fix: prevent ConcurrentModificationException when session events are modified by another thread during iteration

core/src/main/java/com/google/adk/flows/llmflows/Contents.java

@@ -64,14 +64,19 @@ public Single<RequestProcessor.RequestProcessingResult> processRequest(
       modelName = "";
     }
 
+    ImmutableList<Event> sessionEvents;
+    synchronized (context.session().events()) {
+      sessionEvents = ImmutableList.copyOf(context.session().events());
+    }
+
     if (llmAgent.includeContents() == LlmAgent.IncludeContents.NONE) {
       return Single.just(
           RequestProcessor.RequestProcessingResult.create(
               request.toBuilder()
                   .contents(
                       getCurrentTurnContents(
                           context.branch().orElse(null),
-                          context.session().events(),
+                          sessionEvents,
                           context.agent().name(),
                           modelName))
                   .build(),
@@ -80,10 +85,7 @@ public Single<RequestProcessor.RequestProcessingResult> processRequest(
 
     ImmutableList<Content> contents =
         getContents(
-            context.branch().orElse(null),
-            context.session().events(),
-            context.agent().name(),
-            modelName);
+            context.branch().orElse(null), sessionEvents, context.agent().name(), modelName);
 
     return Single.just(
         RequestProcessor.RequestProcessingResult.create(

core/src/test/java/com/google/adk/flows/llmflows/ContentsTest.java

@@ -36,10 +36,13 @@
 import com.google.genai.types.FunctionCall;
 import com.google.genai.types.FunctionResponse;
 import com.google.genai.types.Part;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.atomic.AtomicReference;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -780,6 +783,63 @@ public void processRequest_notEmptyContent() {
     assertThat(contents).containsExactly(e.content().get());
   }
 
+  @Test
+  public void processRequest_concurrentReadAndWrite_noException() throws Exception {
+    LlmAgent agent =
+        LlmAgent.builder().name(AGENT).includeContents(LlmAgent.IncludeContents.DEFAULT).build();
+    Session session =
+        sessionService
+            .createSession("test-app", "test-user", new HashMap<>(), "test-session")
+            .blockingGet();
+
+    // Seed with dummy events to widen the race capability
+    for (int i = 0; i < 5000; i++) {
+      session.events().add(createUserEvent("dummy" + i, "dummy"));
+    }
+
+    InvocationContext context =
+        InvocationContext.builder()
+            .invocationId("test-invocation")
+            .agent(agent)
+            .session(session)
+            .sessionService(sessionService)
+            .build();
+
+    LlmRequest initialRequest = LlmRequest.builder().build();
+
+    AtomicReference<Throwable> writerError = new AtomicReference<>();
+    CountDownLatch startLatch = new CountDownLatch(1);
+
+    Thread writerThread =
+        new Thread(
+            () -> {
+              startLatch.countDown();
+              try {
+                for (int i = 0; i < 2000; i++) {
+                  session.events().add(createUserEvent("writer" + i, "new data"));
+                }
+              } catch (Throwable t) {
+                writerError.set(t);
+              }
+            });
+
+    writerThread.start();
+    startLatch.await(); // wait for writer to be ready
+
+    // Process (read) requests concurrently to trigger race conditions
+    for (int i = 0; i < 200; i++) {
+      var unused = contentsProcessor.processRequest(context, initialRequest).blockingGet();
+      if (writerError.get() != null) {
+        throw new RuntimeException("Writer failed", writerError.get());
+      }
+    }
+
+    writerThread.join();
+    if (writerError.get() != null) {
+      throw new RuntimeException("Writer failed", writerError.get());
+    }
+  }
+
   private static Event createUserEvent(String id, String text) {
     return Event.builder()
         .id(id)