refactor: Introduce a ToolMaker interface

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

@@ -48,6 +48,7 @@
 import com.google.adk.models.Model;
 import com.google.adk.tools.BaseTool;
 import com.google.adk.tools.BaseToolset;
+import com.google.adk.tools.ToolMarker;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
@@ -84,7 +85,7 @@ public enum IncludeContents {
   private final Optional<Model> model;
   private final Instruction instruction;
   private final Instruction globalInstruction;
-  private final List<Object> toolsUnion;
+  private final List<ToolMarker> toolsUnion;
   private final ImmutableList<BaseToolset> toolsets;
   private final Optional<GenerateContentConfig> generateContentConfig;
   // TODO: Remove exampleProvider field - examples should only be provided via ExampleTool
@@ -152,7 +153,7 @@ public static Builder builder() {
   }
 
   /** Extracts BaseToolset instances from the toolsUnion list. */
-  private static ImmutableList<BaseToolset> extractToolsets(List<Object> toolsUnion) {
+  private static ImmutableList<BaseToolset> extractToolsets(List<ToolMarker> toolsUnion) {
     return toolsUnion.stream()
         .filter(obj -> obj instanceof BaseToolset)
         .map(obj -> (BaseToolset) obj)
@@ -165,7 +166,7 @@ public static class Builder extends BaseAgent.Builder<Builder> {
 
     private Instruction instruction;
     private Instruction globalInstruction;
-    private ImmutableList<Object> toolsUnion;
+    private ImmutableList<ToolMarker> toolsUnion;
     private GenerateContentConfig generateContentConfig;
     private BaseExampleProvider exampleProvider;
     private IncludeContents includeContents;
@@ -221,13 +222,13 @@ public Builder globalInstruction(String globalInstruction) {
     }
 
     @CanIgnoreReturnValue
-    public Builder tools(List<?> tools) {
+    public Builder tools(List<? extends ToolMarker> tools) {
       this.toolsUnion = ImmutableList.copyOf(tools);
       return this;
     }
 
     @CanIgnoreReturnValue
-    public Builder tools(Object... tools) {
+    public Builder tools(ToolMarker... tools) {
       this.toolsUnion = ImmutableList.copyOf(tools);
       return this;
     }
@@ -679,7 +680,7 @@ public Single<Map.Entry<String, Boolean>> canonicalGlobalInstruction(ReadonlyCon
    */
   public Flowable<BaseTool> canonicalTools(Optional<ReadonlyContext> context) {
     List<Flowable<BaseTool>> toolFlowables = new ArrayList<>();
-    for (Object toolOrToolset : toolsUnion) {
+    for (ToolMarker toolOrToolset : toolsUnion) {
       if (toolOrToolset instanceof BaseTool baseTool) {
         toolFlowables.add(Flowable.just(baseTool));
       } else if (toolOrToolset instanceof BaseToolset baseToolset) {
@@ -740,7 +741,7 @@ public Single<List<BaseTool>> tools() {
     return canonicalTools().toList();
   }
 
-  public List<Object> toolsUnion() {
+  public List<ToolMarker> toolsUnion() {
     return toolsUnion;
   }
 

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

@@ -23,6 +23,7 @@
 import com.google.adk.tools.BaseTool.ToolArgsConfig;
 import com.google.adk.tools.BaseTool.ToolConfig;
 import com.google.adk.tools.BaseToolset;
+import com.google.adk.tools.ToolMarker;
 import com.google.adk.utils.ComponentRegistry;
 import com.google.common.collect.ImmutableList;
 import java.lang.reflect.Constructor;
@@ -56,14 +57,14 @@ private ToolResolver() {}
    * @throws ConfigurationException if any tool configuration is invalid (e.g., missing name), if a
    *     tool cannot be found by its name or class, or if tool instantiation fails.
    */
-  static ImmutableList<Object> resolveToolsAndToolsets(
+  static ImmutableList<ToolMarker> resolveToolsAndToolsets(
       List<ToolConfig> toolConfigs, String configAbsPath) throws ConfigurationException {
 
     if (toolConfigs == null || toolConfigs.isEmpty()) {
       return ImmutableList.of();
     }
 
-    ImmutableList.Builder<Object> resolvedItems = ImmutableList.builder();
+    ImmutableList.Builder<ToolMarker> resolvedItems = ImmutableList.builder();
 
     for (ToolConfig toolConfig : toolConfigs) {
       try {

core/src/main/java/com/google/adk/tools/BaseTool.java

@@ -44,7 +44,7 @@
 import org.slf4j.LoggerFactory;
 
 /** The base class for all ADK tools. */
-public abstract class BaseTool {
+public abstract class BaseTool implements ToolMarker {
   private final String name;
   private final String description;
   private final boolean isLongRunning;

core/src/main/java/com/google/adk/tools/BaseToolset.java

@@ -6,7 +6,7 @@
 import java.util.Optional;
 
 /** Base interface for toolsets. */
-public interface BaseToolset extends AutoCloseable {
+public interface BaseToolset extends AutoCloseable, ToolMarker {
 
   /**
    * Return all tools in the toolset based on the provided context.

core/src/main/java/com/google/adk/tools/ToolMarker.java

@@ -0,0 +1,19 @@
+/*
+ * 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.tools;
+
+/** Marker interface for BaseTool and BaseToolset. */
+public interface ToolMarker {}