diff --git a/compiler-plugin/src/main/kotlin/com/google/devtools/ksp/Incremental.kt b/compiler-plugin/src/main/kotlin/com/google/devtools/ksp/Incremental.kt
index fb20321428..18bbc51b0b 100644
--- a/compiler-plugin/src/main/kotlin/com/google/devtools/ksp/Incremental.kt
+++ b/compiler-plugin/src/main/kotlin/com/google/devtools/ksp/Incremental.kt
@@ -36,7 +36,9 @@ import org.jetbrains.kotlin.incremental.*
 import org.jetbrains.kotlin.incremental.components.LookupTracker
 import org.jetbrains.kotlin.incremental.components.Position
 import org.jetbrains.kotlin.incremental.components.ScopeKind
-import org.jetbrains.kotlin.incremental.storage.AppendableAbstractBasicMap
+import org.jetbrains.kotlin.incremental.storage.AppendableBasicMap
+import org.jetbrains.kotlin.incremental.storage.AppendableDataExternalizer
+import org.jetbrains.kotlin.incremental.storage.CollectionExternalizer
 import org.jetbrains.kotlin.incremental.storage.FileToPathConverter
 import org.jetbrains.kotlin.resolve.descriptorUtil.getAllSuperclassesWithoutAny
 import org.jetbrains.kotlin.types.KotlinType
@@ -49,19 +51,44 @@ import java.util.*
 abstract class PersistentMap<K : Comparable<K>, V>(
     storageFile: File,
     keyDescriptor: KeyDescriptor<K>,
-    valueExternalizer: DataExternalizer<V>,
+    valueExternalizer: AppendableDataExternalizer<V>,
     icContext: IncrementalCompilationContext,
-) : AppendableAbstractBasicMap<K, V, Collection<V>>(storageFile, keyDescriptor, valueExternalizer, icContext)
+) : AppendableBasicMap<K, V>(storageFile, keyDescriptor, valueExternalizer, icContext) {
+    abstract operator fun get(key: K): V?
+    abstract operator fun set(key: K, value: V)
+    abstract fun remove(key: K)
+}
 
 class FileToSymbolsMap(
     storageFile: File,
     icContext: IncrementalCompilationContext
-) : PersistentMap<File, LookupSymbol>(
+) : PersistentMap<File, Collection<LookupSymbol>>(
     storageFile,
     FileKeyDescriptor,
-    LookupSymbolExternalizer,
+    CollectionExternalizer(LookupSymbolExternalizer, { HashSet() }),
     icContext,
-)
+) {
+    override fun dumpKey(key: File): String = key.toString()
+
+    override fun dumpValue(value: Collection<LookupSymbol>): String = value.toString()
+
+    fun add(file: File, symbol: LookupSymbol) {
+        storage.append(file, listOf(symbol))
+    }
+
+    override operator fun get(key: File): Collection<LookupSymbol>? = storage[key]
+
+    override operator fun set(key: File, symbols: Collection<LookupSymbol>) {
+        storage[key] = symbols
+    }
+
+    override fun remove(key: File) {
+        storage.remove(key)
+    }
+
+    val keys: Collection<File>
+        get() = storage.keys
+}
 
 object FileKeyDescriptor : KeyDescriptor<File> {
     override fun read(input: DataInput): File {
@@ -97,16 +124,30 @@ object FileExternalizer : DataExternalizer<File> {
 class FileToFilesMap(
     storageFile: File,
     icContext: IncrementalCompilationContext
-) : PersistentMap<File, File>(
+) : PersistentMap<File, Collection<File>>(
     storageFile,
     FileKeyDescriptor,
-    FileExternalizer,
+    CollectionExternalizer(FileExternalizer, { HashSet() }),
     icContext,
 ) {
+
+    override operator fun get(key: File): Collection<File>? = storage[key]
+
+    override operator fun set(key: File, value: Collection<File>) {
+        storage[key] = value
+    }
+
     override fun dumpKey(key: File): String = key.path
 
     override fun dumpValue(value: Collection<File>) =
         value.dumpCollection()
+
+    override fun remove(key: File) {
+        storage.remove(key)
+    }
+
+    val keys: Collection<File>
+        get() = storage.keys
 }
 
 object symbolCollector : KSDefaultVisitor<(LookupSymbol) -> Unit, Unit>() {
@@ -453,14 +494,14 @@ class IncrementalContext(
         updateLookupCache(dirtyFiles)
 
         // Update symbolsMap
-        fun <K : Comparable<K>, V> update(m: PersistentMap<K, V>, u: MultiMap<K, V>) {
+        fun <K : Comparable<K>, V> update(m: PersistentMap<K, Collection<V>>, u: MultiMap<K, V>) {
             // Update symbol caches from modified files.
             u.keySet().forEach {
                 m.set(it, u[it].toSet())
             }
         }
 
-        fun <K : Comparable<K>, V> remove(m: PersistentMap<K, V>, removedKeys: Collection<K>) {
+        fun <K : Comparable<K>, V> remove(m: PersistentMap<K, Collection<V>>, removedKeys: Collection<K>) {
             // Remove symbol caches from removed files.
             removedKeys.forEach {
                 m.remove(it)
diff --git a/compiler-plugin/src/main/kotlin/com/google/devtools/ksp/symbol/impl/binary/KSDeclarationDescriptorImpl.kt b/compiler-plugin/src/main/kotlin/com/google/devtools/ksp/symbol/impl/binary/KSDeclarationDescriptorImpl.kt
index 1dfaa219fb..db041b2668 100644
--- a/compiler-plugin/src/main/kotlin/com/google/devtools/ksp/symbol/impl/binary/KSDeclarationDescriptorImpl.kt
+++ b/compiler-plugin/src/main/kotlin/com/google/devtools/ksp/symbol/impl/binary/KSDeclarationDescriptorImpl.kt
@@ -20,10 +20,10 @@ package com.google.devtools.ksp.symbol.impl.binary
 import com.google.devtools.ksp.memoized
 import com.google.devtools.ksp.processing.impl.KSNameImpl
 import com.google.devtools.ksp.symbol.*
+import org.jetbrains.kotlin.backend.common.serialization.findPackage
 import org.jetbrains.kotlin.descriptors.ClassDescriptor
 import org.jetbrains.kotlin.descriptors.DeclarationDescriptor
 import org.jetbrains.kotlin.descriptors.FunctionDescriptor
-import org.jetbrains.kotlin.descriptors.findPackage
 import org.jetbrains.kotlin.load.java.descriptors.JavaClassDescriptor
 import org.jetbrains.kotlin.resolve.descriptorUtil.fqNameSafe
 import org.jetbrains.kotlin.resolve.descriptorUtil.parents
@@ -50,7 +50,7 @@ abstract class KSDeclarationDescriptorImpl(private val descriptor: DeclarationDe
             is ClassDescriptor -> KSClassDeclarationDescriptorImpl.getCached(containingDescriptor)
             is FunctionDescriptor -> KSFunctionDeclarationDescriptorImpl.getCached(containingDescriptor)
             else -> null
-        }
+        } as KSDeclaration?
     }
 
     override val parent: KSNode? by lazy {
diff --git a/gradle.properties b/gradle.properties
index 702717f109..c79458646d 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -1,7 +1,7 @@
 # Copied from kotlinc
 org.gradle.jvmargs=-Duser.country=US -Dkotlin.daemon.jvm.options=-Xmx2200m -Dfile.encoding=UTF-8
 
-kotlinBaseVersion=2.0.0-dev-4562
+kotlinBaseVersion=1.9.30-dev-2548
 agpBaseVersion=7.0.0
 intellijVersion=213.7172.25
 junitVersion=4.13.1
diff --git a/integration-tests/src/test/kotlin/com/google/devtools/ksp/test/KSPCmdLineOptionsIT.kt b/integration-tests/src/test/kotlin/com/google/devtools/ksp/test/KSPCmdLineOptionsIT.kt
index bb6dcc6804..e06136a849 100644
--- a/integration-tests/src/test/kotlin/com/google/devtools/ksp/test/KSPCmdLineOptionsIT.kt
+++ b/integration-tests/src/test/kotlin/com/google/devtools/ksp/test/KSPCmdLineOptionsIT.kt
@@ -38,7 +38,6 @@ class KSPCmdLineOptionsIT(val useKSP2: Boolean) {
         }.maxByOrNull { it.lastModified() }!!
         val compilerArgs = mutableListOf(
             "-no-stdlib",
-            "-language-version", "1.9",
             "-Xplugin=${kspPluginJar.absolutePath}",
             "-Xplugin=${kspApiJar.absolutePath}",
             "-P", "plugin:$kspPluginId:apclasspath=${processorJar.absolutePath}",
diff --git a/integration-tests/src/test/kotlin/com/google/devtools/ksp/test/PlaygroundIT.kt b/integration-tests/src/test/kotlin/com/google/devtools/ksp/test/PlaygroundIT.kt
index f54f0e6150..853ff5393d 100644
--- a/integration-tests/src/test/kotlin/com/google/devtools/ksp/test/PlaygroundIT.kt
+++ b/integration-tests/src/test/kotlin/com/google/devtools/ksp/test/PlaygroundIT.kt
@@ -223,6 +223,7 @@ class PlaygroundIT(val useKSP2: Boolean) {
             Assert.assertTrue(jarFile.getEntry("hello/HELLO.class").size > 0)
             Assert.assertTrue(jarFile.getEntry("com/example/AClassBuilder.class").size > 0)
         }
+        Assert.assertTrue(result.output.contains("w: Language version 2.0 is experimental"))
         project.restore(buildFile.path)
     }
 
@@ -262,6 +263,7 @@ class PlaygroundIT(val useKSP2: Boolean) {
             Assert.assertTrue(jarFile.getEntry("hello/HELLO.class").size > 0)
             Assert.assertTrue(jarFile.getEntry("com/example/AClassBuilder.class").size > 0)
         }
+        Assert.assertTrue(result.output.contains("w: Language version 2.0 is experimental"))
         project.restore(buildFile.path)
         project.restore(gradleProperties.path)
     }
@@ -281,10 +283,7 @@ class PlaygroundIT(val useKSP2: Boolean) {
         gradleRunner.buildAndCheck("clean", "build") { result ->
             Assert.assertTrue(result.output.contains("language version: 1.5"))
             Assert.assertTrue(result.output.contains("api version: 1.5"))
-            if (!useKSP2) {
-                // In case KSP 1 and KSP 2 uses different compiler versions, ignore this test for KSP 2 for now.
-                Assert.assertTrue(result.output.contains("compiler version: $kotlinVersion"))
-            }
+            Assert.assertTrue(result.output.contains("compiler version: $kotlinVersion"))
         }
         project.restore(buildFile.path)
     }
@@ -300,12 +299,12 @@ class PlaygroundIT(val useKSP2: Boolean) {
         gradleRunner.withArguments("build").buildAndFail().let {
             Assert.assertEquals(TaskOutcome.SUCCESS, it.task(":workload:kspKotlin")?.outcome)
             Assert.assertEquals(TaskOutcome.FAILED, it.task(":workload:compileKotlin")?.outcome)
-            Assert.assertTrue("Unresolved reference 'AClassBuilder'" in it.output)
+            Assert.assertTrue("Unresolved reference: AClassBuilder" in it.output)
         }
         gradleRunner.withArguments("build").buildAndFail().let {
             Assert.assertEquals(TaskOutcome.UP_TO_DATE, it.task(":workload:kspKotlin")?.outcome)
             Assert.assertEquals(TaskOutcome.FAILED, it.task(":workload:compileKotlin")?.outcome)
-            Assert.assertTrue("Unresolved reference 'AClassBuilder'" in it.output)
+            Assert.assertTrue("Unresolved reference: AClassBuilder" in it.output)
         }
 
         project.restore("workload/build.gradle.kts")