diff --git a/src/google/adk/apps/llm_event_summarizer.py b/src/google/adk/apps/llm_event_summarizer.py
index 8081dd4f13f..9916c0cace0 100644
--- a/src/google/adk/apps/llm_event_summarizer.py
+++ b/src/google/adk/apps/llm_event_summarizer.py
@@ -13,7 +13,10 @@
 # limitations under the License.
 from __future__ import annotations
 
+import logging
+
 from google.genai.types import Content
+from google.genai.types import FinishReason
 from google.genai.types import Part
 
 from ..apps.base_events_summarizer import BaseEventsSummarizer
@@ -24,6 +27,16 @@
 from ..models.llm_request import LlmRequest
 from ..utils.context_utils import Aclosing
 
+logger = logging.getLogger('google_adk.' + __name__)
+
+# A summary that ended for any other reason (e.g. MAX_TOKENS or SAFETY) is
+# incomplete and must not replace the events it was meant to summarize.
+_COMPLETE_FINISH_REASONS = (
+    None,
+    FinishReason.STOP,
+    FinishReason.FINISH_REASON_UNSPECIFIED,
+)
+
 
 class LlmEventSummarizer(BaseEventsSummarizer):
   """An LLM-based event summarizer for sliding window compaction.
@@ -150,6 +163,7 @@ async def maybe_summarize_events(
     )
     summary_content = None
     summary_usage_metadata = None
+    summary_finish_reason = None
     async with Aclosing(
         self._llm.generate_content_async(llm_request, stream=False)
     ) as agen:
@@ -157,11 +171,20 @@ async def maybe_summarize_events(
         if llm_response.content:
           summary_content = llm_response.content
           summary_usage_metadata = llm_response.usage_metadata
+          summary_finish_reason = llm_response.finish_reason
           break
 
     if summary_content is None:
       return None
 
+    if summary_finish_reason not in _COMPLETE_FINISH_REASONS:
+      logger.warning(
+          'Discarding summary that ended with finish reason %s; keeping the'
+          ' uncompacted history.',
+          summary_finish_reason,
+      )
+      return None
+
     # Ensure the compacted content has the role 'model'
     summary_content.role = 'model'
 
diff --git a/tests/unittests/apps/test_llm_event_summarizer.py b/tests/unittests/apps/test_llm_event_summarizer.py
index 2442167bb18..6d805154fb3 100644
--- a/tests/unittests/apps/test_llm_event_summarizer.py
+++ b/tests/unittests/apps/test_llm_event_summarizer.py
@@ -118,6 +118,64 @@ async def async_gen():
     compacted_event = await self.compactor.maybe_summarize_events(events=events)
     self.assertIsNone(compacted_event)
 
+  async def test_maybe_compact_events_rejects_incomplete_summary(self):
+    events = [
+        self._create_event(1.0, 'The delivery address is 123 Maple Avenue.'),
+        self._create_event(2.0, 'Saved.', 'model'),
+    ]
+    for finish_reason in (
+        types.FinishReason.MAX_TOKENS,
+        types.FinishReason.SAFETY,
+        types.FinishReason.OTHER,
+    ):
+      with self.subTest(finish_reason=finish_reason):
+        llm_response = LlmResponse(
+            content=Content(parts=[Part(text='The delivery address is')]),
+            finish_reason=finish_reason,
+        )
+
+        async def async_gen(response=llm_response):
+          yield response
+
+        self.mock_llm.generate_content_async.return_value = async_gen()
+
+        compacted_event = await self.compactor.maybe_summarize_events(
+            events=events
+        )
+
+        self.assertIsNone(compacted_event)
+
+  async def test_maybe_compact_events_accepts_complete_summary(self):
+    events = [
+        self._create_event(1.0, 'Hello', 'user'),
+        self._create_event(2.0, 'Hi there!', 'model'),
+    ]
+    for finish_reason in (
+        None,
+        types.FinishReason.STOP,
+        types.FinishReason.FINISH_REASON_UNSPECIFIED,
+    ):
+      with self.subTest(finish_reason=finish_reason):
+        llm_response = LlmResponse(
+            content=Content(parts=[Part(text='Summary')]),
+            finish_reason=finish_reason,
+        )
+
+        async def async_gen(response=llm_response):
+          yield response
+
+        self.mock_llm.generate_content_async.return_value = async_gen()
+
+        compacted_event = await self.compactor.maybe_summarize_events(
+            events=events
+        )
+
+        self.assertIsNotNone(compacted_event)
+        self.assertEqual(
+            compacted_event.actions.compaction.compacted_content.parts[0].text,
+            'Summary',
+        )
+
   async def test_maybe_compact_events_includes_usage_metadata(self):
     events = [
         self._create_event(1.0, 'Hello', 'user'),