feat: Introduce the App class for defining agentic applications

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

@@ -16,9 +16,9 @@
 
 package com.google.adk.agents;
 
+import com.google.adk.apps.ResumabilityConfig;
 import com.google.adk.artifacts.BaseArtifactService;
 import com.google.adk.events.Event;
-import com.google.adk.flows.llmflows.ResumabilityConfig;
 import com.google.adk.memory.BaseMemoryService;
 import com.google.adk.models.LlmCallsLimitExceededException;
 import com.google.adk.plugins.Plugin;

core/src/main/java/com/google/adk/apps/App.java

@@ -0,0 +1,145 @@
+/*
+ * Copyright 2026 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.apps;
+
+import com.google.adk.agents.BaseAgent;
+import com.google.adk.plugins.BasePlugin;
+import com.google.adk.summarizer.EventsCompactionConfig;
+import com.google.common.collect.ImmutableList;
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
+import java.util.List;
+import java.util.regex.Pattern;
+import javax.annotation.Nullable;
+
+/**
+ * Represents an LLM-backed agentic application.
+ *
+ * <p>An {@code App} is the top-level container for an agentic system powered by LLMs. It manages a
+ * root agent ({@code rootAgent}), which serves as the root of an agent tree, enabling coordination
+ * and communication across all agents in the hierarchy. The {@code plugins} are application-wide
+ * components that provide shared capabilities and services to the entire system.
+ */
+public class App {
+  private static final Pattern IDENTIFIER_PATTERN = Pattern.compile("[a-zA-Z_][a-zA-Z0-9_]*");
+
+  private final String name;
+  private final BaseAgent rootAgent;
+  private final ImmutableList<BasePlugin> plugins;
+  @Nullable private final EventsCompactionConfig eventsCompactionConfig;
+  @Nullable private final ResumabilityConfig resumabilityConfig;
+
+  private App(
+      String name,
+      BaseAgent rootAgent,
+      List<BasePlugin> plugins,
+      @Nullable EventsCompactionConfig eventsCompactionConfig,
+      @Nullable ResumabilityConfig resumabilityConfig) {
+    this.name = name;
+    this.rootAgent = rootAgent;
+    this.plugins = ImmutableList.copyOf(plugins);
+    this.eventsCompactionConfig = eventsCompactionConfig;
+    this.resumabilityConfig = resumabilityConfig;
+  }
+
+  public String name() {
+    return name;
+  }
+
+  public BaseAgent rootAgent() {
+    return rootAgent;
+  }
+
+  public ImmutableList<BasePlugin> plugins() {
+    return plugins;
+  }
+
+  @Nullable
+  public EventsCompactionConfig eventsCompactionConfig() {
+    return eventsCompactionConfig;
+  }
+
+  @Nullable
+  public ResumabilityConfig resumabilityConfig() {
+    return resumabilityConfig;
+  }
+
+  /** Builder for {@link App}. */
+  public static class Builder {
+    private String name;
+    private BaseAgent rootAgent;
+    private List<BasePlugin> plugins = ImmutableList.of();
+    @Nullable private EventsCompactionConfig eventsCompactionConfig;
+    @Nullable private ResumabilityConfig resumabilityConfig;
+
+    @CanIgnoreReturnValue
+    public Builder name(String name) {
+      this.name = name;
+      return this;
+    }
+
+    @CanIgnoreReturnValue
+    public Builder rootAgent(BaseAgent rootAgent) {
+      this.rootAgent = rootAgent;
+      return this;
+    }
+
+    @CanIgnoreReturnValue
+    public Builder plugins(List<BasePlugin> plugins) {
+      this.plugins = plugins;
+      return this;
+    }
+
+    @CanIgnoreReturnValue
+    public Builder eventsCompactionConfig(EventsCompactionConfig eventsCompactionConfig) {
+      this.eventsCompactionConfig = eventsCompactionConfig;
+      return this;
+    }
+
+    @CanIgnoreReturnValue
+    public Builder resumabilityConfig(ResumabilityConfig resumabilityConfig) {
+      this.resumabilityConfig = resumabilityConfig;
+      return this;
+    }
+
+    public App build() {
+      if (name == null) {
+        throw new IllegalStateException("App name must be provided.");
+      }
+      if (rootAgent == null) {
+        throw new IllegalStateException("Root agent must be provided.");
+      }
+      validateAppName(name);
+      return new App(name, rootAgent, plugins, eventsCompactionConfig, resumabilityConfig);
+    }
+  }
+
+  public static Builder builder() {
+    return new Builder();
+  }
+
+  private static void validateAppName(String name) {
+    if (!IDENTIFIER_PATTERN.matcher(name).matches()) {
+      throw new IllegalArgumentException(
+          "Invalid app name '"
+              + name
+              + "': must be a valid identifier consisting of letters, digits, and underscores.");
+    }
+    if (name.equals("user")) {
+      throw new IllegalArgumentException("App name cannot be 'user'; reserved for end-user input.");
+    }
+  }
+}

core/src/main/java/com/google/adk/flows/llmflows/ResumabilityConfig.java → core/src/main/java/com/google/adk/apps/ResumabilityConfig.java

@@ -9,11 +9,10 @@
  *
  * 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
+ * WITHOUT WARRANTIES OR CONDITIONS language governing permissions and
  * limitations under the License.
  */
-package com.google.adk.flows.llmflows;
+package com.google.adk.apps;
 
 /**
  * An app contains Resumability configuration for the agents.

core/src/main/java/com/google/adk/runner/Runner.java

@@ -23,11 +23,12 @@
 import com.google.adk.agents.LiveRequestQueue;
 import com.google.adk.agents.LlmAgent;
 import com.google.adk.agents.RunConfig;
+import com.google.adk.apps.App;
+import com.google.adk.apps.ResumabilityConfig;
 import com.google.adk.artifacts.BaseArtifactService;
 import com.google.adk.artifacts.InMemoryArtifactService;
 import com.google.adk.events.Event;
 import com.google.adk.events.EventActions;
-import com.google.adk.flows.llmflows.ResumabilityConfig;
 import com.google.adk.memory.BaseMemoryService;
 import com.google.adk.models.Model;
 import com.google.adk.plugins.BasePlugin;
@@ -41,6 +42,7 @@
 import com.google.adk.tools.BaseTool;
 import com.google.adk.tools.FunctionTool;
 import com.google.adk.utils.CollectionUtils;
+import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.genai.types.AudioTranscriptionConfig;
@@ -76,23 +78,31 @@ public class Runner {
 
   /** Builder for {@link Runner}. */
   public static class Builder {
+    private App app;
     private BaseAgent agent;
     private String appName;
     private BaseArtifactService artifactService = new InMemoryArtifactService();
     private BaseSessionService sessionService = new InMemorySessionService();
     @Nullable private BaseMemoryService memoryService = null;
     private List<BasePlugin> plugins = ImmutableList.of();
-    private ResumabilityConfig resumabilityConfig = new ResumabilityConfig();
-    @Nullable private EventsCompactionConfig eventsCompactionConfig;
+
+    @CanIgnoreReturnValue
+    public Builder app(App app) {
+      Preconditions.checkState(this.agent == null, "app() cannot be called when agent() is set.");
+      this.app = app;
+      return this;
+    }
 
     @CanIgnoreReturnValue
     public Builder agent(BaseAgent agent) {
+      Preconditions.checkState(this.app == null, "agent() cannot be called when app is set.");
       this.agent = agent;
       return this;
     }
 
     @CanIgnoreReturnValue
     public Builder appName(String appName) {
+      Preconditions.checkState(this.app == null, "appName() cannot be called when app is set.");
       this.appName = appName;
       return this;
     }
@@ -117,28 +127,46 @@ public Builder memoryService(BaseMemoryService memoryService) {
 
     @CanIgnoreReturnValue
     public Builder plugins(List<BasePlugin> plugins) {
+      Preconditions.checkState(this.app == null, "plugins() cannot be called when app is set.");
       this.plugins = plugins;
       return this;
     }
 
-    @CanIgnoreReturnValue
-    public Builder resumabilityConfig(ResumabilityConfig resumabilityConfig) {
-      this.resumabilityConfig = resumabilityConfig;
-      return this;
-    }
-
-    @CanIgnoreReturnValue
-    public Builder eventsCompactionConfig(EventsCompactionConfig eventsCompactionConfig) {
-      this.eventsCompactionConfig = eventsCompactionConfig;
-      return this;
-    }
-
     public Runner build() {
-      if (agent == null) {
-        throw new IllegalStateException("Agent must be provided.");
+      BaseAgent buildAgent;
+      String buildAppName;
+      List<BasePlugin> buildPlugins;
+      ResumabilityConfig buildResumabilityConfig;
+      EventsCompactionConfig buildEventsCompactionConfig;
+
+      if (this.app != null) {
+        if (this.agent != null) {
+          throw new IllegalStateException("agent() cannot be called when app() is called.");
+        }
+        if (!this.plugins.isEmpty()) {
+          throw new IllegalStateException("plugins() cannot be called when app() is called.");
+        }
+        buildAgent = this.app.rootAgent();
+        buildPlugins = this.app.plugins();
+        buildAppName = this.appName == null ? this.app.name() : this.appName;
+        buildResumabilityConfig =
+            this.app.resumabilityConfig() != null
+                ? this.app.resumabilityConfig()
+                : new ResumabilityConfig();
+        buildEventsCompactionConfig = this.app.eventsCompactionConfig();
+      } else {
+        buildAgent = this.agent;
+        buildAppName = this.appName;
+        buildPlugins = this.plugins;
+        buildResumabilityConfig = new ResumabilityConfig();
+        buildEventsCompactionConfig = null;
+      }
+
+      if (buildAgent == null) {
+        throw new IllegalStateException("Agent must be provided via app() or agent().");
       }
-      if (appName == null) {
-        throw new IllegalStateException("App name must be provided.");
+      if (buildAppName == null) {
+        throw new IllegalStateException("App name must be provided via app() or appName().");
       }
       if (artifactService == null) {
         throw new IllegalStateException("Artifact service must be provided.");
@@ -147,14 +175,14 @@ public Runner build() {
         throw new IllegalStateException("Session service must be provided.");
       }
       return new Runner(
-          agent,
-          appName,
+          buildAgent,
+          buildAppName,
           artifactService,
           sessionService,
           memoryService,
-          plugins,
-          resumabilityConfig,
-          eventsCompactionConfig);
+          buildPlugins,
+          buildResumabilityConfig,
+          buildEventsCompactionConfig);
     }
   }
 

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

@@ -19,9 +19,9 @@
 import static com.google.common.truth.Truth.assertThat;
 import static org.mockito.Mockito.mock;
 
+import com.google.adk.apps.ResumabilityConfig;
 import com.google.adk.artifacts.BaseArtifactService;
 import com.google.adk.events.Event;
-import com.google.adk.flows.llmflows.ResumabilityConfig;
 import com.google.adk.memory.BaseMemoryService;
 import com.google.adk.plugins.PluginManager;
 import com.google.adk.sessions.BaseSessionService;