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 a952d602d..4fc1667d9 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
@@ -45,6 +45,7 @@
 import io.opentelemetry.context.Scope;
 import io.reactivex.rxjava3.core.Flowable;
 import io.reactivex.rxjava3.core.Maybe;
+import io.reactivex.rxjava3.core.Observable;
 import io.reactivex.rxjava3.core.Single;
 import io.reactivex.rxjava3.disposables.Disposable;
 import io.reactivex.rxjava3.functions.Function;
@@ -199,15 +200,16 @@ public static Maybe<Event> handleFunctionCalls(
                   });
         };
 
-    Flowable<Event> functionResponseEventsFlowable;
+    Observable<Event> functionResponseEventsObservable;
     if (invocationContext.runConfig().toolExecutionMode() == ToolExecutionMode.SEQUENTIAL) {
-      functionResponseEventsFlowable =
-          Flowable.fromIterable(functionCalls).concatMapMaybe(functionCallMapper);
+      functionResponseEventsObservable =
+          Observable.fromIterable(functionCalls).concatMapMaybe(functionCallMapper);
     } else {
-      functionResponseEventsFlowable =
-          Flowable.fromIterable(functionCalls).flatMapMaybe(functionCallMapper);
+      functionResponseEventsObservable =
+          Observable.fromIterable(functionCalls)
+              .concatMapEager(call -> functionCallMapper.apply(call).toObservable());
     }
-    return functionResponseEventsFlowable
+    return functionResponseEventsObservable
         .toList()
         .flatMapMaybe(
             events -> {
@@ -310,18 +312,18 @@ public static Maybe<Event> handleFunctionCallsLive(
                   });
         };
 
-    Flowable<Event> responseEventsFlowable;
+    Observable<Event> responseEventsObservable;
 
     if (invocationContext.runConfig().toolExecutionMode() == ToolExecutionMode.SEQUENTIAL) {
-      responseEventsFlowable =
-          Flowable.fromIterable(functionCalls).concatMapMaybe(functionCallMapper);
-
+      responseEventsObservable =
+          Observable.fromIterable(functionCalls).concatMapMaybe(functionCallMapper);
     } else {
-      responseEventsFlowable =
-          Flowable.fromIterable(functionCalls).flatMapMaybe(functionCallMapper);
+      responseEventsObservable =
+          Observable.fromIterable(functionCalls)
+              .concatMapEager(call -> functionCallMapper.apply(call).toObservable());
     }
 
-    return responseEventsFlowable
+    return responseEventsObservable
         .toList()
         .flatMapMaybe(
             events -> {
diff --git a/core/src/test/java/com/google/adk/flows/llmflows/FunctionsTest.java b/core/src/test/java/com/google/adk/flows/llmflows/FunctionsTest.java
index d880d7d85..b9d99d39c 100644
--- a/core/src/test/java/com/google/adk/flows/llmflows/FunctionsTest.java
+++ b/core/src/test/java/com/google/adk/flows/llmflows/FunctionsTest.java
@@ -167,7 +167,8 @@ public void handleFunctionCalls_multipleFunctionCalls() {
                         .name("echo_tool")
                         .response(ImmutableMap.of("result", args2))
                         .build())
-                .build());
+                .build())
+        .inOrder();
   }
 
   @Test
@@ -229,7 +230,9 @@ public void populateClientFunctionCallId_withExistingId_noChange() {
                         .build()))
             .build();
 
-    Functions.populateClientFunctionCallId(event);
-    assertThat(event).isEqualTo(event);
+    Event eventCopy = Event.fromJson(event.toJson());
+    Functions.populateClientFunctionCallId(eventCopy);
+
+    assertThat(eventCopy).isEqualTo(event);
   }
 }