diff --git a/libraries/tools/kotlin-gradle-plugin-idea/src/test/kotlin/org/jetbrains/kotlin/gradle/kpm/idea/BackwardsCompatibilityDeserializationTest.kt b/libraries/tools/kotlin-gradle-plugin-idea/src/test/kotlin/org/jetbrains/kotlin/gradle/kpm/idea/BackwardsCompatibilityDeserializationTest.kt
index 390162de7ba..f1d9270cedf 100644
--- a/libraries/tools/kotlin-gradle-plugin-idea/src/test/kotlin/org/jetbrains/kotlin/gradle/kpm/idea/BackwardsCompatibilityDeserializationTest.kt
+++ b/libraries/tools/kotlin-gradle-plugin-idea/src/test/kotlin/org/jetbrains/kotlin/gradle/kpm/idea/BackwardsCompatibilityDeserializationTest.kt
@@ -12,7 +12,12 @@ import org.gradle.api.internal.project.ProjectInternal
 import org.gradle.configurationcache.extensions.serviceOf
 import org.gradle.kotlin.dsl.create
 import org.gradle.testfixtures.ProjectBuilder
+import org.gradle.tooling.internal.adapter.ProtocolToModelAdapter
 import org.gradle.tooling.provider.model.ToolingModelBuilderRegistry
+import org.jetbrains.kotlin.gradle.kpm.KotlinExternalModelKey
+import org.jetbrains.kotlin.gradle.kpm.KotlinExternalModelSerializer.Companion.serializable
+import org.jetbrains.kotlin.gradle.kpm.external.ExternalVariantApi
+import org.jetbrains.kotlin.gradle.kpm.external.external
 import org.jetbrains.kotlin.gradle.plugin.KotlinPm20PluginWrapper
 import org.jetbrains.kotlin.gradle.plugin.mpp.pm20.KotlinIosX64Variant
 import org.jetbrains.kotlin.gradle.plugin.mpp.pm20.KotlinLinuxX64Variant
@@ -21,9 +26,7 @@ import org.jetbrains.kotlin.gradle.plugin.mpp.pm20.jvm
 import org.junit.Test
 import java.io.*
 import java.net.URLClassLoader
-import kotlin.test.assertEquals
-import kotlin.test.assertNotEquals
-import kotlin.test.assertSame
+import kotlin.test.*
 
 /**
  * This test is designed to test if serialized models can still be deserialized with a older (minimal supported)
@@ -55,42 +58,59 @@ class BackwardsCompatibilityDeserializationTest {
         }
         project.evaluate()
 
-        testDeserializeCompatibility(buildModel(project))
-    }
+        val model = buildModel(project)
+        val deserializedModel = deserializeModelWithBackwardsCompatibleClasses(model)
 
-    private fun buildModel(project: Project): IdeaKotlinProjectModel {
-        return project.serviceOf<ToolingModelBuilderRegistry>().getBuilder(IdeaKotlinProjectModel::class.java.name)
-            .buildAll(IdeaKotlinProjectModel::class.java.name, project) as IdeaKotlinProjectModel
-    }
+        /* Use reflection magic (aka ProtocolToModelAdapter) to proxy the deserialized model */
+        run {
+            val adaptedDeserializedModel = ProtocolToModelAdapter().adapt(IdeaKotlinProjectModel::class.java, deserializedModel)
 
-    private fun testDeserializeCompatibility(model: IdeaKotlinProjectModel) {
-        val serializedModel = ByteArrayOutputStream().run {
-            ObjectOutputStream(this).use { stream -> stream.writeObject(model) }
-            toByteArray()
-        }
+            val mainModule = adaptedDeserializedModel.modules.firstOrNull { it.moduleIdentifier.moduleClassifier == null }
+                ?: fail("Missing main module")
 
-        val backwardsCompatibilityTestClassLoader = getClassLoaderForBackwardsCompatibilityTest()
-        val backwardsCompatibilityObjectInputStream = object : ObjectInputStream(ByteArrayInputStream(serializedModel)) {
-            override fun resolveClass(desc: ObjectStreamClass): Class<*> {
-                return backwardsCompatibilityTestClassLoader.loadClass(desc.name)
+            val testModule = adaptedDeserializedModel.modules.firstOrNull { it.moduleIdentifier.moduleClassifier == "test" }
+                ?: fail("Missing test module")
+
+            listOf(mainModule, testModule).forEach { module ->
+                assertEquals(
+                    model.modules.flatMap { it.fragments }.map { it.name }.toSet(),
+                    module.fragments.map { it.name }.toSet(),
+                    "Expected all fragment names to be present"
+                )
             }
         }
-        val backwardsCompatibilityModel = backwardsCompatibilityObjectInputStream.readObject()
-        assertSame(
-            backwardsCompatibilityModel.javaClass.classLoader,
-            backwardsCompatibilityTestClassLoader,
-            "Expected deserialized model being loaded by 'backwardsCompatibilityTestClassLoader'"
-        )
+    }
 
-        assertNotEquals<Class<*>>(
-            model.javaClass, backwardsCompatibilityModel.javaClass,
-            "Expected deserialized model java class to be different from origin"
-        )
+    @OptIn(ExternalVariantApi::class)
+    @Test
+    fun `test - attaching serializable models`() {
+        data class RetainedModel(val id: Int) : Serializable
+        data class UnretainedModel(val id: Int)
 
-        assertEquals(
-            model.javaClass.name, backwardsCompatibilityModel.javaClass.name,
-            "Expected deserialized model to be same java class as origin"
-        )
+        val retainedModelKey = KotlinExternalModelKey<RetainedModel>(serializable())
+        val unretainedModelKey = KotlinExternalModelKey<UnretainedModel>()
+
+        val project = ProjectBuilder.builder().build() as ProjectInternal
+        project.plugins.apply(KotlinPm20PluginWrapper::class.java)
+
+        /* Setup example project */
+        val kotlinExtension = project.extensions.getByType(KotlinPm20ProjectExtension::class.java)
+        kotlinExtension.main.common.external[retainedModelKey] = RetainedModel(2411)
+        kotlinExtension.main.common.external[unretainedModelKey] = UnretainedModel(510)
+
+        val model = buildModel(project)
+        val deserializedModel = deserializeModelWithBackwardsCompatibleClasses(model)
+        val adaptedDeserializedModel = ProtocolToModelAdapter().adapt(IdeaKotlinProjectModel::class.java, deserializedModel)
+
+        val mainModule = adaptedDeserializedModel.modules.find { it.moduleIdentifier.moduleClassifier == null }
+            ?: fail("Missing main module")
+
+        val commonFragment = mainModule.fragments.find { it.name == "common" }
+            ?: fail("Missing common fragment")
+
+        assertEquals(1, commonFragment.external.ids.size)
+        assertEquals(RetainedModel(2411), commonFragment.external[retainedModelKey])
+        assertNull(commonFragment.external[unretainedModelKey])
     }
 }
 
@@ -107,3 +127,39 @@ private fun getClasspathForBackwardsCompatibilityTest(): List<File> {
         .onEach { file -> if (!file.exists()) println("[WARNING] Missing $file") }
 }
 
+private fun buildModel(project: Project): IdeaKotlinProjectModel {
+    return project.serviceOf<ToolingModelBuilderRegistry>().getBuilder(IdeaKotlinProjectModel::class.java.name)
+        .buildAll(IdeaKotlinProjectModel::class.java.name, project) as IdeaKotlinProjectModel
+}
+
+private fun deserializeModelWithBackwardsCompatibleClasses(model: IdeaKotlinProjectModel): Any {
+    val serializedModel = ByteArrayOutputStream().run {
+        ObjectOutputStream(this).use { stream -> stream.writeObject(model) }
+        toByteArray()
+    }
+
+    val backwardsCompatibilityTestClassLoader = getClassLoaderForBackwardsCompatibilityTest()
+    val backwardsCompatibilityObjectInputStream = object : ObjectInputStream(ByteArrayInputStream(serializedModel)) {
+        override fun resolveClass(desc: ObjectStreamClass): Class<*> {
+            return backwardsCompatibilityTestClassLoader.loadClass(desc.name)
+        }
+    }
+    val backwardsCompatibilityModel = backwardsCompatibilityObjectInputStream.use { it.readObject() }
+    assertSame(
+        backwardsCompatibilityModel.javaClass.classLoader,
+        backwardsCompatibilityTestClassLoader,
+        "Expected deserialized model being loaded by 'backwardsCompatibilityTestClassLoader'"
+    )
+
+    assertNotEquals<Class<*>>(
+        model.javaClass, backwardsCompatibilityModel.javaClass,
+        "Expected deserialized model java class to be different from origin"
+    )
+
+    assertEquals(
+        model.javaClass.name, backwardsCompatibilityModel.javaClass.name,
+        "Expected deserialized model to be same java class as origin"
+    )
+
+    return backwardsCompatibilityModel
+}