fix: adjust default ToolExecutionMode to SEQUENTIAL as it was actual and widely used behavior for all ADK Java users before parallel tool execution fix

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

@@ -39,14 +39,14 @@ public enum StreamingMode {
   }
 
   /**
-   * Tool execution mode for the runner, when they are multiple tools requested (by the models or
-   * callbacks).
+   * Execution mode when the model requests multiple tools.
    *
-   * <p>NONE: default to PARALLEL.
+   * <p>NONE: defaults to SEQUENTIAL.
    *
-   * <p>SEQUENTIAL: Multiple tools are executed in the order they are requested.
+   * <p>SEQUENTIAL: tools execute in request order on the caller thread.
    *
-   * <p>PARALLEL: Multiple tools are executed in parallel.
+   * <p>PARALLEL: tools execute concurrently on worker threads. Tool implementations must be
+   * thread-safe.
    */
   public enum ToolExecutionMode {
     NONE,

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

@@ -236,19 +236,15 @@ public static Maybe<Event> handleFunctionCallsLive(
   }
 
   /**
-   * Builds the tool-execution {@link Observable}.
-   *
-   * <p>SEQUENTIAL (or a single call, where parallelism is moot) runs on the caller thread via
-   * {@code concatMapMaybe} to keep synchronous semantics. PARALLEL with multiple calls dispatches
-   * each tool on a worker so blocking calls run concurrently; {@code concatMapEager} preserves
-   * input order required by {@link #mergeParallelFunctionResponseEvents}.
+   * Sequential by default; only {@link ToolExecutionMode#PARALLEL} with multiple calls dispatches
+   * tools on workers (using {@code concatMapEager} to preserve input order).
    */
   private static Observable<Event> buildToolExecutionObservable(
       InvocationContext invocationContext,
       List<FunctionCall> validFunctionCalls,
       Function<FunctionCall, Maybe<Event>> functionCallMapper) {
     boolean parallel =
-        invocationContext.runConfig().toolExecutionMode() != ToolExecutionMode.SEQUENTIAL
+        invocationContext.runConfig().toolExecutionMode() == ToolExecutionMode.PARALLEL
             && validFunctionCalls.size() > 1;
     if (!parallel) {
       return Observable.fromIterable(validFunctionCalls).concatMapMaybe(functionCallMapper);

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

@@ -398,6 +398,43 @@ public void getAskUserConfirmationFunctionCalls_eventWithConfirmationFunctionCal
     assertThat(result).containsExactly(confirmationCall1, confirmationCall2);
   }
 
+  // Default ToolExecutionMode.NONE must execute tools sequentially.
+  @Test
+  public void handleFunctionCalls_defaultMode_blockingTools_runSequentially() {
+    long sleepMillis = 300L;
+    int toolCount = 2;
+    InvocationContext invocationContext =
+        createInvocationContext(createRootAgent(), RunConfig.builder().build());
+
+    Map<String, BaseTool> tools = new LinkedHashMap<>();
+    List<Part> callParts = new ArrayList<>();
+    for (int i = 1; i <= toolCount; i++) {
+      String toolName = "slow_tool_" + i;
+      tools.put(toolName, new SleepingTool(toolName, sleepMillis));
+      callParts.add(
+          Part.builder()
+              .functionCall(
+                  FunctionCall.builder()
+                      .id("call_" + i)
+                      .name(toolName)
+                      .args(ImmutableMap.of())
+                      .build())
+              .build());
+    }
+    Event event =
+        createEvent("event").toBuilder()
+            .content(Content.fromParts(callParts.toArray(new Part[0])))
+            .build();
+
+    long start = System.currentTimeMillis();
+    Event functionResponseEvent =
+        Functions.handleFunctionCalls(invocationContext, event, tools).blockingGet();
+    long durationMillis = System.currentTimeMillis() - start;
+
+    assertThat(functionResponseEvent).isNotNull();
+    assertThat(durationMillis).isAtLeast((long) toolCount * sleepMillis);
+  }
+
   @Test
   public void handleFunctionCalls_parallel_blockingTools_runConcurrently_twoTools() {
     runParallelBlockingToolsTest(/* toolCount= */ 2);