Fix: Parallel Agent not executing sub agents concurrently by default

core/src/main/java/com/google/adk/agents/ParallelAgent.java

@@ -20,6 +20,8 @@
 import com.google.adk.agents.ConfigAgentUtils.ConfigurationException;
 import com.google.adk.events.Event;
 import io.reactivex.rxjava3.core.Flowable;
+import io.reactivex.rxjava3.core.Scheduler;
+import io.reactivex.rxjava3.schedulers.Schedulers;
 import java.util.ArrayList;
 import java.util.List;
 import org.slf4j.Logger;
@@ -35,6 +37,7 @@
 public class ParallelAgent extends BaseAgent {
 
   private static final Logger logger = LoggerFactory.getLogger(ParallelAgent.class);
+  private final Scheduler scheduler;
 
   /**
    * Constructor for ParallelAgent.
@@ -44,24 +47,34 @@ public class ParallelAgent extends BaseAgent {
    * @param subAgents The list of sub-agents to run in parallel.
    * @param beforeAgentCallback Optional callback before the agent runs.
    * @param afterAgentCallback Optional callback after the agent runs.
+   * @param scheduler The scheduler to use for parallel execution.
    */
   private ParallelAgent(
       String name,
       String description,
       List<? extends BaseAgent> subAgents,
       List<Callbacks.BeforeAgentCallback> beforeAgentCallback,
-      List<Callbacks.AfterAgentCallback> afterAgentCallback) {
+      List<Callbacks.AfterAgentCallback> afterAgentCallback,
+      Scheduler scheduler) {
 
     super(name, description, subAgents, beforeAgentCallback, afterAgentCallback);
+    this.scheduler = scheduler;
   }
 
   /** Builder for {@link ParallelAgent}. */
   public static class Builder extends BaseAgent.Builder<Builder> {
 
+    private Scheduler scheduler = Schedulers.io();
+
+    public Builder scheduler(Scheduler scheduler) {
+      this.scheduler = scheduler;
+      return this;
+    }
+
     @Override
     public ParallelAgent build() {
       return new ParallelAgent(
-          name, description, subAgents, beforeAgentCallback, afterAgentCallback);
+          name, description, subAgents, beforeAgentCallback, afterAgentCallback, scheduler);
     }
   }
 
@@ -131,7 +144,7 @@ protected Flowable<Event> runAsyncImpl(InvocationContext invocationContext) {
 
     List<Flowable<Event>> agentFlowables = new ArrayList<>();
     for (BaseAgent subAgent : currentSubAgents) {
-      agentFlowables.add(subAgent.runAsync(invocationContext));
+      agentFlowables.add(subAgent.runAsync(invocationContext).subscribeOn(scheduler));
     }
     return Flowable.merge(agentFlowables);
   }

core/src/test/java/com/google/adk/agents/ParallelAgentTest.java

@@ -25,7 +25,10 @@
 import com.google.genai.types.Content;
 import com.google.genai.types.Part;
 import io.reactivex.rxjava3.core.Flowable;
+import io.reactivex.rxjava3.core.Scheduler;
 import io.reactivex.rxjava3.schedulers.Schedulers;
+import io.reactivex.rxjava3.schedulers.TestScheduler;
+import io.reactivex.rxjava3.subscribers.TestSubscriber;
 import java.util.List;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -36,10 +39,16 @@ public final class ParallelAgentTest {
 
   static class TestingAgent extends BaseAgent {
     private final long delayMillis;
+    private final Scheduler scheduler;
 
     private TestingAgent(String name, String description, long delayMillis) {
+      this(name, description, delayMillis, Schedulers.computation());
+    }
+
+    private TestingAgent(String name, String description, long delayMillis, Scheduler scheduler) {
       super(name, description, ImmutableList.of(), null, null);
       this.delayMillis = delayMillis;
+      this.scheduler = scheduler;
     }
 
     @Override
@@ -55,7 +64,7 @@ protected Flowable<Event> runAsyncImpl(InvocationContext invocationContext) {
                       .build());
 
       if (delayMillis > 0) {
-        return event.delay(delayMillis, MILLISECONDS, Schedulers.computation());
+        return event.delay(delayMillis, MILLISECONDS, scheduler);
       }
       return event;
     }
@@ -110,4 +119,79 @@ public void runAsync_noSubAgents_returnsEmptyFlowable() {
 
     assertThat(events).isEmpty();
   }
+
+  static class BlockingAgent extends BaseAgent {
+    private final long sleepMillis;
+
+    private BlockingAgent(String name, long sleepMillis) {
+      super(name, "Blocking Agent", ImmutableList.of(), null, null);
+      this.sleepMillis = sleepMillis;
+    }
+
+    @Override
+    protected Flowable<Event> runAsyncImpl(InvocationContext invocationContext) {
+      return Flowable.fromCallable(
+          () -> {
+            Thread.sleep(sleepMillis);
+            return Event.builder()
+                .author(name())
+                .branch(invocationContext.branch().orElse(null))
+                .invocationId(invocationContext.invocationId())
+                .content(Content.fromParts(Part.fromText("Done")))
+                .build();
+          });
+    }
+
+    @Override
+    protected Flowable<Event> runLiveImpl(InvocationContext invocationContext) {
+      throw new UnsupportedOperationException("Not implemented");
+    }
+  }
+
+  @Test
+  public void runAsync_blockingSubAgents_shouldExecuteInParallel() {
+    long sleepTime = 1000;
+    BlockingAgent agent1 = new BlockingAgent("agent1", sleepTime);
+    BlockingAgent agent2 = new BlockingAgent("agent2", sleepTime);
+
+    ParallelAgent parallelAgent =
+        ParallelAgent.builder().name("parallel_agent").subAgents(agent1, agent2).build();
+
+    InvocationContext invocationContext = createInvocationContext(parallelAgent);
+
+    long startTime = System.currentTimeMillis();
+    List<Event> events = parallelAgent.runAsync(invocationContext).toList().blockingGet();
+    long duration = System.currentTimeMillis() - startTime;
+
+    assertThat(events).hasSize(2);
+    // If parallel, duration should be less than 1.5 * sleepTime (1500ms).
+    assertThat(duration).isAtLeast(sleepTime);
+    assertThat(duration).isLessThan((long) (1.5 * sleepTime));
+  }
+
+  @Test
+  public void runAsync_withTestScheduler_usesVirtualTime() {
+    TestScheduler testScheduler = new TestScheduler();
+    long delayMillis = 1000;
+    TestingAgent agent =
+        new TestingAgent("delayed_agent", "Delayed Agent", delayMillis, testScheduler);
+
+    ParallelAgent parallelAgent =
+        ParallelAgent.builder()
+            .name("parallel_agent")
+            .subAgents(agent)
+            .scheduler(testScheduler)
+            .build();
+
+    InvocationContext invocationContext = createInvocationContext(parallelAgent);
+
+    TestSubscriber<Event> testSubscriber = parallelAgent.runAsync(invocationContext).test();
+
+    testScheduler.advanceTimeBy(delayMillis - 100, MILLISECONDS);
+    testSubscriber.assertNoValues();
+    testSubscriber.assertNotComplete();
+    testScheduler.advanceTimeBy(200, MILLISECONDS);
+    testSubscriber.assertValueCount(1);
+    testSubscriber.assertComplete();
+  }
 }