diff --git a/tests/aidl/Android.bp b/tests/aidl/Android.bp
index d59ca7e..ed4e8ff 100644
--- a/tests/aidl/Android.bp
+++ b/tests/aidl/Android.bp
@@ -6,6 +6,10 @@
     name: "com.android.microdroid.testservice",
     srcs: ["com/android/microdroid/testservice/**/*.aidl"],
     unstable: true,
+    flags: [
+        "-Werror",
+        "-Wno-mixed-oneway",
+    ],
     backend: {
         java: {
             gen_rpc: true,
diff --git a/tests/aidl/com/android/microdroid/testservice/IAppCallback.aidl b/tests/aidl/com/android/microdroid/testservice/IAppCallback.aidl
new file mode 100644
index 0000000..9859090
--- /dev/null
+++ b/tests/aidl/com/android/microdroid/testservice/IAppCallback.aidl
@@ -0,0 +1,31 @@
+/*
+ * Copyright 2023 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 com.android.microdroid.testservice;
+
+import com.android.microdroid.testservice.IVmCallback;
+
+/**
+ * An interface exposed by the app for callbacks from the VM.
+ *
+ * {@hide}
+ */
+interface IAppCallback {
+    /** Invites the app to call vmCallback#echoMessage() */
+    void setVmCallback(IVmCallback vmCallback);
+
+    /** Asynchronusly called by the VM in response to a call to echoMessage(). */
+    void onEchoRequestReceived(String message);
+}
diff --git a/tests/aidl/com/android/microdroid/testservice/ITestService.aidl b/tests/aidl/com/android/microdroid/testservice/ITestService.aidl
index 66dbe4b..36c3aaf 100644
--- a/tests/aidl/com/android/microdroid/testservice/ITestService.aidl
+++ b/tests/aidl/com/android/microdroid/testservice/ITestService.aidl
@@ -15,7 +15,12 @@
  */
 package com.android.microdroid.testservice;
 
-/** {@hide} */
+import com.android.microdroid.testservice.IAppCallback;
+
+/**
+ * This is the service exposed by the test payload, called by the test app.
+ * {@hide}
+ */
 interface ITestService {
     const long SERVICE_PORT = 5678;
 
@@ -62,6 +67,9 @@
     /** Returns flags for the given mountPoint. */
     int getMountFlags(String mountPoint);
 
+    /** Requests the VM to asynchronously call appCallback.setVmCallback() */
+    void requestCallback(IAppCallback appCallback);
+
     /**
      * Request the service to exit, triggering the termination of the VM. This may cause any
      * requests in flight to fail.
diff --git a/tests/aidl/com/android/microdroid/testservice/IVmCallback.aidl b/tests/aidl/com/android/microdroid/testservice/IVmCallback.aidl
new file mode 100644
index 0000000..617d184
--- /dev/null
+++ b/tests/aidl/com/android/microdroid/testservice/IVmCallback.aidl
@@ -0,0 +1,26 @@
+/*
+ * Copyright 2023 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 com.android.microdroid.testservice;
+
+/**
+ * An interface exposed by the VM for callbacks from the app.
+ *
+ * {@hide}
+ */
+interface IVmCallback {
+    /** Requests the VM to asynchronously call the app's onEchoRequestReceived() callback. */
+    void echoMessage(String message);
+}