(input: DataInput): T = decodeFrom(input, serializer())
 
 fun ByteArray.toAsciiHexString() = joinToString("") {
@@ -1320,6 +1376,7 @@ used in the [CBOR (experimental)](#cbor-experimental) and [ProtoBuf (experimenta
 @Serializable
 data class Project(val name: String, val language: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization", "Kotlin")
     val output = ByteArrayOutputStream()
@@ -1383,6 +1440,7 @@ we add a trivial implementation of `encodeCompactSize` function that uses only o
 a size of up to 254 bytes.  
 
 <!--- INCLUDE
+@ExperimentalSerializationApi
 class DataOutputEncoder(val output: DataOutput) : AbstractEncoder() {
     override val serializersModule: SerializersModule = EmptySerializersModule()
     override fun encodeBoolean(value: Boolean) = output.writeByte(if (value) 1 else 0)
@@ -1431,13 +1489,16 @@ class DataOutputEncoder(val output: DataOutput) : AbstractEncoder() {
 <!--- INCLUDE
 }
 
+@ExperimentalSerializationApi
 fun <T> encodeTo(output: DataOutput, serializer: SerializationStrategy<T>, value: T) {
     val encoder = DataOutputEncoder(output)
     encoder.encodeSerializableValue(serializer, value)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> encodeTo(output: DataOutput, value: T) = encodeTo(output, serializer(), value)
 
+@ExperimentalSerializationApi
 class DataInputDecoder(val input: DataInput, var elementsCount: Int = 0) : AbstractDecoder() {
     private var elementIndex = 0
     override val serializersModule: SerializersModule = EmptySerializersModule()
@@ -1495,11 +1556,13 @@ the [decodeSerializableValue][Decoder.decodeSerializableValue] function.
 <!--- INCLUDE
 }
 
+@ExperimentalSerializationApi
 fun <T> decodeFrom(input: DataInput, deserializer: DeserializationStrategy<T>): T {
     val decoder = DataInputDecoder(input)
     return decoder.decodeSerializableValue(deserializer)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> decodeFrom(input: DataInput): T = decodeFrom(input, serializer())
 
 fun ByteArray.toAsciiHexString() = joinToString("") {
@@ -1514,6 +1577,7 @@ Now everything is ready to perform serialization of some byte arrays.
 @Serializable
 data class Project(val name: String, val attachment: ByteArray)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization", byteArrayOf(0x0A, 0x0B, 0x0C, 0x0D))
     val output = ByteArrayOutputStream()
diff --git a/docs/json.md b/docs/json.md
index d009b4090..234d7c927 100644
--- a/docs/json.md
+++ b/docs/json.md
@@ -172,6 +172,7 @@ but such renaming blocks the ability to decode data with the old name.
 To support multiple JSON names for the one Kotlin property, there is the [JsonNames] annotation:
 
 ```kotlin
+@OptIn(ExperimentalSerializationApi::class) // JsonNames is an experimental annotation for now
 @Serializable
 data class Project(@JsonNames("title") val name: String)
 
@@ -462,6 +463,7 @@ control over the resulting JSON object:
 It is also possible to specify different class discriminators for different hierarchies. Instead of Json instance property, use [JsonClassDiscriminator] annotation directly on base serializable class:
 
 ```kotlin
+@OptIn(ExperimentalSerializationApi::class) // JsonClassDiscriminator is an experimental annotation for now
 @Serializable
 @JsonClassDiscriminator("message_type")
 sealed class Base
@@ -524,6 +526,7 @@ addition of the class discriminator with the [JsonBuilder.classDiscriminatorMode
 For example, [ClassDiscriminatorMode.NONE] does not add class discriminator at all, in case the receiving party is not interested in Kotlin types:
 
 ```kotlin
+@OptIn(ExperimentalSerializationApi::class) // classDiscriminatorMode is an experimental setting for now
 val format = Json { classDiscriminatorMode = ClassDiscriminatorMode.NONE }
 
 @Serializable
@@ -562,8 +565,10 @@ However, sometimes third-party JSONs have such values named in lowercase or some
 In this case, it is possible to decode enum values in a case-insensitive manner using [JsonBuilder.decodeEnumsCaseInsensitive] property:
 
 ```kotlin
+@OptIn(ExperimentalSerializationApi::class) // decodeEnumsCaseInsensitive is an experimental setting for now
 val format = Json { decodeEnumsCaseInsensitive = true }
 
+@OptIn(ExperimentalSerializationApi::class) // JsonNames is an experimental annotation for now
 enum class Cases { VALUE_A, @JsonNames("Alternative") VALUE_B }
 
 @Serializable
@@ -598,6 +603,7 @@ for a [Json] instance. `kotlinx.serialization` provides one strategy implementat
 @Serializable
 data class Project(val projectName: String, val projectOwner: String)
 
+@OptIn(ExperimentalSerializationApi::class) // namingStrategy is an experimental setting for now
 val format = Json { namingStrategy = JsonNamingStrategy.SnakeCase }
 
 fun main() {
@@ -929,6 +935,7 @@ fun main() {
     val pi = BigDecimal("3.141592653589793238462643383279")
 
     // use JsonUnquotedLiteral to encode raw JSON content
+    @OptIn(ExperimentalSerializationApi::class)
     val piJsonLiteral = JsonUnquotedLiteral(pi.toString())
 
     val piJsonDouble = JsonPrimitive(pi.toDouble())
@@ -1000,6 +1007,7 @@ To avoid creating an inconsistent state, encoding a String equal to `"null"` is
 Use [JsonNull] or [JsonPrimitive] instead.
 
 ```kotlin
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     // caution: creating null with JsonUnquotedLiteral will cause an exception! 
     JsonUnquotedLiteral("null")
@@ -1269,6 +1277,8 @@ For example, to add a preliminary modification of JSON elements or to add proces
 
 In this case, you can mark the serializable class with the [`@KeepGeneratedSerializer`][KeepGeneratedSerializer] annotation and get the generated serializer using the `generatedSerializer()` function.
 
+> This annotation is currently experimental. Kotlin 2.0.20 or higher is required for this feature to work.
+
 Here is an example of the simultaneous use of [JsonTransformingSerializer] and polymorphism.
 In this example, we use `transformDeserialize` function to rename `basic-name` key into `name` so it matches the `abstract val name` property from the `Project` supertype.
 ```kotlin
@@ -1277,6 +1287,7 @@ sealed class Project {
     abstract val name: String
 }
 
+@OptIn(ExperimentalSerializationApi::class)
 @KeepGeneratedSerializer
 @Serializable(with = BasicProjectSerializer::class)
 @SerialName("basic")
@@ -1345,7 +1356,7 @@ sealed class Response<out T> {
 }
 
 class ResponseSerializer<T>(private val dataSerializer: KSerializer<T>) : KSerializer<Response<T>> {
-    override val descriptor: SerialDescriptor = buildSerialDescriptor("Response", PolymorphicKind.SEALED) {
+    override val descriptor: SerialDescriptor = buildClassSerialDescriptor("Response") {
         element("Ok", dataSerializer.descriptor)
         element("Error", buildClassSerialDescriptor("Error") {
           element<String>("message")
diff --git a/docs/serializers.md b/docs/serializers.md
index 742806818..df40863ac 100644
--- a/docs/serializers.md
+++ b/docs/serializers.md
@@ -397,6 +397,7 @@ import kotlinx.serialization.builtins.IntArraySerializer
 
 class ColorIntArraySerializer : KSerializer<Color> {
     private val delegateSerializer = IntArraySerializer()
+    @OptIn(ExperimentalSerializationApi::class)
     override val descriptor = SerialDescriptor("Color", delegateSerializer.descriptor)
 
     override fun serialize(encoder: Encoder, value: Color) {
@@ -644,7 +645,8 @@ object ColorAsObjectSerializer : KSerializer<Color> {
         decoder.decodeStructure(descriptor) {
             var r = -1
             var g = -1
-            var b = -1     
+            var b = -1
+            @OptIn(ExperimentalSerializationApi::class)
             if (decodeSequentially()) { // sequential decoding protocol
                 r = decodeIntElement(descriptor, 0)           
                 g = decodeIntElement(descriptor, 1)  
@@ -984,6 +986,8 @@ The most common examples are: using a plugin-generated serializer for fallback s
 In order for the plugin to continue generating the serializer, you must specify the `@KeepGeneratedSerializer` annotation in the type declaration.
 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.
+
 Annotation `@KeepGeneratedSerializer` is not allowed on classes involved in polymorphic serialization: interfaces, sealed classes, abstract classes, classes marked by [Polymorphic].
 
 An example of using two serializers at once:
@@ -1005,6 +1009,7 @@ object ColorAsStringSerializer : KSerializer<Color> {
 -->
 
 ```kotlin
+@OptIn(ExperimentalSerializationApi::class)
 @KeepGeneratedSerializer
 @Serializable(with = ColorAsStringSerializer::class)
 class Color(val rgb: Int)
@@ -1170,7 +1175,8 @@ using the [Serializer] annotation on an object with the [`forClass`][Serializer.
 ```kotlin         
 // NOT @Serializable
 class Project(val name: String, val language: String)
-                           
+
+@OptIn(ExperimentalSerializationApi::class)
 @Serializer(forClass = Project::class)
 object ProjectSerializer
 ```
@@ -1215,8 +1221,9 @@ class Project(
         get() = "kotlin/$name"                                         
 
     private var locked: Boolean = false // private, not accessible -- not serialized 
-}              
+}
 
+@OptIn(ExperimentalSerializationApi::class)
 @Serializer(forClass = Project::class)
 object ProjectSerializer
 
diff --git a/formats/hocon/build.gradle.kts b/formats/hocon/build.gradle.kts
index 71a02fcbf..e2670528e 100644
--- a/formats/hocon/build.gradle.kts
+++ b/formats/hocon/build.gradle.kts
@@ -14,6 +14,7 @@ plugins {
 
 kotlin {
     jvmToolchain(jdkToolchainVersion)
+
     compilerOptions {
         jvmTarget = JvmTarget.JVM_1_8
         if (overriddenLanguageVersion != null) {
@@ -22,6 +23,14 @@ kotlin {
         }
         freeCompilerArgs.add("-Xjdk-release=1.8")
     }
+
+    sourceSets.all {
+        languageSettings {
+            progressiveMode = true
+
+            optIn("kotlinx.serialization.InternalSerializationApi")
+        }
+    }
 }
 
 // Only main
diff --git a/guide/build.gradle.kts b/guide/build.gradle.kts
index a69992b50..5bafa9bb2 100644
--- a/guide/build.gradle.kts
+++ b/guide/build.gradle.kts
@@ -14,6 +14,7 @@ kotlin {
     jvmToolchain(8)
 
     compilerOptions {
+        allWarningsAsErrors = true
         if (overriddenLanguageVersion != null) {
             languageVersion = KotlinVersion.fromVersion(overriddenLanguageVersion!!)
             freeCompilerArgs.add("-Xsuppress-version-warnings")
diff --git a/guide/example/example-classes-10.kt b/guide/example/example-classes-10.kt
index c5a1f739c..941bde468 100644
--- a/guide/example/example-classes-10.kt
+++ b/guide/example/example-classes-10.kt
@@ -5,6 +5,7 @@ import kotlinx.serialization.*
 import kotlinx.serialization.json.*
 
 @Serializable
+@OptIn(ExperimentalSerializationApi::class) // EncodeDefault is an experimental annotation for now
 data class Project(
     val name: String,
     @EncodeDefault val language: String = "Kotlin"
@@ -12,6 +13,7 @@ data class Project(
 
 
 @Serializable
+@OptIn(ExperimentalSerializationApi::class) // EncodeDefault is an experimental annotation for now
 data class User(
     val name: String,
     @EncodeDefault(EncodeDefault.Mode.NEVER) val projects: List<Project> = emptyList()
diff --git a/guide/example/example-formats-01.kt b/guide/example/example-formats-01.kt
index ad2b3e6d7..446bc3a34 100644
--- a/guide/example/example-formats-01.kt
+++ b/guide/example/example-formats-01.kt
@@ -12,6 +12,7 @@ fun ByteArray.toAsciiHexString() = joinToString("") {
 @Serializable
 data class Project(val name: String, val language: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization", "Kotlin") 
     val bytes = Cbor.encodeToByteArray(data)   
diff --git a/guide/example/example-formats-02.kt b/guide/example/example-formats-02.kt
index c95e53294..cddccf691 100644
--- a/guide/example/example-formats-02.kt
+++ b/guide/example/example-formats-02.kt
@@ -4,13 +4,14 @@ package example.exampleFormats02
 import kotlinx.serialization.*
 import kotlinx.serialization.cbor.*
 
-val format = Cbor { ignoreUnknownKeys = true }
-
 @Serializable
 data class Project(val name: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
-    val data = format.decodeFromHexString<Project>(
+  val format = Cbor { ignoreUnknownKeys = true }
+  
+  val data = format.decodeFromHexString<Project>(
         "bf646e616d65756b6f746c696e782e73657269616c697a6174696f6e686c616e6775616765664b6f746c696eff"
     )
     println(data)
diff --git a/guide/example/example-formats-03.kt b/guide/example/example-formats-03.kt
index d2191f6a0..b48811d6f 100644
--- a/guide/example/example-formats-03.kt
+++ b/guide/example/example-formats-03.kt
@@ -10,12 +10,14 @@ fun ByteArray.toAsciiHexString() = joinToString("") {
 }
 
 @Serializable
+@OptIn(ExperimentalSerializationApi::class)
 data class Data(
     @ByteString
     val type2: ByteArray, // CBOR Major type 2
     val type4: ByteArray  // CBOR Major type 4
-)        
+)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Data(byteArrayOf(1, 2, 3, 4), byteArrayOf(5, 6, 7, 8)) 
     val bytes = Cbor.encodeToByteArray(data)   
diff --git a/guide/example/example-formats-04.kt b/guide/example/example-formats-04.kt
index 9f6610bc1..0ff0289fe 100644
--- a/guide/example/example-formats-04.kt
+++ b/guide/example/example-formats-04.kt
@@ -12,6 +12,7 @@ fun ByteArray.toAsciiHexString() = joinToString("") {
 @Serializable
 data class Project(val name: String, val language: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization", "Kotlin") 
     val bytes = ProtoBuf.encodeToByteArray(data)   
diff --git a/guide/example/example-formats-05.kt b/guide/example/example-formats-05.kt
index 796e3eb07..b65e1cd1e 100644
--- a/guide/example/example-formats-05.kt
+++ b/guide/example/example-formats-05.kt
@@ -9,6 +9,7 @@ fun ByteArray.toAsciiHexString() = joinToString("") {
         "{${it.toUByte().toString(16).padStart(2, '0').uppercase()}}"
 }
 
+@OptIn(ExperimentalSerializationApi::class)
 @Serializable
 data class Project(
     @ProtoNumber(1)
@@ -17,6 +18,7 @@ data class Project(
     val language: String
 )
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization", "Kotlin") 
     val bytes = ProtoBuf.encodeToByteArray(data)   
diff --git a/guide/example/example-formats-06.kt b/guide/example/example-formats-06.kt
index 5e4fbc501..d675df117 100644
--- a/guide/example/example-formats-06.kt
+++ b/guide/example/example-formats-06.kt
@@ -9,6 +9,7 @@ fun ByteArray.toAsciiHexString() = joinToString("") {
         "{${it.toUByte().toString(16).padStart(2, '0').uppercase()}}"
 }
 
+@OptIn(ExperimentalSerializationApi::class)
 @Serializable
 class Data(
     @ProtoType(ProtoIntegerType.DEFAULT)
@@ -19,6 +20,7 @@ class Data(
     val c: Int
 )
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Data(1, -2, 3) 
     println(ProtoBuf.encodeToByteArray(data).toAsciiHexString())
diff --git a/guide/example/example-formats-07.kt b/guide/example/example-formats-07.kt
index 52bc826b7..c219b8ddc 100644
--- a/guide/example/example-formats-07.kt
+++ b/guide/example/example-formats-07.kt
@@ -15,6 +15,7 @@ data class Data(
     val b: List<Int> = emptyList()
 )
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Data(listOf(1, 2, 3), listOf())
     val bytes = ProtoBuf.encodeToByteArray(data)
diff --git a/guide/example/example-formats-08.kt b/guide/example/example-formats-08.kt
index 1316aa0d3..23f3ccf31 100644
--- a/guide/example/example-formats-08.kt
+++ b/guide/example/example-formats-08.kt
@@ -5,6 +5,7 @@ import kotlinx.serialization.*
 import kotlinx.serialization.protobuf.*
 
 // The outer class
+@OptIn(ExperimentalSerializationApi::class)
 @Serializable
 data class Data(
     @ProtoNumber(1) val name: String,
@@ -15,11 +16,14 @@ data class Data(
 @Serializable sealed interface IPhoneType
 
 // Message holder for home_phone
+@OptIn(ExperimentalSerializationApi::class)
 @Serializable @JvmInline value class HomePhone(@ProtoNumber(2) val number: String): IPhoneType
 
 // Message holder for work_phone. Can also be a value class, but we leave it as `data` to demonstrate that both variants can be used.
+@OptIn(ExperimentalSerializationApi::class)
 @Serializable data class WorkPhone(@ProtoNumber(3) val number: String): IPhoneType
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
   val dataTom = Data("Tom", HomePhone("123"))
   val stringTom = ProtoBuf.encodeToHexString(dataTom)
diff --git a/guide/example/example-formats-09.kt b/guide/example/example-formats-09.kt
index 387061f08..8c1ee2238 100644
--- a/guide/example/example-formats-09.kt
+++ b/guide/example/example-formats-09.kt
@@ -11,6 +11,8 @@ data class SampleData(
     val description: String?,
     val department: String = "QA"
 )
+
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
   val descriptors = listOf(SampleData.serializer().descriptor)
   val schemas = ProtoBufSchemaGenerator.generateSchemaText(descriptors)
diff --git a/guide/example/example-formats-10.kt b/guide/example/example-formats-10.kt
index 020f175da..6941e4597 100644
--- a/guide/example/example-formats-10.kt
+++ b/guide/example/example-formats-10.kt
@@ -11,6 +11,7 @@ class Project(val name: String, val owner: User)
 @Serializable
 class User(val name: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization",  User("kotlin"))
     val map = Properties.encodeToMap(data)
diff --git a/guide/example/example-formats-11.kt b/guide/example/example-formats-11.kt
index 9fa4180d8..6b8ccd117 100644
--- a/guide/example/example-formats-11.kt
+++ b/guide/example/example-formats-11.kt
@@ -6,6 +6,7 @@ import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
 import kotlinx.serialization.modules.*
 
+@ExperimentalSerializationApi
 class ListEncoder : AbstractEncoder() {
     val list = mutableListOf<Any>()
 
@@ -16,12 +17,14 @@ class ListEncoder : AbstractEncoder() {
     }
 }
 
+@ExperimentalSerializationApi
 fun <T> encodeToList(serializer: SerializationStrategy<T>, value: T): List<Any> {
     val encoder = ListEncoder()
     encoder.encodeSerializableValue(serializer, value)
     return encoder.list
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> encodeToList(value: T) = encodeToList(serializer(), value)
 
 @Serializable
@@ -30,6 +33,7 @@ data class Project(val name: String, val owner: User, val votes: Int)
 @Serializable
 data class User(val name: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization",  User("kotlin"), 9000)
     println(encodeToList(data))
diff --git a/guide/example/example-formats-12.kt b/guide/example/example-formats-12.kt
index 803f331e6..5f087d94d 100644
--- a/guide/example/example-formats-12.kt
+++ b/guide/example/example-formats-12.kt
@@ -6,6 +6,7 @@ import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
 import kotlinx.serialization.modules.*
 
+@ExperimentalSerializationApi
 class ListEncoder : AbstractEncoder() {
     val list = mutableListOf<Any>()
 
@@ -16,14 +17,17 @@ class ListEncoder : AbstractEncoder() {
     }
 }
 
+@ExperimentalSerializationApi
 fun <T> encodeToList(serializer: SerializationStrategy<T>, value: T): List<Any> {
     val encoder = ListEncoder()
     encoder.encodeSerializableValue(serializer, value)
     return encoder.list
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> encodeToList(value: T) = encodeToList(serializer(), value)
 
+@ExperimentalSerializationApi
 class ListDecoder(val list: ArrayDeque<Any>) : AbstractDecoder() {
     private var elementIndex = 0
 
@@ -40,11 +44,13 @@ class ListDecoder(val list: ArrayDeque<Any>) : AbstractDecoder() {
         ListDecoder(list)
 }
 
+@ExperimentalSerializationApi
 fun <T> decodeFromList(list: List<Any>, deserializer: DeserializationStrategy<T>): T {
     val decoder = ListDecoder(ArrayDeque(list))
     return decoder.decodeSerializableValue(deserializer)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> decodeFromList(list: List<Any>): T = decodeFromList(list, serializer())
 
 @Serializable
@@ -53,6 +59,7 @@ data class Project(val name: String, val owner: User, val votes: Int)
 @Serializable
 data class User(val name: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization",  User("kotlin"), 9000)
     val list = encodeToList(data)
diff --git a/guide/example/example-formats-13.kt b/guide/example/example-formats-13.kt
index 568017765..f2915f645 100644
--- a/guide/example/example-formats-13.kt
+++ b/guide/example/example-formats-13.kt
@@ -6,6 +6,7 @@ import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
 import kotlinx.serialization.modules.*
 
+@ExperimentalSerializationApi
 class ListEncoder : AbstractEncoder() {
     val list = mutableListOf<Any>()
 
@@ -16,14 +17,17 @@ class ListEncoder : AbstractEncoder() {
     }
 }
 
+@ExperimentalSerializationApi
 fun <T> encodeToList(serializer: SerializationStrategy<T>, value: T): List<Any> {
     val encoder = ListEncoder()
     encoder.encodeSerializableValue(serializer, value)
     return encoder.list
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> encodeToList(value: T) = encodeToList(serializer(), value)
 
+@ExperimentalSerializationApi
 class ListDecoder(val list: ArrayDeque<Any>) : AbstractDecoder() {
     private var elementIndex = 0
 
@@ -42,11 +46,13 @@ class ListDecoder(val list: ArrayDeque<Any>) : AbstractDecoder() {
     override fun decodeSequentially(): Boolean = true
 }        
 
+@ExperimentalSerializationApi
 fun <T> decodeFromList(list: List<Any>, deserializer: DeserializationStrategy<T>): T {
     val decoder = ListDecoder(ArrayDeque(list))
     return decoder.decodeSerializableValue(deserializer)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> decodeFromList(list: List<Any>): T = decodeFromList(list, serializer())
 
 @Serializable
@@ -55,6 +61,7 @@ data class Project(val name: String, val owner: User, val votes: Int)
 @Serializable
 data class User(val name: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization",  User("kotlin"), 9000)
     val list = encodeToList(data)
diff --git a/guide/example/example-formats-14.kt b/guide/example/example-formats-14.kt
index 3f88d51ca..782022a2e 100644
--- a/guide/example/example-formats-14.kt
+++ b/guide/example/example-formats-14.kt
@@ -6,6 +6,7 @@ import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
 import kotlinx.serialization.modules.*
 
+@ExperimentalSerializationApi
 class ListEncoder : AbstractEncoder() {
     val list = mutableListOf<Any>()
 
@@ -21,14 +22,17 @@ class ListEncoder : AbstractEncoder() {
     }                                                
 }
 
+@ExperimentalSerializationApi
 fun <T> encodeToList(serializer: SerializationStrategy<T>, value: T): List<Any> {
     val encoder = ListEncoder()
     encoder.encodeSerializableValue(serializer, value)
     return encoder.list
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> encodeToList(value: T) = encodeToList(serializer(), value)
 
+@ExperimentalSerializationApi
 class ListDecoder(val list: ArrayDeque<Any>, var elementsCount: Int = 0) : AbstractDecoder() {
     private var elementIndex = 0
 
@@ -50,11 +54,13 @@ class ListDecoder(val list: ArrayDeque<Any>, var elementsCount: Int = 0) : Abstr
         decodeInt().also { elementsCount = it }
 }
 
+@ExperimentalSerializationApi
 fun <T> decodeFromList(list: List<Any>, deserializer: DeserializationStrategy<T>): T {
     val decoder = ListDecoder(ArrayDeque(list))
     return decoder.decodeSerializableValue(deserializer)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> decodeFromList(list: List<Any>): T = decodeFromList(list, serializer())
 
 @Serializable
@@ -63,6 +69,7 @@ data class Project(val name: String, val owners: List<User>, val votes: Int)
 @Serializable
 data class User(val name: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization",  listOf(User("kotlin"), User("jetbrains")), 9000)
     val list = encodeToList(data)
diff --git a/guide/example/example-formats-15.kt b/guide/example/example-formats-15.kt
index c48c18e4f..354ea1cba 100644
--- a/guide/example/example-formats-15.kt
+++ b/guide/example/example-formats-15.kt
@@ -6,6 +6,7 @@ import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
 import kotlinx.serialization.modules.*
 
+@ExperimentalSerializationApi
 class ListEncoder : AbstractEncoder() {
     val list = mutableListOf<Any>()
 
@@ -24,14 +25,17 @@ class ListEncoder : AbstractEncoder() {
     override fun encodeNotNullMark() = encodeValue("!!")
 }
 
+@ExperimentalSerializationApi
 fun <T> encodeToList(serializer: SerializationStrategy<T>, value: T): List<Any> {
     val encoder = ListEncoder()
     encoder.encodeSerializableValue(serializer, value)
     return encoder.list
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> encodeToList(value: T) = encodeToList(serializer(), value)
 
+@ExperimentalSerializationApi
 class ListDecoder(val list: ArrayDeque<Any>, var elementsCount: Int = 0) : AbstractDecoder() {
     private var elementIndex = 0
     
@@ -55,11 +59,13 @@ class ListDecoder(val list: ArrayDeque<Any>, var elementsCount: Int = 0) : Abstr
     override fun decodeNotNullMark(): Boolean = decodeString() != "NULL"
 }
 
+@ExperimentalSerializationApi
 fun <T> decodeFromList(list: List<Any>, deserializer: DeserializationStrategy<T>): T {
     val decoder = ListDecoder(ArrayDeque(list))
     return decoder.decodeSerializableValue(deserializer)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> decodeFromList(list: List<Any>): T = decodeFromList(list, serializer())
 
 @Serializable
@@ -68,6 +74,7 @@ data class Project(val name: String, val owner: User?, val votes: Int?)
 @Serializable
 data class User(val name: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization",  User("kotlin") , null)
     val list = encodeToList(data)
diff --git a/guide/example/example-formats-16.kt b/guide/example/example-formats-16.kt
index 5bf0e4451..7a93ce271 100644
--- a/guide/example/example-formats-16.kt
+++ b/guide/example/example-formats-16.kt
@@ -8,6 +8,7 @@ import kotlinx.serialization.encoding.*
 import kotlinx.serialization.modules.*
 import java.io.*
 
+@ExperimentalSerializationApi
 class DataOutputEncoder(val output: DataOutput) : AbstractEncoder() {
     override val serializersModule: SerializersModule = EmptySerializersModule()
     override fun encodeBoolean(value: Boolean) = output.writeByte(if (value) 1 else 0)
@@ -30,13 +31,16 @@ class DataOutputEncoder(val output: DataOutput) : AbstractEncoder() {
     override fun encodeNotNullMark() = encodeBoolean(true)
 }
 
+@ExperimentalSerializationApi
 fun <T> encodeTo(output: DataOutput, serializer: SerializationStrategy<T>, value: T) {
     val encoder = DataOutputEncoder(output)
     encoder.encodeSerializableValue(serializer, value)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> encodeTo(output: DataOutput, value: T) = encodeTo(output, serializer(), value)
 
+@ExperimentalSerializationApi
 class DataInputDecoder(val input: DataInput, var elementsCount: Int = 0) : AbstractDecoder() {
     private var elementIndex = 0
     override val serializersModule: SerializersModule = EmptySerializersModule()
@@ -67,11 +71,13 @@ class DataInputDecoder(val input: DataInput, var elementsCount: Int = 0) : Abstr
     override fun decodeNotNullMark(): Boolean = decodeBoolean()
 }
 
+@ExperimentalSerializationApi
 fun <T> decodeFrom(input: DataInput, deserializer: DeserializationStrategy<T>): T {
     val decoder = DataInputDecoder(input)
     return decoder.decodeSerializableValue(deserializer)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> decodeFrom(input: DataInput): T = decodeFrom(input, serializer())
 
 fun ByteArray.toAsciiHexString() = joinToString("") {
@@ -82,6 +88,7 @@ fun ByteArray.toAsciiHexString() = joinToString("") {
 @Serializable
 data class Project(val name: String, val language: String)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization", "Kotlin")
     val output = ByteArrayOutputStream()
diff --git a/guide/example/example-formats-17.kt b/guide/example/example-formats-17.kt
index 9a1c5a21d..ab16bf7e3 100644
--- a/guide/example/example-formats-17.kt
+++ b/guide/example/example-formats-17.kt
@@ -9,6 +9,7 @@ import kotlinx.serialization.encoding.*
 import java.io.*
 
 private val byteArraySerializer = serializer<ByteArray>()
+@ExperimentalSerializationApi
 class DataOutputEncoder(val output: DataOutput) : AbstractEncoder() {
     override val serializersModule: SerializersModule = EmptySerializersModule()
     override fun encodeBoolean(value: Boolean) = output.writeByte(if (value) 1 else 0)
@@ -52,13 +53,16 @@ class DataOutputEncoder(val output: DataOutput) : AbstractEncoder() {
     }            
 }
 
+@ExperimentalSerializationApi
 fun <T> encodeTo(output: DataOutput, serializer: SerializationStrategy<T>, value: T) {
     val encoder = DataOutputEncoder(output)
     encoder.encodeSerializableValue(serializer, value)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> encodeTo(output: DataOutput, value: T) = encodeTo(output, serializer(), value)
 
+@ExperimentalSerializationApi
 class DataInputDecoder(val input: DataInput, var elementsCount: Int = 0) : AbstractDecoder() {
     private var elementIndex = 0
     override val serializersModule: SerializersModule = EmptySerializersModule()
@@ -108,11 +112,13 @@ class DataInputDecoder(val input: DataInput, var elementsCount: Int = 0) : Abstr
     }
 }
 
+@ExperimentalSerializationApi
 fun <T> decodeFrom(input: DataInput, deserializer: DeserializationStrategy<T>): T {
     val decoder = DataInputDecoder(input)
     return decoder.decodeSerializableValue(deserializer)
 }
 
+@ExperimentalSerializationApi
 inline fun <reified T> decodeFrom(input: DataInput): T = decodeFrom(input, serializer())
 
 fun ByteArray.toAsciiHexString() = joinToString("") {
@@ -123,6 +129,7 @@ fun ByteArray.toAsciiHexString() = joinToString("") {
 @Serializable
 data class Project(val name: String, val attachment: ByteArray)
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     val data = Project("kotlinx.serialization", byteArrayOf(0x0A, 0x0B, 0x0C, 0x0D))
     val output = ByteArrayOutputStream()
diff --git a/guide/example/example-json-04.kt b/guide/example/example-json-04.kt
index a8ae148cc..92d036728 100644
--- a/guide/example/example-json-04.kt
+++ b/guide/example/example-json-04.kt
@@ -4,6 +4,7 @@ package example.exampleJson04
 import kotlinx.serialization.*
 import kotlinx.serialization.json.*
 
+@OptIn(ExperimentalSerializationApi::class) // JsonNames is an experimental annotation for now
 @Serializable
 data class Project(@JsonNames("title") val name: String)
 
diff --git a/guide/example/example-json-12.kt b/guide/example/example-json-12.kt
index da76d4c39..f3f11a678 100644
--- a/guide/example/example-json-12.kt
+++ b/guide/example/example-json-12.kt
@@ -4,6 +4,7 @@ package example.exampleJson12
 import kotlinx.serialization.*
 import kotlinx.serialization.json.*
 
+@OptIn(ExperimentalSerializationApi::class) // JsonClassDiscriminator is an experimental annotation for now
 @Serializable
 @JsonClassDiscriminator("message_type")
 sealed class Base
diff --git a/guide/example/example-json-13.kt b/guide/example/example-json-13.kt
index 5da592e74..9794230c6 100644
--- a/guide/example/example-json-13.kt
+++ b/guide/example/example-json-13.kt
@@ -4,6 +4,7 @@ package example.exampleJson13
 import kotlinx.serialization.*
 import kotlinx.serialization.json.*
 
+@OptIn(ExperimentalSerializationApi::class) // classDiscriminatorMode is an experimental setting for now
 val format = Json { classDiscriminatorMode = ClassDiscriminatorMode.NONE }
 
 @Serializable
diff --git a/guide/example/example-json-14.kt b/guide/example/example-json-14.kt
index 67d2e2d32..f0def0e4b 100644
--- a/guide/example/example-json-14.kt
+++ b/guide/example/example-json-14.kt
@@ -4,8 +4,10 @@ package example.exampleJson14
 import kotlinx.serialization.*
 import kotlinx.serialization.json.*
 
+@OptIn(ExperimentalSerializationApi::class) // decodeEnumsCaseInsensitive is an experimental setting for now
 val format = Json { decodeEnumsCaseInsensitive = true }
 
+@OptIn(ExperimentalSerializationApi::class) // JsonNames is an experimental annotation for now
 enum class Cases { VALUE_A, @JsonNames("Alternative") VALUE_B }
 
 @Serializable
diff --git a/guide/example/example-json-15.kt b/guide/example/example-json-15.kt
index db504cbff..267d5cc2e 100644
--- a/guide/example/example-json-15.kt
+++ b/guide/example/example-json-15.kt
@@ -7,6 +7,7 @@ import kotlinx.serialization.json.*
 @Serializable
 data class Project(val projectName: String, val projectOwner: String)
 
+@OptIn(ExperimentalSerializationApi::class) // namingStrategy is an experimental setting for now
 val format = Json { namingStrategy = JsonNamingStrategy.SnakeCase }
 
 fun main() {
diff --git a/guide/example/example-json-22.kt b/guide/example/example-json-22.kt
index 952ca30cf..f334ce5f9 100644
--- a/guide/example/example-json-22.kt
+++ b/guide/example/example-json-22.kt
@@ -12,6 +12,7 @@ fun main() {
     val pi = BigDecimal("3.141592653589793238462643383279")
 
     // use JsonUnquotedLiteral to encode raw JSON content
+    @OptIn(ExperimentalSerializationApi::class)
     val piJsonLiteral = JsonUnquotedLiteral(pi.toString())
 
     val piJsonDouble = JsonPrimitive(pi.toDouble())
diff --git a/guide/example/example-json-24.kt b/guide/example/example-json-24.kt
index c9138d243..3452c6ce1 100644
--- a/guide/example/example-json-24.kt
+++ b/guide/example/example-json-24.kt
@@ -4,6 +4,7 @@ package example.exampleJson24
 import kotlinx.serialization.*
 import kotlinx.serialization.json.*
 
+@OptIn(ExperimentalSerializationApi::class)
 fun main() {
     // caution: creating null with JsonUnquotedLiteral will cause an exception! 
     JsonUnquotedLiteral("null")
diff --git a/guide/example/example-json-29.kt b/guide/example/example-json-29.kt
index e72ca6135..41245ffb1 100644
--- a/guide/example/example-json-29.kt
+++ b/guide/example/example-json-29.kt
@@ -9,6 +9,7 @@ sealed class Project {
     abstract val name: String
 }
 
+@OptIn(ExperimentalSerializationApi::class)
 @KeepGeneratedSerializer
 @Serializable(with = BasicProjectSerializer::class)
 @SerialName("basic")
diff --git a/guide/example/example-json-30.kt b/guide/example/example-json-30.kt
index 2c3ec3cb2..fe379dff6 100644
--- a/guide/example/example-json-30.kt
+++ b/guide/example/example-json-30.kt
@@ -14,7 +14,7 @@ sealed class Response<out T> {
 }
 
 class ResponseSerializer<T>(private val dataSerializer: KSerializer<T>) : KSerializer<Response<T>> {
-    override val descriptor: SerialDescriptor = buildSerialDescriptor("Response", PolymorphicKind.SEALED) {
+    override val descriptor: SerialDescriptor = buildClassSerialDescriptor("Response") {
         element("Ok", dataSerializer.descriptor)
         element("Error", buildClassSerialDescriptor("Error") {
           element<String>("message")
diff --git a/guide/example/example-serializer-10.kt b/guide/example/example-serializer-10.kt
index 2b756524d..a69deceb4 100644
--- a/guide/example/example-serializer-10.kt
+++ b/guide/example/example-serializer-10.kt
@@ -10,6 +10,7 @@ import kotlinx.serialization.builtins.IntArraySerializer
 
 class ColorIntArraySerializer : KSerializer<Color> {
     private val delegateSerializer = IntArraySerializer()
+    @OptIn(ExperimentalSerializationApi::class)
     override val descriptor = SerialDescriptor("Color", delegateSerializer.descriptor)
 
     override fun serialize(encoder: Encoder, value: Color) {
diff --git a/guide/example/example-serializer-13.kt b/guide/example/example-serializer-13.kt
index f2b088828..8de0c8e52 100644
--- a/guide/example/example-serializer-13.kt
+++ b/guide/example/example-serializer-13.kt
@@ -26,7 +26,8 @@ object ColorAsObjectSerializer : KSerializer<Color> {
         decoder.decodeStructure(descriptor) {
             var r = -1
             var g = -1
-            var b = -1     
+            var b = -1
+            @OptIn(ExperimentalSerializationApi::class)
             if (decodeSequentially()) { // sequential decoding protocol
                 r = decodeIntElement(descriptor, 0)           
                 g = decodeIntElement(descriptor, 1)  
diff --git a/guide/example/example-serializer-20.kt b/guide/example/example-serializer-20.kt
index 4d5cc991b..812c05b85 100644
--- a/guide/example/example-serializer-20.kt
+++ b/guide/example/example-serializer-20.kt
@@ -20,6 +20,7 @@ object ColorAsStringSerializer : KSerializer<Color> {
     }
 }
 
+@OptIn(ExperimentalSerializationApi::class)
 @KeepGeneratedSerializer
 @Serializable(with = ColorAsStringSerializer::class)
 class Color(val rgb: Int)
diff --git a/guide/example/example-serializer-23.kt b/guide/example/example-serializer-23.kt
index 9f01476f0..78d537f50 100644
--- a/guide/example/example-serializer-23.kt
+++ b/guide/example/example-serializer-23.kt
@@ -8,7 +8,8 @@ import kotlinx.serialization.descriptors.*
 
 // NOT @Serializable
 class Project(val name: String, val language: String)
-                           
+
+@OptIn(ExperimentalSerializationApi::class)
 @Serializer(forClass = Project::class)
 object ProjectSerializer
 
diff --git a/guide/example/example-serializer-24.kt b/guide/example/example-serializer-24.kt
index 99b0c2d7c..52b5be75e 100644
--- a/guide/example/example-serializer-24.kt
+++ b/guide/example/example-serializer-24.kt
@@ -17,8 +17,9 @@ class Project(
         get() = "kotlin/$name"                                         
 
     private var locked: Boolean = false // private, not accessible -- not serialized 
-}              
+}
 
+@OptIn(ExperimentalSerializationApi::class)
 @Serializer(forClass = Project::class)
 object ProjectSerializer