fix: make a mutable copy of function args for the beforeToolCallback invocations

PiperOrigin-RevId: 876191407

Author: tilgalas

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

@@ -253,7 +253,8 @@ private static Function<FunctionCall, Maybe<Event>> getFunctionCallMapper(
                             functionCall.id().map(toolConfirmations::get).orElse(null))
                         .build();
 
-                Map<String, Object> functionArgs = functionCall.args().orElse(new HashMap<>());
+                Map<String, Object> functionArgs =
+                    functionCall.args().map(HashMap::new).orElse(new HashMap<>());
 
                 Maybe<Map<String, Object>> maybeFunctionResult =
                     maybeInvokeBeforeToolCall(invocationContext, tool, functionArgs, toolContext)
@@ -482,12 +483,8 @@ private static Maybe<Map<String, Object>> maybeInvokeBeforeToolCall(
     if (invocationContext.agent() instanceof LlmAgent) {
       LlmAgent agent = (LlmAgent) invocationContext.agent();
 
-      HashMap<String, Object> mutableFunctionArgs = new HashMap<>(functionArgs);
-
       Maybe<Map<String, Object>> pluginResult =
-          invocationContext
-              .pluginManager()
-              .beforeToolCallback(tool, mutableFunctionArgs, toolContext);
+          invocationContext.pluginManager().beforeToolCallback(tool, functionArgs, toolContext);
 
       List<? extends BeforeToolCallback> callbacks = agent.canonicalBeforeToolCallbacks();
       if (callbacks.isEmpty()) {
@@ -500,8 +497,7 @@ private static Maybe<Map<String, Object>> maybeInvokeBeforeToolCall(
                   Flowable.fromIterable(callbacks)
                       .concatMapMaybe(
                           callback ->
-                              callback.call(
-                                  invocationContext, tool, mutableFunctionArgs, toolContext))
+                              callback.call(invocationContext, tool, functionArgs, toolContext))
                       .firstElement());
 
       return pluginResult.switchIfEmpty(callbackResult);

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

@@ -1172,10 +1172,51 @@ public Maybe<Map<String, Object>> beforeToolCallback(
                 event,
                 ImmutableMap.of("echo_tool", new TestUtils.FailingEchoTool()))
             .blockingGet();
-
     assertThat(getFunctionResponse(functionResponseEvent)).isEqualTo(responseFromAgentCb);
   }
 
+  @Test
+  public void handleFunctionCalls_withBeforeToolCallback_modifiesArgs() {
+    ImmutableMap<String, Object> originalArgs = ImmutableMap.of("arg1", "val1");
+    ImmutableMap<String, Object> modifiedArgs = ImmutableMap.of("arg1", "val1", "arg2", "val2");
+
+    Callbacks.BeforeToolCallbackSync cb1 =
+        (invocationContext, tool, input, toolContext) -> {
+          input.put("arg2", "val2");
+          return Optional.empty();
+        };
+
+    TestUtils.EchoTool echoTool = new TestUtils.EchoTool();
+
+    InvocationContext invocationContext =
+        createInvocationContext(
+            createTestAgentBuilder(createTestLlm(LlmResponse.builder().build()))
+                .beforeToolCallbackSync(cb1)
+                .build());
+
+    Event event =
+        createEvent("event").toBuilder()
+            .content(
+                Content.fromParts(
+                    Part.fromText("..."),
+                    Part.builder()
+                        .functionCall(
+                            FunctionCall.builder()
+                                .id("fc_id")
+                                .name("echo_tool")
+                                .args(originalArgs)
+                                .build())
+                        .build()))
+            .build();
+
+    Event functionResponseEvent =
+        Functions.handleFunctionCalls(
+                invocationContext, event, ImmutableMap.of("echo_tool", echoTool))
+            .blockingGet();
+
+    assertThat(getFunctionResponse(functionResponseEvent)).containsExactly("result", modifiedArgs);
+  }
+
   @Test
   public void agentRunAsync_withToolCallbacks_inspectsArgsAndReturnsResponse() {
     TestUtils.EchoTool echoTool = new TestUtils.EchoTool();