refactor: centralize the client function-call-id prefix and helpers

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

@@ -29,6 +29,7 @@
 import com.google.adk.events.Event;
 import com.google.adk.events.EventActions;
 import com.google.adk.events.ToolConfirmation;
+import com.google.adk.models.FunctionCallIds;
 import com.google.adk.telemetry.Instrumentation;
 import com.google.adk.telemetry.Instrumentation.ToolExecution;
 import com.google.adk.telemetry.Tracing;
@@ -60,7 +61,6 @@
 import java.util.Map;
 import java.util.Optional;
 import java.util.Set;
-import java.util.UUID;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -72,12 +72,11 @@ public final class Functions {
   /** Session state key for storing the security policy outcomes for tool calls. */
   public static final String TOOL_CALL_SECURITY_STATES = "adk_tool_call_security_states";
 
-  private static final String AF_FUNCTION_CALL_ID_PREFIX = "adk-";
   private static final Logger logger = LoggerFactory.getLogger(Functions.class);
 
   /** Generates a unique ID for a function call. */
   public static String generateClientFunctionCallId() {
-    return AF_FUNCTION_CALL_ID_PREFIX + UUID.randomUUID();
+    return FunctionCallIds.generateClientFunctionCallId();
   }
 
   /**

core/src/main/java/com/google/adk/models/FunctionCallIds.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.adk.models;
+
+import java.util.UUID;
+
+/** Constants and helpers for ADK-generated function call IDs. */
+public final class FunctionCallIds {
+
+  /** Prefix marking function call IDs the ADK generated client-side. */
+  private static final String AF_FUNCTION_CALL_ID_PREFIX = "adk-";
+
+  /** Returns a new client-side function call ID with the ADK prefix. */
+  public static String generateClientFunctionCallId() {
+    return AF_FUNCTION_CALL_ID_PREFIX + UUID.randomUUID();
+  }
+
+  /** Returns whether {@code id} was generated client-side by the ADK. */
+  public static boolean isClientGeneratedFunctionCallId(String id) {
+    return id != null && id.startsWith(AF_FUNCTION_CALL_ID_PREFIX);
+  }
+
+  private FunctionCallIds() {}
+}

core/src/main/java/com/google/adk/models/Gemini.java

@@ -43,7 +43,6 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
-import java.util.UUID;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ExecutorService;
 import okhttp3.OkHttpClient;
@@ -401,13 +400,9 @@ private static List<Part> ensureFunctionCallIds(List<Part> parts) {
       return result;
     }
 
-    /**
-     * Generates a unique client-side function-call ID. Format matches {@code
-     * com.google.adk.flows.llmflows.Functions#generateClientFunctionCallId()} so downstream code
-     * that already sees IDs with the {@code "adk-"} prefix continues to work.
-     */
+    /** Generates a unique client-side function-call ID. */
     private static String generateClientFunctionCallId() {
-      return "adk-" + UUID.randomUUID();
+      return FunctionCallIds.generateClientFunctionCallId();
     }
 
     /**

core/src/main/java/com/google/adk/models/GeminiUtil.java

@@ -38,8 +38,6 @@ public final class GeminiUtil {
       "Continue output. DO NOT look at this line. ONLY look at the content before this line and"
           + " system instruction.";
 
-  private static final String AF_FUNCTION_CALL_ID_PREFIX = "adk-";
-
   private GeminiUtil() {}
 
   /**
@@ -174,14 +172,15 @@ public static LlmRequest removeClientFunctionCallId(LlmRequest llmRequest) {
   private static Part removeClientFunctionCallIdFromPart(Part part) {
     if (part.functionCall().isPresent()
         && part.functionCall().get().id().isPresent()
-        && part.functionCall().get().id().get().startsWith(AF_FUNCTION_CALL_ID_PREFIX)) {
+        && FunctionCallIds.isClientGeneratedFunctionCallId(part.functionCall().get().id().get())) {
       return part.toBuilder()
           .functionCall(part.functionCall().get().toBuilder().clearId().build())
           .build();
     }
     if (part.functionResponse().isPresent()
         && part.functionResponse().get().id().isPresent()
-        && part.functionResponse().get().id().get().startsWith(AF_FUNCTION_CALL_ID_PREFIX)) {
+        && FunctionCallIds.isClientGeneratedFunctionCallId(
+            part.functionResponse().get().id().get())) {
       return part.toBuilder()
           .functionResponse(part.functionResponse().get().toBuilder().clearId().build())
           .build();

core/src/test/java/com/google/adk/models/FunctionCallIdsTest.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.adk.models;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public final class FunctionCallIdsTest {
+
+  @Test
+  public void generatedId_isRecognizedAsClientGenerated() {
+    String id = FunctionCallIds.generateClientFunctionCallId();
+
+    assertThat(FunctionCallIds.isClientGeneratedFunctionCallId(id)).isTrue();
+  }
+
+  @Test
+  public void generateClientFunctionCallId_returnsUniqueIds() {
+    String first = FunctionCallIds.generateClientFunctionCallId();
+    String second = FunctionCallIds.generateClientFunctionCallId();
+
+    assertThat(first).isNotEqualTo(second);
+  }
+
+  @Test
+  public void isClientGeneratedFunctionCallId_falseForModelGeneratedId() {
+    assertThat(FunctionCallIds.isClientGeneratedFunctionCallId("call_123")).isFalse();
+  }
+
+  @Test
+  public void isClientGeneratedFunctionCallId_falseForNullOrEmpty() {
+    assertThat(FunctionCallIds.isClientGeneratedFunctionCallId(null)).isFalse();
+    assertThat(FunctionCallIds.isClientGeneratedFunctionCallId("")).isFalse();
+  }
+}