feat: Allow EventsCompactionConfig to have a null summarizer initially

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

@@ -29,12 +29,14 @@
 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;
 import com.google.adk.plugins.PluginManager;
 import com.google.adk.sessions.BaseSessionService;
 import com.google.adk.sessions.InMemorySessionService;
 import com.google.adk.sessions.Session;
 import com.google.adk.summarizer.EventsCompactionConfig;
+import com.google.adk.summarizer.LlmEventSummarizer;
 import com.google.adk.summarizer.SlidingWindowEventCompactor;
 import com.google.adk.tools.BaseTool;
 import com.google.adk.tools.FunctionTool;
@@ -252,7 +254,7 @@ protected Runner(
     this.memoryService = memoryService;
     this.pluginManager = new PluginManager(plugins);
     this.resumabilityConfig = resumabilityConfig;
-    this.eventsCompactionConfig = eventsCompactionConfig;
+    this.eventsCompactionConfig = createEventsCompactionConfig(agent, eventsCompactionConfig);
   }
 
   /**
@@ -786,5 +788,26 @@ private boolean hasLiveRequestQueueParameter(FunctionTool functionTool) {
         .anyMatch(parameter -> parameter.getType().equals(LiveRequestQueue.class));
   }
 
+  @Nullable
+  private static EventsCompactionConfig createEventsCompactionConfig(
+      BaseAgent agent, @Nullable EventsCompactionConfig config) {
+    if (config == null || config.summarizer() != null) {
+      return config;
+    }
+    LlmEventSummarizer summarizer =
+        Optional.of(agent)
+            .filter(LlmAgent.class::isInstance)
+            .map(LlmAgent.class::cast)
+            .flatMap(LlmAgent::model)
+            .flatMap(Model::model)
+            .map(LlmEventSummarizer::new)
+            .orElseThrow(
+                () ->
+                    new IllegalArgumentException(
+                        "No BaseLlm model available for event compaction"));
+    return new EventsCompactionConfig(
+        config.compactionInterval(), config.overlapSize(), summarizer);
+  }
+
   // TODO: run statelessly
 }

core/src/main/java/com/google/adk/summarizer/EventsCompactionConfig.java

@@ -16,6 +16,8 @@
 
 package com.google.adk.summarizer;
 
+import javax.annotation.Nullable;
+
 /**
  * Configuration for event compaction.
  *
@@ -27,4 +29,9 @@
  * @param summarizer An event summarizer to use for compaction.
  */
 public record EventsCompactionConfig(
-    int compactionInterval, int overlapSize, BaseEventSummarizer summarizer) {}
+    int compactionInterval, int overlapSize, @Nullable BaseEventSummarizer summarizer) {
+
+  public EventsCompactionConfig(int compactionInterval, int overlapSize) {
+    this(compactionInterval, overlapSize, null);
+  }
+}

core/src/main/java/com/google/adk/summarizer/SlidingWindowEventCompactor.java

@@ -16,6 +16,8 @@
 
 package com.google.adk.summarizer;
 
+import static com.google.common.base.Preconditions.checkArgument;
+
 import com.google.adk.events.Event;
 import com.google.adk.events.EventCompaction;
 import com.google.adk.sessions.BaseSessionService;
@@ -97,11 +99,13 @@ public SlidingWindowEventCompactor(EventsCompactionConfig config) {
    */
   @Override
   public Completable compact(Session session, BaseSessionService sessionService) {
+    BaseEventSummarizer summarizer = config.summarizer();
+    checkArgument(summarizer != null, "Missing BaseEventSummarizer for event compaction");
     logger.debug("Running event compaction for session {}", session.id());
 
     return Completable.fromMaybe(
         getCompactionEvents(session)
-            .flatMap(config.summarizer()::summarizeEvents)
+            .flatMap(summarizer::summarizeEvents)
             .flatMapSingle(e -> sessionService.appendEvent(session, e)));
   }
 

core/src/test/java/com/google/adk/runner/RunnerTest.java

@@ -40,7 +40,6 @@
 import com.google.adk.plugins.BasePlugin;
 import com.google.adk.sessions.Session;
 import com.google.adk.summarizer.EventsCompactionConfig;
-import com.google.adk.summarizer.LlmEventSummarizer;
 import com.google.adk.testing.TestLlm;
 import com.google.adk.testing.TestUtils;
 import com.google.adk.testing.TestUtils.EchoTool;
@@ -134,8 +133,7 @@ public void eventsCompaction_enabled() {
 
     Runner runner =
         Runner.builder()
-            .eventsCompactionConfig(
-                new EventsCompactionConfig(1, 0, new LlmEventSummarizer(testLlm)))
+            .eventsCompactionConfig(new EventsCompactionConfig(1, 0))
             .agent(agent)
             .sessionService(this.runner.sessionService())
             .appName(this.runner.appName())