diff --git a/core/src/main/java/com/google/adk/runner/Runner.java b/core/src/main/java/com/google/adk/runner/Runner.java
index f89e49d6b..00a3a18f5 100644
--- a/core/src/main/java/com/google/adk/runner/Runner.java
+++ b/core/src/main/java/com/google/adk/runner/Runner.java
@@ -38,6 +38,7 @@
 import io.reactivex.rxjava3.core.Flowable;
 import io.reactivex.rxjava3.core.Maybe;
 import io.reactivex.rxjava3.core.Single;
+import io.reactivex.rxjava3.core.Completable;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
@@ -83,7 +84,7 @@ public BaseSessionService sessionService() {
    *
    * @throws IllegalArgumentException if message has no parts.
    */
-  private void appendNewMessageToSession(
+  private Completable appendNewMessageToSession(
       Session session,
       Content newMessage,
       InvocationContext invocationContext,
@@ -123,7 +124,7 @@ private void appendNewMessageToSession(
             .author("user")
             .content(Optional.of(newMessage))
             .build();
-    this.sessionService.appendEvent(session, event);
+    return this.sessionService.appendEvent(session, event).ignoreElement();
   }
 
   /**
@@ -190,14 +191,20 @@ public Flowable<Event> runAsync(Session session, Content newMessage, RunConfig r
                         newMessage,
                         runConfig);
 
+                Completable newMessageCompletable = Completable.complete();
                 if (newMessage != null) {
-                  appendNewMessageToSession(
-                      sess, newMessage, invocationContext, runConfig.saveInputBlobsAsArtifacts());
+                  newMessageCompletable =
+                      appendNewMessageToSession(
+                          sess,
+                          newMessage,
+                          invocationContext,
+                          runConfig.saveInputBlobsAsArtifacts());
                 }
 
                 invocationContext.agent(this.findAgentToRun(sess, rootAgent));
                 Flowable<Event> events = invocationContext.agent().runAsync(invocationContext);
-                return events.doOnNext(event -> this.sessionService.appendEvent(sess, event));
+                return newMessageCompletable.andThen(
+                    events.concatMapSingle(event -> this.sessionService.appendEvent(sess, event)));
               })
           .doOnError(
               throwable -> {