diff --git a/thread/tests/cts/src/android/net/thread/cts/ThreadNetworkExceptionTest.java b/thread/tests/cts/src/android/net/thread/cts/ThreadNetworkExceptionTest.java
index 6770175..7d9ae81 100644
--- a/thread/tests/cts/src/android/net/thread/cts/ThreadNetworkExceptionTest.java
+++ b/thread/tests/cts/src/android/net/thread/cts/ThreadNetworkExceptionTest.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2023 The Android Open Source Project
+ * Copyright (C) 2024 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -32,7 +32,7 @@
 import org.junit.Test;
 import org.junit.runner.RunWith;
 
-/** Tests for {@link ThreadNetworkException}. */
+/** CTS tests for {@link ThreadNetworkException}. */
 @SmallTest
 @RunWith(AndroidJUnit4.class)
 public final class ThreadNetworkExceptionTest {
@@ -57,8 +57,9 @@
     }
 
     @Test
-    public void constructor_invalidErrorCode_throwsIllegalArgumentException() throws Exception {
+    public void constructor_tooSmallErrorCode_throwsIllegalArgumentException() throws Exception {
         assertThrows(IllegalArgumentException.class, () -> new ThreadNetworkException(0, "0"));
-        assertThrows(IllegalArgumentException.class, () -> new ThreadNetworkException(13, "13"));
+        // TODO: add argument check for too large error code when mainline CTS is ready. This was
+        // not added here for CTS forward copatibility.
     }
 }
diff --git a/thread/tests/unit/src/android/net/thread/ThreadNetworkExceptionTest.java b/thread/tests/unit/src/android/net/thread/ThreadNetworkExceptionTest.java
new file mode 100644
index 0000000..f62b437
--- /dev/null
+++ b/thread/tests/unit/src/android/net/thread/ThreadNetworkExceptionTest.java
@@ -0,0 +1,37 @@
+/*
+ * Copyright (C) 2024 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package android.net.thread;
+
+import static org.junit.Assert.assertThrows;
+
+import androidx.test.ext.junit.runners.AndroidJUnit4;
+import androidx.test.filters.SmallTest;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+/** Unit tests for {@link ThreadNetworkException} to cover what is not covered in CTS tests. */
+@SmallTest
+@RunWith(AndroidJUnit4.class)
+public final class ThreadNetworkExceptionTest {
+    @Test
+    public void constructor_tooLargeErrorCode_throwsIllegalArgumentException() throws Exception {
+        // TODO (b/323791003): move this test case to cts/ThreadNetworkExceptionTest when mainline
+        // CTS is ready.
+        assertThrows(IllegalArgumentException.class, () -> new ThreadNetworkException(13, "13"));
+    }
+}