diff --git a/compiler/tests-common-new/tests/org/jetbrains/kotlin/test/builders/RegisteredDirectivesBuilder.kt b/compiler/tests-common-new/tests/org/jetbrains/kotlin/test/builders/RegisteredDirectivesBuilder.kt
index ab9ae680a6d..10951e7f25d 100644
--- a/compiler/tests-common-new/tests/org/jetbrains/kotlin/test/builders/RegisteredDirectivesBuilder.kt
+++ b/compiler/tests-common-new/tests/org/jetbrains/kotlin/test/builders/RegisteredDirectivesBuilder.kt
@@ -65,8 +65,13 @@ class RegisteredDirectivesBuilder private constructor(
     }
 
     private fun <K : Directive, V> MutableMap<K, V>.putWithExistsCheck(key: K, value: V) {
-        val alreadyRegistered = put(key, value)
-        if (alreadyRegistered != null) {
+        val alreadyRegistered = get(key)
+        if (alreadyRegistered == null) {
+            put(key, value)
+        } else if (alreadyRegistered is List<Any?> && value is List<Any?>) {
+            @Suppress("UNCHECKED_CAST")
+            put(key, (alreadyRegistered + value) as V)
+        } else {
             error("Default values for $key directive already registered")
         }
     }
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/DuplicateSerialName.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/DuplicateSerialName.fir.kt
index 808573495ce..5d9186265cf 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/DuplicateSerialName.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/DuplicateSerialName.fir.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // FILE: test.kt
 import kotlinx.serialization.*
@@ -7,4 +6,4 @@ import kotlinx.serialization.*
 open class Parent(open val arg: Int)
 
 @Serializable
-class Derived(override val arg: Int): Parent(arg)
\ No newline at end of file
+class Derived(override val arg: Int): Parent(arg)
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/DuplicateSerialName.kt b/plugins/kotlinx-serialization/testData/diagnostics/DuplicateSerialName.kt
index fba32c83982..88498ea9db9 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/DuplicateSerialName.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/DuplicateSerialName.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // FILE: test.kt
 import kotlinx.serialization.*
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/EnumDuplicateSerialName.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/EnumDuplicateSerialName.fir.kt
index 6bd4954094e..5601a709a4b 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/EnumDuplicateSerialName.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/EnumDuplicateSerialName.fir.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // SKIP_TXT
 import kotlinx.serialization.*
@@ -26,4 +25,4 @@ enum class ReversedExplicitlyDuplicated {
     FIRST,
     SECOND,
     THIRD
-}
\ No newline at end of file
+}
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/EnumDuplicateSerialName.kt b/plugins/kotlinx-serialization/testData/diagnostics/EnumDuplicateSerialName.kt
index 26654963040..89ab930ebe3 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/EnumDuplicateSerialName.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/EnumDuplicateSerialName.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // SKIP_TXT
 import kotlinx.serialization.*
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/ExternalSerializers.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/ExternalSerializers.fir.kt
index ed54229944f..97831db6625 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/ExternalSerializers.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/ExternalSerializers.fir.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE, -OPT_IN_USAGE
 // SKIP_TXT
 
 // FILE: test.kt
@@ -7,4 +6,4 @@ import kotlinx.serialization.*
 class Foo(i: Int, val j: Int)
 
 @Serializer(forClass = Foo::class)
-object ExternalSerializer
\ No newline at end of file
+object ExternalSerializer
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/ExternalSerializers.kt b/plugins/kotlinx-serialization/testData/diagnostics/ExternalSerializers.kt
index d3b4f709b41..81702f971a8 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/ExternalSerializers.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/ExternalSerializers.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE, -OPT_IN_USAGE
 // SKIP_TXT
 
 // FILE: test.kt
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient.fir.kt
index ee0e7ee3b94..ce7bac3efe0 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient.fir.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // SKIP_TXT
 
@@ -9,4 +8,4 @@ import java.io.Serializable as JavaSerializable
 class Data(val x: Int, @Transient val y: String)
 
 @Serializable
-class Data2(val x: Int, @Transient val y: String) : JavaSerializable
\ No newline at end of file
+class Data2(val x: Int, @Transient val y: String) : JavaSerializable
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient.kt b/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient.kt
index acee89cdcf3..e350948f1b7 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // SKIP_TXT
 
@@ -9,4 +8,4 @@ import java.io.Serializable as JavaSerializable
 class Data(val x: Int, <!INCORRECT_TRANSIENT!>@Transient<!> val y: String)
 
 @Serializable
-class Data2(val x: Int, @Transient val y: String) : JavaSerializable
\ No newline at end of file
+class Data2(val x: Int, @Transient val y: String) : JavaSerializable
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient2.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient2.fir.kt
index c5e287d89de..5b4ce4a6d4e 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient2.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient2.fir.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // SKIP_TXT
 
@@ -17,4 +16,4 @@ class Data2(val x: Int, @Transient val y: String = "a") : JavaSerializable
 class Data3(val x: Int, @Transient @JavaTransient val y: String = "a") : JavaSerializable
 
 @Serializable
-class Data4(val x: Int, @JavaTransient val y: String)
\ No newline at end of file
+class Data4(val x: Int, @JavaTransient val y: String)
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient2.kt b/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient2.kt
index 821b2795244..b870d8f86f4 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient2.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/IncorrectTransient2.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // SKIP_TXT
 
@@ -17,4 +16,4 @@ class Data2(val x: Int, @Transient val y: String = "a") : JavaSerializable
 class Data3(val x: Int, @Transient @JavaTransient val y: String = "a") : JavaSerializable
 
 @Serializable
-class Data4(val x: Int, <!INCORRECT_TRANSIENT!>@JavaTransient<!> val y: String)
\ No newline at end of file
+class Data4(val x: Int, <!INCORRECT_TRANSIENT!>@JavaTransient<!> val y: String)
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/InheritableInfo.kt b/plugins/kotlinx-serialization/testData/diagnostics/InheritableInfo.kt
index cdf122eb157..b7e705bfc95 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/InheritableInfo.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/InheritableInfo.kt
@@ -1,5 +1,4 @@
 // FIR_IDENTICAL
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // SKIP_TXT
 // !USE_EXPERIMENTAL: kotlinx.serialization.ExperimentalSerializationApi
@@ -47,4 +46,4 @@ open class B2: A()
 @Serializable
 //@I("b")
 //@I2(E.A, E::class)
-open class C: B()
\ No newline at end of file
+open class C: B()
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/LazyRecursionBug.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/LazyRecursionBug.fir.kt
index e0cd23e5dce..afce707a688 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/LazyRecursionBug.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/LazyRecursionBug.fir.kt
@@ -1,10 +1,9 @@
 // This test enshures that analysis ends up without compiler exceptions
-// !DIAGNOSTICS: -OPT_IN_USAGE
 
 import kotlinx.serialization.*
 
 @Serializable
 class Digest() {
-    @<!OPT_IN_USAGE_ERROR!>Serializer<!>(forClass = Digest::class)
+    @Serializer(forClass = Digest::class)
     companion <!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>object<!> : KSerializer<Digest> {}
 }
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/LazyRecursionBug.kt b/plugins/kotlinx-serialization/testData/diagnostics/LazyRecursionBug.kt
index ed8ebc080b7..d2455d7f342 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/LazyRecursionBug.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/LazyRecursionBug.kt
@@ -1,5 +1,4 @@
 // This test enshures that analysis ends up without compiler exceptions
-// !DIAGNOSTICS: -OPT_IN_USAGE
 
 import kotlinx.serialization.*
 
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/LocalAndAnonymous.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/LocalAndAnonymous.fir.kt
index 071b465334a..6b52bd6bb95 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/LocalAndAnonymous.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/LocalAndAnonymous.fir.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -OPT_IN_USAGE_ERROR, -OPT_IN_USAGE
 // WITH_STDLIB
 // SKIP_TXT
 
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/LocalAndAnonymous.kt b/plugins/kotlinx-serialization/testData/diagnostics/LocalAndAnonymous.kt
index 6774edbfc8f..207ccf55629 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/LocalAndAnonymous.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/LocalAndAnonymous.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -OPT_IN_USAGE_ERROR, -OPT_IN_USAGE
 // WITH_STDLIB
 // SKIP_TXT
 
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/NoSuitableCtorInParent.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/NoSuitableCtorInParent.fir.kt
index 194cf5167d9..c15f8293837 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/NoSuitableCtorInParent.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/NoSuitableCtorInParent.fir.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // FILE: test.kt
 import kotlinx.serialization.*
@@ -6,4 +5,4 @@ import kotlinx.serialization.*
 open class NonSerializableParent(val arg: Int)
 
 @Serializable
-class Derived(val someData: String): NonSerializableParent(42)
\ No newline at end of file
+class Derived(val someData: String): NonSerializableParent(42)
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/NoSuitableCtorInParent.kt b/plugins/kotlinx-serialization/testData/diagnostics/NoSuitableCtorInParent.kt
index 5b511798ce5..0b7e907d781 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/NoSuitableCtorInParent.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/NoSuitableCtorInParent.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // FILE: test.kt
 import kotlinx.serialization.*
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/NonSerializable.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/NonSerializable.fir.kt
index ac5fda18ed8..27b49634951 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/NonSerializable.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/NonSerializable.fir.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // FILE: test.kt
 import kotlinx.serialization.*
@@ -14,4 +13,4 @@ class Inside(val foo: List<NonSerializable>)
 @Serializable
 class WithImplicitType {
     val foo = NonSerializable()
-}
\ No newline at end of file
+}
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/NonSerializable.kt b/plugins/kotlinx-serialization/testData/diagnostics/NonSerializable.kt
index 255bee34899..8daa9cc5102 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/NonSerializable.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/NonSerializable.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // FILE: test.kt
 import kotlinx.serialization.*
@@ -14,4 +13,4 @@ class Inside(val foo: List<<!SERIALIZER_NOT_FOUND("NonSerializable")!>NonSeriali
 @Serializable
 class WithImplicitType {
     <!SERIALIZER_NOT_FOUND("NonSerializable")!>val foo = NonSerializable()<!>
-}
\ No newline at end of file
+}
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/ParamIsNotProperty.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/ParamIsNotProperty.fir.kt
index 1bca02e11a3..1136d34460e 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/ParamIsNotProperty.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/ParamIsNotProperty.fir.kt
@@ -1,7 +1,6 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // FILE: test.kt
 import kotlinx.serialization.*
 
 @Serializable
-class Test(val someData: String, cantBeDeserialized: Int)
\ No newline at end of file
+class Test(val someData: String, cantBeDeserialized: Int)
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/ParamIsNotProperty.kt b/plugins/kotlinx-serialization/testData/diagnostics/ParamIsNotProperty.kt
index 549d73d53aa..05d011fdc19 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/ParamIsNotProperty.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/ParamIsNotProperty.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // FILE: test.kt
 import kotlinx.serialization.*
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/SerializableEnums.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/SerializableEnums.fir.kt
index b6e92f8c6e7..b3303fd6bb6 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/SerializableEnums.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/SerializableEnums.fir.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // FILE: test.kt
 import kotlinx.serialization.*
@@ -21,4 +20,4 @@ object EnumSerializer: KSerializer<ExplicitlyMarkedEnumCustom> {
 }
 
 @Serializable
-data class EnumUsage(val s: SimpleEnum, val m: MarkedNameEnum, val e: ExplicitlyMarkedEnum)
\ No newline at end of file
+data class EnumUsage(val s: SimpleEnum, val m: MarkedNameEnum, val e: ExplicitlyMarkedEnum)
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/SerializableEnums.kt b/plugins/kotlinx-serialization/testData/diagnostics/SerializableEnums.kt
index b58904aa427..d09709832ce 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/SerializableEnums.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/SerializableEnums.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // FILE: test.kt
 import kotlinx.serialization.*
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/SerializableIgnored.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/SerializableIgnored.fir.kt
index a1635f8dc77..5a9faf9a06f 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/SerializableIgnored.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/SerializableIgnored.fir.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // SKIP_TXT
 
 // FILE: test.kt
@@ -8,4 +7,4 @@ import kotlinx.serialization.*
 interface INonSerializable
 
 @Serializable
-sealed interface SealedSerializable
\ No newline at end of file
+sealed interface SealedSerializable
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/SerializableIgnored.kt b/plugins/kotlinx-serialization/testData/diagnostics/SerializableIgnored.kt
index cb512efc24d..bfcb9ea8bb6 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/SerializableIgnored.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/SerializableIgnored.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // SKIP_TXT
 
 // FILE: test.kt
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeCompatibleForSpecials.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeCompatibleForSpecials.fir.kt
index a835d3413bd..f88873b8dba 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeCompatibleForSpecials.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeCompatibleForSpecials.fir.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -OPT_IN_USAGE
 // WITH_STDLIB
 // SKIP_TXT
 
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeCompatibleForSpecials.kt b/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeCompatibleForSpecials.kt
index e0e4e97ab0a..037cdaa70f0 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeCompatibleForSpecials.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeCompatibleForSpecials.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -OPT_IN_USAGE
 // WITH_STDLIB
 // SKIP_TXT
 
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeIncompatible.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeIncompatible.fir.kt
index 131648e958f..21137764862 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeIncompatible.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeIncompatible.fir.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -OPT_IN_USAGE
 // WITH_STDLIB
 // SKIP_TXT
 
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeIncompatible.kt b/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeIncompatible.kt
index 6b09e3e48c3..e150e583c8b 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeIncompatible.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/SerializerTypeIncompatible.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -OPT_IN_USAGE
 // WITH_STDLIB
 // SKIP_TXT
 
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/Transients.fir.kt b/plugins/kotlinx-serialization/testData/diagnostics/Transients.fir.kt
index d26139a5dd6..bcf1b38b130 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/Transients.fir.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/Transients.fir.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // FILE: test.kt
 import kotlinx.serialization.*
@@ -9,4 +8,4 @@ data class WithTransients(@Transient val missing: Int) {
 
     @Transient
     lateinit var allowTransientLateinitWithoutInitializer: String
-}
\ No newline at end of file
+}
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/Transients.kt b/plugins/kotlinx-serialization/testData/diagnostics/Transients.kt
index ab738456ec9..6336b5b95b4 100644
--- a/plugins/kotlinx-serialization/testData/diagnostics/Transients.kt
+++ b/plugins/kotlinx-serialization/testData/diagnostics/Transients.kt
@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // FILE: test.kt
 import kotlinx.serialization.*
diff --git a/plugins/kotlinx-serialization/tests/org/jetbrains/kotlinx/serialization/runners/DiagnosticTests.kt b/plugins/kotlinx-serialization/tests/org/jetbrains/kotlinx/serialization/runners/DiagnosticTests.kt
index c91fa2dac8c..24865205d45 100644
--- a/plugins/kotlinx-serialization/tests/org/jetbrains/kotlinx/serialization/runners/DiagnosticTests.kt
+++ b/plugins/kotlinx-serialization/tests/org/jetbrains/kotlinx/serialization/runners/DiagnosticTests.kt
@@ -6,6 +6,7 @@
 package org.jetbrains.kotlinx.serialization.runners
 
 import org.jetbrains.kotlin.test.builders.TestConfigurationBuilder
+import org.jetbrains.kotlin.test.directives.DiagnosticsDirectives.DIAGNOSTICS
 import org.jetbrains.kotlin.test.directives.FirDiagnosticsDirectives.FIR_DUMP
 import org.jetbrains.kotlin.test.runners.AbstractDiagnosticTest
 import org.jetbrains.kotlin.test.runners.AbstractFirDiagnosticTest
@@ -15,7 +16,10 @@ import org.jetbrains.kotlinx.serialization.configureForKotlinxSerialization
 abstract class AbstractSerializationPluginDiagnosticTest : AbstractDiagnosticTest() {
     override fun configure(builder: TestConfigurationBuilder) {
         super.configure(builder)
-        builder.configureForKotlinxSerialization()
+        with(builder) {
+            configureForKotlinxSerialization()
+            disableOptInErrors()
+        }
     }
 }
 
@@ -24,6 +28,7 @@ abstract class AbstractSerializationFirDiagnosticTest : AbstractFirDiagnosticTes
         super.configure(builder)
         with(builder) {
             configureForKotlinxSerialization()
+            disableOptInErrors()
 
             forTestsMatching("*/diagnostics/*") {
                 configurationForClassicAndFirTestsAlongside()
@@ -37,3 +42,9 @@ abstract class AbstractSerializationFirDiagnosticTest : AbstractFirDiagnosticTes
         }
     }
 }
+
+private fun TestConfigurationBuilder.disableOptInErrors() {
+    defaultDirectives {
+        DIAGNOSTICS with listOf("-OPT_IN_USAGE", "-OPT_IN_USAGE_ERROR")
+    }
+}