diff --git a/ktor-http/api/ktor-http.api b/ktor-http/api/ktor-http.api
index 3b3da432dff..3f490eb1bf2 100644
--- a/ktor-http/api/ktor-http.api
+++ b/ktor-http/api/ktor-http.api
@@ -1062,7 +1062,7 @@ public final class io/ktor/http/UrlKt {
 }
 
 public final class io/ktor/http/UrlSerializer : kotlinx/serialization/KSerializer {
-	public fun <init> ()V
+	public static final field INSTANCE Lio/ktor/http/UrlSerializer;
 	public fun deserialize (Lkotlinx/serialization/encoding/Decoder;)Lio/ktor/http/Url;
 	public synthetic fun deserialize (Lkotlinx/serialization/encoding/Decoder;)Ljava/lang/Object;
 	public fun getDescriptor ()Lkotlinx/serialization/descriptors/SerialDescriptor;
diff --git a/ktor-http/api/ktor-http.klib.api b/ktor-http/api/ktor-http.klib.api
index 7ab441b73c7..c4372c4f9a4 100644
--- a/ktor-http/api/ktor-http.klib.api
+++ b/ktor-http/api/ktor-http.klib.api
@@ -1134,16 +1134,6 @@ final class io.ktor.http/Url : io.ktor.utils.io/JvmSerializable { // io.ktor.htt
     }
 }
 
-final class io.ktor.http/UrlSerializer : kotlinx.serialization/KSerializer<io.ktor.http/Url> { // io.ktor.http/UrlSerializer|null[0]
-    constructor <init>() // io.ktor.http/UrlSerializer.<init>|<init>(){}[0]
-
-    final val descriptor // io.ktor.http/UrlSerializer.descriptor|{}descriptor[0]
-        final fun <get-descriptor>(): kotlinx.serialization.descriptors/SerialDescriptor // io.ktor.http/UrlSerializer.descriptor.<get-descriptor>|<get-descriptor>(){}[0]
-
-    final fun deserialize(kotlinx.serialization.encoding/Decoder): io.ktor.http/Url // io.ktor.http/UrlSerializer.deserialize|deserialize(kotlinx.serialization.encoding.Decoder){}[0]
-    final fun serialize(kotlinx.serialization.encoding/Encoder, io.ktor.http/Url) // io.ktor.http/UrlSerializer.serialize|serialize(kotlinx.serialization.encoding.Encoder;io.ktor.http.Url){}[0]
-}
-
 sealed class io.ktor.http.auth/HttpAuthHeader { // io.ktor.http.auth/HttpAuthHeader|null[0]
     final val authScheme // io.ktor.http.auth/HttpAuthHeader.authScheme|{}authScheme[0]
         final fun <get-authScheme>(): kotlin/String // io.ktor.http.auth/HttpAuthHeader.authScheme.<get-authScheme>|<get-authScheme>(){}[0]
@@ -1592,6 +1582,14 @@ final object io.ktor.http/HttpHeaders { // io.ktor.http/HttpHeaders|null[0]
     final fun isUnsafe(kotlin/String): kotlin/Boolean // io.ktor.http/HttpHeaders.isUnsafe|isUnsafe(kotlin.String){}[0]
 }
 
+final object io.ktor.http/UrlSerializer : kotlinx.serialization/KSerializer<io.ktor.http/Url> { // io.ktor.http/UrlSerializer|null[0]
+    final val descriptor // io.ktor.http/UrlSerializer.descriptor|{}descriptor[0]
+        final fun <get-descriptor>(): kotlinx.serialization.descriptors/SerialDescriptor // io.ktor.http/UrlSerializer.descriptor.<get-descriptor>|<get-descriptor>(){}[0]
+
+    final fun deserialize(kotlinx.serialization.encoding/Decoder): io.ktor.http/Url // io.ktor.http/UrlSerializer.deserialize|deserialize(kotlinx.serialization.encoding.Decoder){}[0]
+    final fun serialize(kotlinx.serialization.encoding/Encoder, io.ktor.http/Url) // io.ktor.http/UrlSerializer.serialize|serialize(kotlinx.serialization.encoding.Encoder;io.ktor.http.Url){}[0]
+}
+
 final const val io.ktor.http/DEFAULT_PORT // io.ktor.http/DEFAULT_PORT|{}DEFAULT_PORT[0]
     final fun <get-DEFAULT_PORT>(): kotlin/Int // io.ktor.http/DEFAULT_PORT.<get-DEFAULT_PORT>|<get-DEFAULT_PORT>(){}[0]
 
diff --git a/ktor-http/common/src/io/ktor/http/Cookie.kt b/ktor-http/common/src/io/ktor/http/Cookie.kt
index fcf2b0ee11b..6872c1d466a 100644
--- a/ktor-http/common/src/io/ktor/http/Cookie.kt
+++ b/ktor-http/common/src/io/ktor/http/Cookie.kt
@@ -2,6 +2,8 @@
  * Copyright 2014-2024 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
  */
 
+@file:OptIn(InternalAPI::class)
+
 package io.ktor.http
 
 import io.ktor.util.*
diff --git a/ktor-http/common/src/io/ktor/http/URLProtocol.kt b/ktor-http/common/src/io/ktor/http/URLProtocol.kt
index 513e6287014..b6f9fc9e141 100644
--- a/ktor-http/common/src/io/ktor/http/URLProtocol.kt
+++ b/ktor-http/common/src/io/ktor/http/URLProtocol.kt
@@ -12,6 +12,7 @@ import io.ktor.utils.io.*
  * @property name of protocol (schema)
  * @property defaultPort default port for protocol or `-1` if not known
  */
+@OptIn(InternalAPI::class)
 public data class URLProtocol(val name: String, val defaultPort: Int) : JvmSerializable {
     init {
         require(name.all { it.isLowerCase() }) { "All characters should be lower case" }
diff --git a/ktor-http/common/src/io/ktor/http/Url.kt b/ktor-http/common/src/io/ktor/http/Url.kt
index 60c94e0aeec..8f492789b28 100644
--- a/ktor-http/common/src/io/ktor/http/Url.kt
+++ b/ktor-http/common/src/io/ktor/http/Url.kt
@@ -2,6 +2,8 @@
  * Copyright 2014-2021 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
  */
 
+@file:OptIn(InternalAPI::class)
+
 package io.ktor.http
 
 import io.ktor.utils.io.*
@@ -263,8 +265,9 @@ internal val Url.encodedUserAndPassword: String
         appendUserAndPassword(encodedUser, encodedPassword)
     }
 
-public class UrlSerializer : KSerializer<Url> {
-    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("Url", PrimitiveKind.STRING)
+public object UrlSerializer : KSerializer<Url> {
+    override val descriptor: SerialDescriptor =
+        PrimitiveSerialDescriptor("io.ktor.http.Url", PrimitiveKind.STRING)
 
     override fun deserialize(decoder: Decoder): Url =
         Url(decoder.decodeString())
diff --git a/ktor-http/jvm/test/io/ktor/tests/http/SerializableTest.kt b/ktor-http/jvm/test/io/ktor/tests/http/SerializableTest.kt
index bc2e21fc599..e3df54915e2 100644
--- a/ktor-http/jvm/test/io/ktor/tests/http/SerializableTest.kt
+++ b/ktor-http/jvm/test/io/ktor/tests/http/SerializableTest.kt
@@ -12,13 +12,11 @@ import kotlin.test.*
 class SerializableTest {
     @Test
     fun urlTest() {
-        val url = Url("https://localhost/path?key=value#fragment")
-        assertEquals(url, assertSerializable(url))
+        assertSerializable(Url("https://localhost/path?key=value#fragment"))
     }
 
     @Test
     fun cookieTest() {
-        val cookie = Cookie("key", "value")
-        assertEquals(cookie, assertSerializable(cookie))
+        assertSerializable(Cookie("key", "value"))
     }
 }
diff --git a/ktor-io/common/src/io/ktor/utils/io/JvmSerializable.kt b/ktor-io/common/src/io/ktor/utils/io/JvmSerializable.kt
index 490fdef56c8..ae6f583e692 100644
--- a/ktor-io/common/src/io/ktor/utils/io/JvmSerializable.kt
+++ b/ktor-io/common/src/io/ktor/utils/io/JvmSerializable.kt
@@ -4,13 +4,17 @@
 
 package io.ktor.utils.io
 
+/** Alias for `java.io.Serializable` on JVM. Empty interface otherwise. */
+@InternalAPI
 public expect interface JvmSerializable
 
+@InternalAPI
 public interface JvmSerializer<T> : JvmSerializable {
     public fun jvmSerialize(value: T): ByteArray
     public fun jvmDeserialize(value: ByteArray): T
 }
 
+@InternalAPI
 public expect fun <T : Any> JvmSerializerReplacement(serializer: JvmSerializer<T>, value: T): Any
 
 internal object DummyJvmSimpleSerializerReplacement
diff --git a/ktor-io/jsAndWasmShared/src/io/ktor/utils/io/JvmSerializable.jsAndWasmShared.kt b/ktor-io/jsAndWasmShared/src/io/ktor/utils/io/JvmSerializable.jsAndWasmShared.kt
index 5fcedcdd9b3..44454f494a0 100644
--- a/ktor-io/jsAndWasmShared/src/io/ktor/utils/io/JvmSerializable.jsAndWasmShared.kt
+++ b/ktor-io/jsAndWasmShared/src/io/ktor/utils/io/JvmSerializable.jsAndWasmShared.kt
@@ -4,8 +4,9 @@
 
 package io.ktor.utils.io
 
-/** Alias for `java.io.Serializable` on JVM. Empty interface otherwise. */
+@InternalAPI
 public actual interface JvmSerializable
 
+@InternalAPI
 public actual fun <T : Any> JvmSerializerReplacement(serializer: JvmSerializer<T>, value: T): Any =
     DummyJvmSimpleSerializerReplacement
diff --git a/ktor-io/jvm/src/io/ktor/utils/io/JvmSerializable.jvm.kt b/ktor-io/jvm/src/io/ktor/utils/io/JvmSerializable.jvm.kt
index 875bf5b8edb..608cc65229a 100644
--- a/ktor-io/jvm/src/io/ktor/utils/io/JvmSerializable.jvm.kt
+++ b/ktor-io/jvm/src/io/ktor/utils/io/JvmSerializable.jvm.kt
@@ -6,12 +6,15 @@ package io.ktor.utils.io
 
 import java.io.*
 
+@InternalAPI
 public actual typealias JvmSerializable = Serializable
 
 @Suppress("UNCHECKED_CAST")
+@InternalAPI
 public actual fun <T : Any> JvmSerializerReplacement(serializer: JvmSerializer<T>, value: T): Any =
     DefaultJvmSerializerReplacement(serializer, value)
 
+@OptIn(InternalAPI::class)
 @PublishedApi // IMPORTANT: changing the class name would result in serialization incompatibility
 internal class DefaultJvmSerializerReplacement<T : Any>(
     private var serializer: JvmSerializer<T>?,
diff --git a/ktor-io/posix/src/io/ktor/utils/io/JvmSerializable.posix.kt b/ktor-io/posix/src/io/ktor/utils/io/JvmSerializable.posix.kt
index 5fcedcdd9b3..44454f494a0 100644
--- a/ktor-io/posix/src/io/ktor/utils/io/JvmSerializable.posix.kt
+++ b/ktor-io/posix/src/io/ktor/utils/io/JvmSerializable.posix.kt
@@ -4,8 +4,9 @@
 
 package io.ktor.utils.io
 
-/** Alias for `java.io.Serializable` on JVM. Empty interface otherwise. */
+@InternalAPI
 public actual interface JvmSerializable
 
+@InternalAPI
 public actual fun <T : Any> JvmSerializerReplacement(serializer: JvmSerializer<T>, value: T): Any =
     DummyJvmSimpleSerializerReplacement
diff --git a/ktor-shared/ktor-junit/jvm/src/io/ktor/junit/Assertions.kt b/ktor-shared/ktor-junit/jvm/src/io/ktor/junit/Assertions.kt
index f3857c1e453..9e1545f0fb4 100644
--- a/ktor-shared/ktor-junit/jvm/src/io/ktor/junit/Assertions.kt
+++ b/ktor-shared/ktor-junit/jvm/src/io/ktor/junit/Assertions.kt
@@ -5,6 +5,7 @@
 package io.ktor.junit
 
 import java.io.*
+import kotlin.test.*
 
 /**
  * Convenience function for asserting on all elements of a collection.
@@ -32,9 +33,13 @@ fun <T> assertAll(collection: Iterable<T>, assertion: (T) -> Unit) {
     )
 }
 
-inline fun <reified T : Any> assertSerializable(obj: T): T {
+inline fun <reified T : Any> assertSerializable(obj: T, checkEquality: Boolean = true): T {
     val encoded = ByteArrayOutputStream().also {
         ObjectOutputStream(it).writeObject(obj)
     }.toByteArray()
-    return ObjectInputStream(encoded.inputStream()).readObject() as T
+    val decoded = ObjectInputStream(encoded.inputStream()).readObject() as T
+    if (checkEquality) {
+        assertEquals(obj, decoded, "deserialized object must be equal to original object")
+    }
+    return decoded
 }