diff --git a/core/src/main/java/com/google/adk/agents/RunConfig.java b/core/src/main/java/com/google/adk/agents/RunConfig.java
index f5fe5cb34..2f8e417d7 100644
--- a/core/src/main/java/com/google/adk/agents/RunConfig.java
+++ b/core/src/main/java/com/google/adk/agents/RunConfig.java
@@ -38,6 +38,22 @@ public enum StreamingMode {
     BIDI
   }
 
+  /**
+   * Tool execution mode for the runner, when they are multiple tools requested (by the models or
+   * callbacks).
+   *
+   * <p>NONE: default to PARALLEL.
+   *
+   * <p>SEQUENTIAL: Multiple tools are executed in the order they are requested.
+   *
+   * <p>PARALLEL: Multiple tools are executed in parallel.
+   */
+  public enum ToolExecutionMode {
+    NONE,
+    SEQUENTIAL,
+    PARALLEL
+  }
+
   public abstract @Nullable SpeechConfig speechConfig();
 
   public abstract ImmutableList<Modality> responseModalities();
@@ -46,6 +62,8 @@ public enum StreamingMode {
 
   public abstract StreamingMode streamingMode();
 
+  public abstract ToolExecutionMode toolExecutionMode();
+
   public abstract @Nullable AudioTranscriptionConfig outputAudioTranscription();
 
   public abstract @Nullable AudioTranscriptionConfig inputAudioTranscription();
@@ -59,6 +77,7 @@ public static Builder builder() {
         .setSaveInputBlobsAsArtifacts(false)
         .setResponseModalities(ImmutableList.of())
         .setStreamingMode(StreamingMode.NONE)
+        .setToolExecutionMode(ToolExecutionMode.NONE)
         .setMaxLlmCalls(500);
   }
 
@@ -66,6 +85,7 @@ public static Builder builder(RunConfig runConfig) {
     return new AutoValue_RunConfig.Builder()
         .setSaveInputBlobsAsArtifacts(runConfig.saveInputBlobsAsArtifacts())
         .setStreamingMode(runConfig.streamingMode())
+        .setToolExecutionMode(runConfig.toolExecutionMode())
         .setMaxLlmCalls(runConfig.maxLlmCalls())
         .setResponseModalities(runConfig.responseModalities())
         .setSpeechConfig(runConfig.speechConfig())
@@ -89,6 +109,9 @@ public abstract static class Builder {
     @CanIgnoreReturnValue
     public abstract Builder setStreamingMode(StreamingMode streamingMode);
 
+    @CanIgnoreReturnValue
+    public abstract Builder setToolExecutionMode(ToolExecutionMode toolExecutionMode);
+
     @CanIgnoreReturnValue
     public abstract Builder setOutputAudioTranscription(
         @Nullable AudioTranscriptionConfig outputAudioTranscription);
diff --git a/core/src/main/java/com/google/adk/flows/llmflows/Functions.java b/core/src/main/java/com/google/adk/flows/llmflows/Functions.java
index 234283c64..61cbab5a1 100644
--- a/core/src/main/java/com/google/adk/flows/llmflows/Functions.java
+++ b/core/src/main/java/com/google/adk/flows/llmflows/Functions.java
@@ -25,6 +25,7 @@
 import com.google.adk.agents.Callbacks.BeforeToolCallback;
 import com.google.adk.agents.InvocationContext;
 import com.google.adk.agents.LlmAgent;
+import com.google.adk.agents.RunConfig.ToolExecutionMode;
 import com.google.adk.events.Event;
 import com.google.adk.events.EventActions;
 import com.google.adk.tools.BaseTool;
@@ -198,7 +199,13 @@ public static Maybe<Event> handleFunctionCalls(
       functionResponseEvents.add(maybeFunctionResponseEvent);
     }
 
-    return Maybe.merge(functionResponseEvents)
+    Flowable<Event> functionResponseEventsFlowable;
+    if (invocationContext.runConfig().toolExecutionMode() == ToolExecutionMode.SEQUENTIAL) {
+      functionResponseEventsFlowable = Maybe.concat(functionResponseEvents);
+    } else {
+      functionResponseEventsFlowable = Maybe.merge(functionResponseEvents);
+    }
+    return functionResponseEventsFlowable
         .toList()
         .flatMapMaybe(
             events -> {
@@ -296,7 +303,13 @@ public static Maybe<Event> handleFunctionCallsLive(
       responseEvents.add(maybeFunctionResponseEvent);
     }
 
-    return Maybe.merge(responseEvents)
+    Flowable<Event> responseEventsFlowable;
+    if (invocationContext.runConfig().toolExecutionMode() == ToolExecutionMode.SEQUENTIAL) {
+      responseEventsFlowable = Maybe.concat(responseEvents);
+    } else {
+      responseEventsFlowable = Maybe.merge(responseEvents);
+    }
+    return responseEventsFlowable
         .toList()
         .flatMapMaybe(
             events -> {