diff --git a/core/src/main/java/com/google/adk/summarizer/TailRetentionEventCompactor.java b/core/src/main/java/com/google/adk/summarizer/TailRetentionEventCompactor.java
index 4159ad2cd..2945c9a85 100644
--- a/core/src/main/java/com/google/adk/summarizer/TailRetentionEventCompactor.java
+++ b/core/src/main/java/com/google/adk/summarizer/TailRetentionEventCompactor.java
@@ -22,7 +22,6 @@
 import com.google.adk.events.EventCompaction;
 import com.google.adk.sessions.BaseSessionService;
 import com.google.adk.sessions.Session;
-import com.google.common.collect.Lists;
 import com.google.genai.types.GenerateContentResponseUsageMetadata;
 import io.reactivex.rxjava3.core.Completable;
 import io.reactivex.rxjava3.core.Maybe;
@@ -41,7 +40,10 @@
  *   <li>Keeps the {@code retentionSize} most recent events raw.
  *   <li>Compacts all events that never compacted and older than the retained tail, including the
  *       most recent compaction event, into a new summary event.
- *   <li>Triggers compaction only if the prompt token count exceeds the {@code tokenThreshold}.
+ *   <li>Triggers compaction only if the estimated prompt token count exceeds the {@code
+ *       tokenThreshold}: the last reported prompt token count plus about one token per four
+ *       characters of the response that reported it and the events after it. Without a count
+ *       reported since the latest compaction, all events since then are estimated this way.
  *   <li>The new summary event is generated by the {@link BaseEventSummarizer}.
  *   <li>Appends this new summary event to the end of the event stream.
  * </ul>
@@ -140,10 +142,11 @@ public Completable compact(Session session, BaseSessionService sessionService) {
    * @param events The list of events to process.
    */
   private Maybe<List<Event>> getCompactionEvents(List<Event> events) {
-    Optional<Integer> count = getLatestPromptTokenCount(events);
+    Optional<Integer> count = getEstimatedPromptTokenCount(events);
     if (count.isPresent() && count.get() <= tokenThreshold) {
       logger.debug(
-          "Skipping compaction. Prompt token count {} is within threshold {}",
+          "Skipping compaction. Prompt token count {} (last reported plus estimate) is within"
+              + " threshold {}",
           count.get(),
           tokenThreshold);
       return Maybe.empty();
@@ -226,13 +229,28 @@ private int estimateTokenCount(List<Event> events) {
     return events.stream().mapToInt(event -> event.stringifyContent().length()).sum() / 4;
   }
 
-  private Optional<Integer> getLatestPromptTokenCount(List<Event> events) {
-    return Lists.reverse(events).stream()
-        .map(Event::usageMetadata)
-        .flatMap(Optional::stream)
-        .map(GenerateContentResponseUsageMetadata::promptTokenCount)
-        .flatMap(Optional::stream)
-        .findFirst();
+  /**
+   * Returns the latest reported prompt token count plus an estimate for the events it does not
+   * cover, or empty if there is no such count after the latest compaction.
+   */
+  private Optional<Integer> getEstimatedPromptTokenCount(List<Event> events) {
+    ListIterator<Event> iter = events.listIterator(events.size());
+    while (iter.hasPrevious()) {
+      Event event = iter.previous();
+      if (isCompactEvent(event)) {
+        // A count reported before a compaction describes a prompt that the compaction replaced.
+        return Optional.empty();
+      }
+      Optional<Integer> promptTokenCount =
+          event.usageMetadata().flatMap(GenerateContentResponseUsageMetadata::promptTokenCount);
+      if (promptTokenCount.isPresent()) {
+        // The count excludes this response and every event appended after it.
+        return Optional.of(
+            promptTokenCount.get()
+                + estimateTokenCount(events.subList(iter.nextIndex(), events.size())));
+      }
+    }
+    return Optional.empty();
   }
 
   private static boolean isCompactEvent(Event event) {
diff --git a/core/src/test/java/com/google/adk/flows/llmflows/CompactionTest.java b/core/src/test/java/com/google/adk/flows/llmflows/CompactionTest.java
index 3ceba5641..a5017beb6 100644
--- a/core/src/test/java/com/google/adk/flows/llmflows/CompactionTest.java
+++ b/core/src/test/java/com/google/adk/flows/llmflows/CompactionTest.java
@@ -31,7 +31,10 @@
 import com.google.adk.summarizer.BaseEventSummarizer;
 import com.google.adk.summarizer.EventsCompactionConfig;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import com.google.genai.types.Content;
 import com.google.genai.types.GenerateContentResponseUsageMetadata;
+import com.google.genai.types.Part;
 import io.reactivex.rxjava3.core.Maybe;
 import io.reactivex.rxjava3.core.Single;
 import java.util.Optional;
@@ -82,13 +85,9 @@ public void processRequest_withConfig_triggersCompaction() {
     when(context.eventsCompactionConfig()).thenReturn(Optional.of(config));
 
     // Setup events with usage > 100 to trigger compaction
-    Event event1 = mock(Event.class);
-    Event event2 = mock(Event.class);
-    Event event3 = mock(Event.class);
-    when(event3.usageMetadata())
-        .thenReturn(
-            Optional.of(
-                GenerateContentResponseUsageMetadata.builder().promptTokenCount(200).build()));
+    Event event1 = Event.builder().timestamp(1).build();
+    Event event2 = Event.builder().timestamp(2).build();
+    Event event3 = eventWithPromptTokenCount(3, 200);
 
     session =
         Session.builder("test-session").events(ImmutableList.of(event1, event2, event3)).build();
@@ -117,11 +116,7 @@ public void processRequest_withConfig_skipsCompactionIfBelowThreshold() {
     when(context.eventsCompactionConfig()).thenReturn(Optional.of(config));
 
     // Setup events with usage 200 (below 500)
-    Event event3 = mock(Event.class);
-    when(event3.usageMetadata())
-        .thenReturn(
-            Optional.of(
-                GenerateContentResponseUsageMetadata.builder().promptTokenCount(200).build()));
+    Event event3 = eventWithPromptTokenCount(3, 200);
 
     session = Session.builder("test-session").events(ImmutableList.of(event3)).build();
     when(context.session()).thenReturn(session);
@@ -137,6 +132,41 @@ public void processRequest_withConfig_skipsCompactionIfBelowThreshold() {
     verify(sessionService, never()).appendEvent(any(), any());
   }
 
+  @Test
+  public void processRequest_withConfig_toolResponseAfterTokenUsage_triggersCompaction() {
+    // Threshold 300: the reported count of 200 alone is within it, but the function response
+    // appended after it (an 800-character result, over 200 tokens) pushes the estimate above.
+    EventsCompactionConfig config = new EventsCompactionConfig(5, 1, summarizer, 300, 2);
+    when(context.eventsCompactionConfig()).thenReturn(Optional.of(config));
+
+    Event event1 = Event.builder().timestamp(1).build();
+    Event event2 = eventWithPromptTokenCount(2, 200);
+    Event event3 =
+        Event.builder()
+            .timestamp(3)
+            .content(
+                Content.fromParts(
+                    Part.fromFunctionResponse("tool", ImmutableMap.of("result", "x".repeat(800)))))
+            .build();
+
+    session =
+        Session.builder("test-session").events(ImmutableList.of(event1, event2, event3)).build();
+    when(context.session()).thenReturn(session);
+
+    Event summaryEvent = mock(Event.class);
+    when(summarizer.summarizeEvents(any())).thenReturn(Maybe.just(summaryEvent));
+    when(sessionService.appendEvent(any(), any())).thenReturn(Single.just(summaryEvent));
+
+    Compaction compaction = new Compaction();
+    compaction
+        .processRequest(context, request)
+        .test()
+        .assertNoErrors()
+        .assertValue(r -> r.updatedRequest() == request);
+
+    verify(sessionService).appendEvent(eq(session), eq(summaryEvent));
+  }
+
   @Test
   public void processRequest_withConfig_nullRetentionSize_doesNothing() {
     // Setup config with retentionSize = null
@@ -154,4 +184,14 @@ public void processRequest_withConfig_nullRetentionSize_doesNothing() {
     verify(sessionService, never()).appendEvent(any(), any());
     verify(context, never()).session();
   }
+
+  private static Event eventWithPromptTokenCount(long timestamp, int promptTokenCount) {
+    return Event.builder()
+        .timestamp(timestamp)
+        .usageMetadata(
+            GenerateContentResponseUsageMetadata.builder()
+                .promptTokenCount(promptTokenCount)
+                .build())
+        .build();
+  }
 }
diff --git a/core/src/test/java/com/google/adk/summarizer/TailRetentionEventCompactorTest.java b/core/src/test/java/com/google/adk/summarizer/TailRetentionEventCompactorTest.java
index 7a4a3ddb9..c3d8840a4 100644
--- a/core/src/test/java/com/google/adk/summarizer/TailRetentionEventCompactorTest.java
+++ b/core/src/test/java/com/google/adk/summarizer/TailRetentionEventCompactorTest.java
@@ -30,6 +30,7 @@
 import com.google.adk.sessions.BaseSessionService;
 import com.google.adk.sessions.Session;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.genai.types.Content;
 import com.google.genai.types.GenerateContentResponseUsageMetadata;
 import com.google.genai.types.Part;
@@ -160,6 +161,118 @@ public void compaction_happensWhenTokenUsageAboveThreshold() {
     verify(mockSessionService).appendEvent(eq(session), eq(summaryEvent));
   }
 
+  @Test
+  public void compaction_happensWhenEventsAfterTokenUsagePushAboveThreshold() {
+    // Threshold is 300, the last model call used 200 prompt tokens, and a tool response of
+    // 800 characters (about 200 tokens) arrived after it: 200 + 200 > 300.
+    EventCompactor compactor = new TailRetentionEventCompactor(mockSummarizer, 1, 300);
+    ImmutableList<Event> events =
+        ImmutableList.of(
+            createEvent(1, "Event1"),
+            withUsage(createEvent(2, "Call"), 200),
+            createEvent(3, "x".repeat(800)));
+    Session session = Session.builder("id").events(events).build();
+    Event summaryEvent = createEvent(4, "Summary");
+
+    when(mockSummarizer.summarizeEvents(any())).thenReturn(Maybe.just(summaryEvent));
+    when(mockSessionService.appendEvent(any(), any())).thenReturn(Single.just(summaryEvent));
+
+    compactor.compact(session, mockSessionService).blockingSubscribe();
+
+    verify(mockSummarizer).summarizeEvents(eventListCaptor.capture());
+    assertThat(
+            eventListCaptor.getValue().stream().map(TailRetentionEventCompactorTest::getPromptText))
+        .containsExactly("Event1", "Call")
+        .inOrder();
+    verify(mockSessionService).appendEvent(eq(session), eq(summaryEvent));
+  }
+
+  @Test
+  public void compaction_happensWhenToolResponseAfterTokenUsagePushesAboveThreshold() {
+    // Threshold is 300, the last model call used 200 prompt tokens, and a function response with
+    // an 800-character result (over 200 tokens once stringified) arrived after it.
+    EventCompactor compactor = new TailRetentionEventCompactor(mockSummarizer, 1, 300);
+    ImmutableList<Event> events =
+        ImmutableList.of(
+            createEvent(1, "Event1"),
+            withUsage(createEvent(2, "Call"), 200),
+            createToolResponseEvent(3, "x".repeat(800)));
+    Session session = Session.builder("id").events(events).build();
+    Event summaryEvent = createEvent(4, "Summary");
+
+    when(mockSummarizer.summarizeEvents(any())).thenReturn(Maybe.just(summaryEvent));
+    when(mockSessionService.appendEvent(any(), any())).thenReturn(Single.just(summaryEvent));
+
+    compactor.compact(session, mockSessionService).blockingSubscribe();
+
+    verify(mockSummarizer).summarizeEvents(eventListCaptor.capture());
+    assertThat(
+            eventListCaptor.getValue().stream().map(TailRetentionEventCompactorTest::getPromptText))
+        .containsExactly("Event1", "Call")
+        .inOrder();
+    verify(mockSessionService).appendEvent(eq(session), eq(summaryEvent));
+  }
+
+  @Test
+  public void compaction_skippedWhenEventsAfterTokenUsageStayBelowThreshold() {
+    // Threshold is 300, the last model call used 200 prompt tokens, and a tool response of
+    // 200 characters (about 50 tokens) arrived after it: 200 + 50 <= 300.
+    EventCompactor compactor = new TailRetentionEventCompactor(mockSummarizer, 1, 300);
+    ImmutableList<Event> events =
+        ImmutableList.of(
+            createEvent(1, "Event1"),
+            withUsage(createEvent(2, "Call"), 200),
+            createEvent(3, "x".repeat(200)));
+    Session session = Session.builder("id").events(events).build();
+
+    compactor.compact(session, mockSessionService).blockingSubscribe();
+
+    verify(mockSummarizer, never()).summarizeEvents(any());
+    verify(mockSessionService, never()).appendEvent(any(), any());
+  }
+
+  @Test
+  public void compaction_countsTheResponseThatReportedTokenUsage() {
+    // promptTokenCount excludes the response that reports it. Its 40 characters (~10 tokens) are
+    // what push 200 + 100 (the 400-character tool response) past the threshold of 300.
+    EventCompactor compactor = new TailRetentionEventCompactor(mockSummarizer, 1, 300);
+    ImmutableList<Event> events =
+        ImmutableList.of(
+            createEvent(1, "Event1"),
+            withUsage(createEvent(2, "y".repeat(40)), 200),
+            createEvent(3, "x".repeat(400)));
+    Session session = Session.builder("id").events(events).build();
+    Event summaryEvent = createEvent(4, "Summary");
+
+    when(mockSummarizer.summarizeEvents(any())).thenReturn(Maybe.just(summaryEvent));
+    when(mockSessionService.appendEvent(any(), any())).thenReturn(Single.just(summaryEvent));
+
+    compactor.compact(session, mockSessionService).blockingSubscribe();
+
+    verify(mockSummarizer).summarizeEvents(any());
+  }
+
+  @Test
+  public void compaction_ignoresTokenUsageReportedBeforeLatestCompaction() {
+    // The 1000-token count describes the prompt that the compaction replaced. What remains
+    // ("Summary", "Retain1", "Retain2") is ~5 estimated tokens, within the threshold of 300.
+    EventCompactor compactor = new TailRetentionEventCompactor(mockSummarizer, 1, 300);
+    ImmutableList<Event> events =
+        ImmutableList.of(
+            createEvent(1, "Event1"),
+            withUsage(createEvent(2, "Call"), 1000),
+            createEvent(3, "Retain1"),
+            createCompactedEvent(
+                /* startTimestamp= */ 1, /* endTimestamp= */ 2, "Summary", /* eventTimestamp= */ 4),
+            createEvent(5, "Retain2"));
+    Session session = Session.builder("id").events(events).build();
+
+    compactor.compact(session, mockSessionService).blockingSubscribe();
+
+    verify(mockSummarizer, never()).summarizeEvents(any());
+    verify(mockSessionService, never()).appendEvent(any(), any());
+  }
+
   @Test
   public void compact_notEnoughEvents_doesNothing() {
     ImmutableList<Event> events =
@@ -330,6 +443,14 @@ private static Event createEvent(long timestamp, String text) {
         .build();
   }
 
+  private static Event createToolResponseEvent(long timestamp, String result) {
+    return Event.builder()
+        .timestamp(timestamp)
+        .content(
+            Content.fromParts(Part.fromFunctionResponse("tool", ImmutableMap.of("result", result))))
+        .build();
+  }
+
   private static String getPromptText(Event event) {
     return event
         .content()