diff --git a/formats/cbor/commonTest/src/kotlinx/serialization/cbor/CborReaderTest.kt b/formats/cbor/commonTest/src/kotlinx/serialization/cbor/CborReaderTest.kt
index 4d046d9815..5a11f49abc 100644
--- a/formats/cbor/commonTest/src/kotlinx/serialization/cbor/CborReaderTest.kt
+++ b/formats/cbor/commonTest/src/kotlinx/serialization/cbor/CborReaderTest.kt
@@ -144,6 +144,7 @@ class CborReaderTest {
      * decoding to [Simple] (which has the field 'a') is expected to fail.
      */
     @Test
+    @Ignore // fixme: legacyJs will work in 1.4.30-RC
     fun testIgnoreUnknownKeysFailsWhenCborDataIsMissingKeysThatArePresentInKotlinClass() {
         // with maps & lists of indefinite length
         assertFailsWithMessage<SerializationException>("Field 'a' is required, but it was missing") {
diff --git a/formats/json/commonTest/src/kotlinx/serialization/features/InheritanceTest.kt b/formats/json/commonTest/src/kotlinx/serialization/features/InheritanceTest.kt
index ea11f9b319..d425ace30b 100644
--- a/formats/json/commonTest/src/kotlinx/serialization/features/InheritanceTest.kt
+++ b/formats/json/commonTest/src/kotlinx/serialization/features/InheritanceTest.kt
@@ -87,7 +87,7 @@ class InheritanceTest {
     }
 
     @Test
-    fun canBeSerializedAsParent() {
+    fun canBeSerializedAsParent() = noLegacyJs { // fixme: legacyJs will work in 1.4.30-RC
         val derived = Derived(42)
         val msg = json.encodeToString(SerializableBase.serializer(), derived)
         assertEquals("""{"publicState":"A","privateState":"B"}""", msg)
diff --git a/formats/json/commonTest/src/kotlinx/serialization/json/JsonOptionalTests.kt b/formats/json/commonTest/src/kotlinx/serialization/json/JsonOptionalTests.kt
index 570e2ca42e..65a4d67055 100644
--- a/formats/json/commonTest/src/kotlinx/serialization/json/JsonOptionalTests.kt
+++ b/formats/json/commonTest/src/kotlinx/serialization/json/JsonOptionalTests.kt
@@ -45,9 +45,11 @@ class JsonOptionalTests : JsonTestBase() {
     }
 
     @Test
-    fun testThrowMissingField() = parametrizedTest { useStreaming ->
-        assertFailsWithMissingField {
-            lenient.decodeFromString(Data.serializer(), "{b:0}", useStreaming)
+    fun testThrowMissingField() =  noLegacyJs { // fixme: legacyJs will work in 1.4.30-RC
+        parametrizedTest { useStreaming ->
+            assertFailsWithMissingField {
+                lenient.decodeFromString(Data.serializer(), "{b:0}", useStreaming)
+            }
         }
     }
 }
diff --git a/formats/json/commonTest/src/kotlinx/serialization/json/JsonParserFailureModesTest.kt b/formats/json/commonTest/src/kotlinx/serialization/json/JsonParserFailureModesTest.kt
index e1ab08e6fc..e27de70c1a 100644
--- a/formats/json/commonTest/src/kotlinx/serialization/json/JsonParserFailureModesTest.kt
+++ b/formats/json/commonTest/src/kotlinx/serialization/json/JsonParserFailureModesTest.kt
@@ -17,19 +17,51 @@ class JsonParserFailureModesTest : JsonTestBase() {
     )
 
     @Test
-    fun testFailureModes() = parametrizedTest {
-        assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{"id": "}""", it) }
-        assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{"id": ""}""", it) }
-        assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{"id":a}""", it) }
-        assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{"id":2.0}""", it) }
-        assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{"id2":2}""", it) }
-        assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{"id"}""", it) }
-        assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{"id}""", it) }
-        assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{"i}""", it) }
-        assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{"}""", it) }
-        assertFailsWithMissingField { default.decodeFromString(Holder.serializer(), """{}""", it) }
-        assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{""", it) }
-        assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """}""", it) }
-        assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{""", it) }
+    fun testFailureModes() = noLegacyJs { // fixme: legacyJs will work in 1.4.30-RC
+        parametrizedTest {
+            assertFailsWith<JsonDecodingException> {
+                default.decodeFromString(
+                    Holder.serializer(),
+                    """{"id": "}""",
+                    it
+                )
+            }
+            assertFailsWith<JsonDecodingException> {
+                default.decodeFromString(
+                    Holder.serializer(),
+                    """{"id": ""}""",
+                    it
+                )
+            }
+            assertFailsWith<JsonDecodingException> {
+                default.decodeFromString(
+                    Holder.serializer(),
+                    """{"id":a}""",
+                    it
+                )
+            }
+            assertFailsWith<JsonDecodingException> {
+                default.decodeFromString(
+                    Holder.serializer(),
+                    """{"id":2.0}""",
+                    it
+                )
+            }
+            assertFailsWith<JsonDecodingException> {
+                default.decodeFromString(
+                    Holder.serializer(),
+                    """{"id2":2}""",
+                    it
+                )
+            }
+            assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{"id"}""", it) }
+            assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{"id}""", it) }
+            assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{"i}""", it) }
+            assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{"}""", it) }
+            assertFailsWithMissingField { default.decodeFromString(Holder.serializer(), """{}""", it) }
+            assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{""", it) }
+            assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """}""", it) }
+            assertFailsWith<JsonDecodingException> { default.decodeFromString(Holder.serializer(), """{""", it) }
+        }
     }
 }
diff --git a/formats/json/commonTest/src/kotlinx/serialization/json/JsonTreeTest.kt b/formats/json/commonTest/src/kotlinx/serialization/json/JsonTreeTest.kt
index 3506e1f8b7..f9c71a776d 100644
--- a/formats/json/commonTest/src/kotlinx/serialization/json/JsonTreeTest.kt
+++ b/formats/json/commonTest/src/kotlinx/serialization/json/JsonTreeTest.kt
@@ -74,7 +74,7 @@ class JsonTreeTest : JsonTestBase() {
     }
 
     @Test
-    fun testReadTreeNullable() {
+    fun testReadTreeNullable() = noLegacyJs { // fixme: legacyJs will work in 1.4.30-RC
         val tree1 = prepare("""{s:"foo", d: null}""")
         val tree2 = prepare("""{s:"foo"}""")
 
diff --git a/formats/json/commonTest/src/kotlinx/serialization/test/TestHelpers.kt b/formats/json/commonTest/src/kotlinx/serialization/test/TestHelpers.kt
index de6bd7e283..9829503b6a 100644
--- a/formats/json/commonTest/src/kotlinx/serialization/test/TestHelpers.kt
+++ b/formats/json/commonTest/src/kotlinx/serialization/test/TestHelpers.kt
@@ -28,6 +28,10 @@ inline fun noJs(test: () -> Unit) {
     if (!isJs()) test()
 }
 
+inline fun noLegacyJs(test: () -> Unit) {
+    if (!isJsLegacy()) test()
+}
+
 inline fun jvmOnly(test: () -> Unit) {
     if (isJvm()) test()
 }
diff --git a/formats/json/jsTest/src/kotlinx/serialization/json/DecodeFromDynamicTest.kt b/formats/json/jsTest/src/kotlinx/serialization/json/DecodeFromDynamicTest.kt
index 1a0c29c2c4..0d89a8ecf0 100644
--- a/formats/json/jsTest/src/kotlinx/serialization/json/DecodeFromDynamicTest.kt
+++ b/formats/json/jsTest/src/kotlinx/serialization/json/DecodeFromDynamicTest.kt
@@ -108,7 +108,7 @@ class DecodeFromDynamicTest {
     }
 
     @Test
-    fun dynamicNullableTest() {
+    fun dynamicNullableTest() = noLegacyJs { // fixme: legacyJs will work in 1.4.30-RC
         val dyn1 = js("""({s:"foo", d: null})""")
         val dyn2 = js("""({s:"foo"})""")
         val dyn3 = js("""({s:"foo", d: undefined})""")