From 10b2bebfafe5ccaceddda5a66d915169788747e4 Mon Sep 17 00:00:00 2001
From: HYYAlex <232531662+HYYAlex@users.noreply.github.com>
Date: Fri, 13 Nov 2026 04:02:36 -0800
Subject: [PATCH] fix(apps): keep history when the compaction summary ends
 early

LlmEventSummarizer accepted any summary response with content, even when
the model stopped with MAX_TOKENS or SAFETY. The truncated or refused text
was published as an EventCompaction and replaced the original events in
every later model request, so facts from the compacted range were lost.

A summary that ends with a finish reason other than STOP (or no reason) is
now treated as a failed compaction: it is logged and None is returned, as
the BaseEventsSummarizer contract specifies, so the uncompacted history is
kept.
---
 src/google/adk/apps/llm_event_summarizer.py   | 23 ++++++++
 .../apps/test_llm_event_summarizer.py         | 58 +++++++++++++++++++
 2 files changed, 81 insertions(+)

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'),