diff --git a/core/src/commonMain/kotlin/cn/rdlevel/rdkt/core/serialization/FieldFlatteningSerializer.kt b/core/src/commonMain/kotlin/cn/rdlevel/rdkt/core/serialization/FieldFlatteningSerializer.kt
new file mode 100644
index 0000000..6eee9fe
--- /dev/null
+++ b/core/src/commonMain/kotlin/cn/rdlevel/rdkt/core/serialization/FieldFlatteningSerializer.kt
@@ -0,0 +1,91 @@
+package cn.rdlevel.rdkt.core.serialization
+
+import cn.rdlevel.rdkt.core.annotations.RDKTInternalAPI
+import kotlinx.serialization.ExperimentalSerializationApi
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.SerialInfo
+import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.json.JsonTransformingSerializer
+import kotlinx.serialization.json.buildJsonObject
+import kotlinx.serialization.json.jsonObject
+
+/**
+ * Annotation to indicate that a field should be flattened when serialized.
+ */
+@OptIn(ExperimentalSerializationApi::class)
+@SerialInfo
+@RDKTInternalAPI
+@Target(AnnotationTarget.PROPERTY)
+@Retention(AnnotationRetention.RUNTIME)
+@MustBeDocumented
+public annotation class Flatten
+
+/**
+ * A serializer that can handle flattening of fields annotated with [Flatten].
+ *
+ * When serializing, the fields annotated with [Flatten] will be flattened into the parent JSON object.
+ * When deserializing, the fields annotated with [Flatten] will be deserialized from the parent JSON object,
+ * with elements that do not belong to any non-flattened fields.
+ *
+ * It's recommended to annotate classes that need to be flattened with [JsonIgnoreUnknownKeys][kotlinx.serialization.json.JsonIgnoreUnknownKeys]
+ * when there are multiple flattened fields, so that fields that only belong to some classes can be ignored when deserializing other classes.
+ *
+ * This serializer only supports JSON encoding and decoding.
+ *
+ * @param tSerializer The serializer, usually the [generated one][kotlinx.serialization.KeepGeneratedSerializer], for the type T that this serializer will delegate to.
+ */
+@RDKTInternalAPI
+public class FieldFlatteningSerializer<T>(
+    tSerializer: KSerializer<T>
+) : JsonTransformingSerializer<T>(tSerializer) {
+    private val flattenFieldNames: MutableList<String> = mutableListOf()
+    private val notFlattenFieldNames: MutableList<String> = mutableListOf()
+
+    init {
+        (0..<descriptor.elementsCount).forEach {
+            val name = descriptor.getElementName(it)
+            if (descriptor.getElementAnnotations(it).filterIsInstance<Flatten>().isEmpty()) {
+                notFlattenFieldNames += name
+            } else {
+                flattenFieldNames += name
+            }
+        }
+    }
+
+    override fun transformSerialize(element: JsonElement): JsonElement {
+        return buildJsonObject {
+            element.jsonObject.forEach { (k, v) ->
+                if (k !in flattenFieldNames) {
+                    put(k, v)
+                } else {
+                    v.jsonObject.forEach { (k2, v2) ->
+                        put(k2, v2)
+                    }
+                }
+            }
+        }
+    }
+
+    override fun transformDeserialize(element: JsonElement): JsonElement {
+        return buildJsonObject base@{
+            val flattened = buildJsonObject flattened@{
+                element.jsonObject.forEach { (k, v) ->
+                    if (k in notFlattenFieldNames) {
+                        this@base.put(k, v)
+                    } else {
+                        put(k, v)
+                    }
+                }
+            }
+            flattenFieldNames.forEach {
+                put(it, flattened)
+            }
+        }
+    }
+}
+
+/**
+ * Converts a [KSerializer] into a [FieldFlatteningSerializer] that can handle flattening of fields annotated with [Flatten].
+ */
+@RDKTInternalAPI
+public fun <T> KSerializer<T>.flatten(): FieldFlatteningSerializer<T> = FieldFlatteningSerializer(this)
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diff --git a/core/src/commonMain/kotlin/cn/rdlevel/rdkt/core/serialization/LegacyFieldFlatteningSerializer.kt b/core/src/commonMain/kotlin/cn/rdlevel/rdkt/core/serialization/LegacyFieldFlatteningSerializer.kt
new file mode 100644
index 0000000..248481f
--- /dev/null
+++ b/core/src/commonMain/kotlin/cn/rdlevel/rdkt/core/serialization/LegacyFieldFlatteningSerializer.kt
@@ -0,0 +1,101 @@
+package cn.rdlevel.rdkt.core.serialization
+
+import cn.rdlevel.rdkt.core.annotations.RDKTInternalAPI
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
+import kotlinx.serialization.json.*
+import kotlinx.serialization.serializer
+import kotlin.reflect.KMutableProperty1
+
+/**
+ * A [KSerializer] that flattens the fields of the serialized type into the same level as the base type.
+ *
+ * Fields to be flattened should be annotated with [kotlinx.serialization.Transient],
+ * and types to be flattened or serialized should be annotated with [JsonIgnoreUnknownKeys].
+ *
+ * This serializer only supports JSON encoding and decoding.
+ *
+ * This serializer is legacy, but I want to keep it because I think it's cool.
+ *
+ * @property tSerializer The original serializer for the base type.
+ */
+@RDKTInternalAPI
+public class LegacyFieldFlatteningSerializer<Type>(
+    private val tSerializer: KSerializer<Type>,
+) : KSerializer<Type> {
+    public class Entry<T, V>(
+        private val serializer: KSerializer<V>,
+        private val supplier: (T) -> V,
+        private val consumer: (T, V) -> Unit,
+    ) {
+        public fun encodeEntry(builder: JsonObjectBuilder, json: Json, baseValue: T) {
+            json.encodeToJsonElement(serializer, supplier(baseValue)).jsonObject.forEach { (k, v) ->
+                builder.put(k, v)
+            }
+        }
+
+        public fun decodeEntry(json: Json, baseValue: T, element: JsonElement) {
+            consumer(baseValue, json.decodeFromJsonElement(serializer, element))
+        }
+    }
+
+    private val flatteningEntries: MutableList<Entry<Type, *>> = mutableListOf()
+
+    public fun flatten(entry: Entry<Type, *>) {
+        flatteningEntries.add(entry)
+    }
+
+    override val descriptor: SerialDescriptor get() = tSerializer.descriptor
+
+    override fun serialize(encoder: Encoder, value: Type) {
+        val jsonEncoder = encoder as? JsonEncoder
+            ?: error("${LegacyFieldFlatteningSerializer::class.simpleName} only supports JSON encoding.")
+
+        val element = buildJsonObject {
+            val json = jsonEncoder.json
+            json.encodeToJsonElement(tSerializer, value).jsonObject.forEach { (k, v) ->
+                put(k, v)
+            }
+            flatteningEntries.forEach { it.encodeEntry(this, json, value) }
+        }
+        jsonEncoder.encodeJsonElement(element)
+    }
+
+    override fun deserialize(decoder: Decoder): Type {
+        val jsonDecoder = decoder as? JsonDecoder
+            ?: error("${LegacyFieldFlatteningSerializer::class.simpleName} only supports JSON decoding.")
+        val element = jsonDecoder.decodeJsonElement()
+        val json = jsonDecoder.json
+        val value = json.decodeFromJsonElement(tSerializer, element)
+        flatteningEntries.forEach { it.decodeEntry(json, value, element) }
+        return value
+    }
+}
+
+/**
+ * Adds a flattened field to the serializer.
+ */
+@RDKTInternalAPI
+public inline fun <Ser : LegacyFieldFlatteningSerializer<T>, T, reified F> Ser.flatten(
+    property: KMutableProperty1<T, F>,
+    fieldSerializer: KSerializer<F>? = null,
+): Ser = apply {
+    flatten(
+        LegacyFieldFlatteningSerializer.Entry(
+            fieldSerializer ?: serializer<F>(),
+            property::get,
+            property::set,
+        )
+    )
+}
+
+/**
+ * Converts a [KSerializer] to a [LegacyFieldFlatteningSerializer] and adds a flattened field to it.
+ */
+@RDKTInternalAPI
+public inline fun <T, reified F> KSerializer<T>.flatten(
+    property: KMutableProperty1<T, F>,
+    fieldSerializer: KSerializer<F>? = null,
+): LegacyFieldFlatteningSerializer<T> = LegacyFieldFlatteningSerializer(this).flatten(property, fieldSerializer)
\ No newline at end of file
diff --git a/core/src/commonTest/kotlin/cn/rdlevel/rdkt/core/test/FieldFlatteningSerializerTest.kt b/core/src/commonTest/kotlin/cn/rdlevel/rdkt/core/test/FieldFlatteningSerializerTest.kt
new file mode 100644
index 0000000..d4cdaf7
--- /dev/null
+++ b/core/src/commonTest/kotlin/cn/rdlevel/rdkt/core/test/FieldFlatteningSerializerTest.kt
@@ -0,0 +1,42 @@
+@file:OptIn(RDKTInternalAPI::class, ExperimentalSerializationApi::class)
+
+package cn.rdlevel.rdkt.core.test
+
+import cn.rdlevel.rdkt.core.annotations.RDKTInternalAPI
+import cn.rdlevel.rdkt.core.serialization.Flatten
+import cn.rdlevel.rdkt.core.serialization.flatten
+import kotlinx.serialization.ExperimentalSerializationApi
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.KeepGeneratedSerializer
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class FieldFlatteningSerializerTest {
+    @Serializable
+    data class Child(
+        val a: Int,
+        val b: String,
+    )
+
+    @Serializable(Parent.Serializer::class)
+    @KeepGeneratedSerializer
+    data class Parent(
+        val c: Double,
+        val d: Boolean,
+        @Flatten
+        val child: Child,
+    ) {
+        object Serializer : KSerializer<Parent> by generatedSerializer().flatten()
+    }
+
+    @Test
+    fun test() {
+        val data = Parent(1.23, true, Child(42, "hello"))
+        val json = Json.encodeToString(Parent.Serializer, data)
+        assertEquals("""{"c":1.23,"d":true,"a":42,"b":"hello"}""", json)
+        val data2: Parent = Json.decodeFromString(json)
+        assertEquals(data2, data2)
+    }
+}
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