diff --git a/build-tools/build.gradle.kts b/build-tools/build.gradle.kts
index d4f690d22cf..f8df91799d4 100644
--- a/build-tools/build.gradle.kts
+++ b/build-tools/build.gradle.kts
@@ -44,7 +44,7 @@ dependencies {
     implementation("org.jetbrains.kotlin:kotlin-stdlib:$kotlinVersion")
     implementation("org.jetbrains.kotlin:kotlin-reflect:$kotlinVersion")
     implementation("com.ullink.slack:simpleslackapi:1.2.0")
-    implementation("org.jetbrains.kotlinx:kotlinx-serialization-runtime:0.14.0")
+    implementation("org.jetbrains.kotlinx:kotlinx-serialization-runtime:0.20.0")
 
     implementation("io.ktor:ktor-client-auth:1.2.1")
     implementation("io.ktor:ktor-client-core:1.2.1")
diff --git a/build-tools/src/main/kotlin/org/jetbrains/kotlin/ExecutorService.kt b/build-tools/src/main/kotlin/org/jetbrains/kotlin/ExecutorService.kt
index 821ec385b91..4ed4863611e 100644
--- a/build-tools/src/main/kotlin/org/jetbrains/kotlin/ExecutorService.kt
+++ b/build-tools/src/main/kotlin/org/jetbrains/kotlin/ExecutorService.kt
@@ -17,8 +17,9 @@
 package org.jetbrains.kotlin
 
 import groovy.lang.Closure
-import kotlinx.serialization.*
-import kotlinx.serialization.json.*
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonConfiguration
 import org.gradle.api.Action
 import org.gradle.api.Project
 import org.gradle.process.ExecResult
@@ -420,14 +421,14 @@ private fun deviceLauncher(project: Project) = object : ExecutorService {
         }
         return out.toString().run {
             check(isNotEmpty())
-            @kotlinx.serialization.Serializable
+            @Serializable
             data class DeviceTarget(val name: String, val udid: String, val state: String, val type: String)
             split("\n")
                     .filter { it.isNotEmpty() }
-                    .map {
-                        Json(JsonConfiguration.Stable.copy(strictMode = false))
+                    .map { Json(JsonConfiguration.Stable.copy(ignoreUnknownKeys = false))
                             .parse(DeviceTarget.serializer(), it)
-                    }.first {
+                    }
+                    .first {
                         it.type == "device" && deviceName?.run { this == it.name } ?: true
                     }
                     .udid
diff --git a/build-tools/src/main/kotlin/org/jetbrains/kotlin/ExternalReportUtils.kt b/build-tools/src/main/kotlin/org/jetbrains/kotlin/ExternalReportUtils.kt
index ee426c8f3e0..6cc32de0299 100644
--- a/build-tools/src/main/kotlin/org/jetbrains/kotlin/ExternalReportUtils.kt
+++ b/build-tools/src/main/kotlin/org/jetbrains/kotlin/ExternalReportUtils.kt
@@ -1,13 +1,16 @@
 package org.jetbrains.kotlin
 
-import kotlinx.io.PrintWriter
-import kotlinx.serialization.*
-import kotlinx.serialization.json.*
+import kotlinx.serialization.ImplicitReflectionSerializer
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.parse
+import kotlinx.serialization.stringify
 import java.io.File
 import java.io.FileReader
+import java.io.PrintWriter
 
 
-@kotlinx.serialization.Serializable
+@Serializable
 data class ExternalTestReport(val statistics: Statistics, val groups: List<KonanTestGroupReport>)
 
 @ImplicitReflectionSerializer
diff --git a/build-tools/src/main/kotlin/org/jetbrains/kotlin/KonanTestSuiteReport.kt b/build-tools/src/main/kotlin/org/jetbrains/kotlin/KonanTestSuiteReport.kt
index da2412caa2c..3534825ba54 100644
--- a/build-tools/src/main/kotlin/org/jetbrains/kotlin/KonanTestSuiteReport.kt
+++ b/build-tools/src/main/kotlin/org/jetbrains/kotlin/KonanTestSuiteReport.kt
@@ -1,8 +1,7 @@
 package org.jetbrains.kotlin
 
-import kotlinx.io.PrintWriter
-import kotlinx.io.StringWriter
-import kotlinx.serialization.Optional
+import java.io.PrintWriter
+import java.io.StringWriter
 import kotlinx.serialization.Serializable
 import org.gradle.api.Project
 
@@ -13,7 +12,7 @@ enum class TestStatus {
     SKIPPED
 }
 
-@kotlinx.serialization.Serializable
+@Serializable
 data class Statistics(
         var passed: Int = 0,
         var failed: Int = 0,
@@ -42,20 +41,20 @@ val Statistics.total: Int
 
 class TestFailedException(msg:String) : RuntimeException(msg)
 
-@kotlinx.serialization.Serializable
-data class KonanTestGroupReport(val name:String, val suites:List<KonanTestSuiteReport>)
+@Serializable
+data class KonanTestGroupReport(val name: String, val suites: List<KonanTestSuiteReport>)
 
-@kotlinx.serialization.Serializable
+@Serializable
 data class KonanTestSuiteReport(val name: String, val tests: List<KonanTestCaseReport>)
 
-@kotlinx.serialization.Serializable
-data class KonanTestCaseReport(val name:String, val status: TestStatus, @Optional val comment:String? = null)
+@Serializable
+data class KonanTestCaseReport(val name: String, val status: TestStatus, val comment: String? = null)
 
-class KonanTestSuiteReportEnvironment(val name:String, val project: Project, val statistics: Statistics) {
+class KonanTestSuiteReportEnvironment(val name: String, val project: Project, val statistics: Statistics) {
     private val tc = if (Tc.enabled) TeamCityTestPrinter(project) else null
     val tests = mutableListOf<KonanTestCaseReport>()
     fun executeTest(testName: String, action:() -> Unit) {
-        var test:KonanTestCaseReport? = null
+        var test: KonanTestCaseReport?
         try {
             tc?.startTest(testName)
             action()
diff --git a/build-tools/src/main/kotlin/org/jetbrains/kotlin/XcRunRuntimeUtils.kt b/build-tools/src/main/kotlin/org/jetbrains/kotlin/XcRunRuntimeUtils.kt
index 235d8ef6628..6eb3dd9067c 100644
--- a/build-tools/src/main/kotlin/org/jetbrains/kotlin/XcRunRuntimeUtils.kt
+++ b/build-tools/src/main/kotlin/org/jetbrains/kotlin/XcRunRuntimeUtils.kt
@@ -2,6 +2,7 @@ package org.jetbrains.kotlin
 
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonConfiguration
 import org.jetbrains.kotlin.konan.target.KonanTarget
 import org.jetbrains.kotlin.konan.target.Xcode
 import kotlin.math.min
@@ -25,7 +26,8 @@ private fun compareStringsAsVersions(version1: String, version2: String): Int {
  * Returns parsed output of `xcrun simctl list runtimes -j`.
  */
 private fun Xcode.getSimulatorRuntimeDescriptors(): List<SimulatorRuntimeDescriptor> =
-     Json.nonstrict.parse(ListRuntimesReport.serializer(), this.simulatorRuntimes).runtimes
+     Json(JsonConfiguration.Stable.copy(ignoreUnknownKeys = true, useArrayPolymorphism = true))
+             .parse(ListRuntimesReport.serializer(), this.simulatorRuntimes).runtimes
 
 /**
  * Returns first available simulator runtime for [target] with at least [osMinVersion] OS version.