diff --git a/src/agents/memory/openai_responses_compaction_session.py b/src/agents/memory/openai_responses_compaction_session.py
index fed2d89ff4..f716b5be10 100644
--- a/src/agents/memory/openai_responses_compaction_session.py
+++ b/src/agents/memory/openai_responses_compaction_session.py
@@ -508,7 +508,19 @@ async def clear_session(self) -> None:
         async with self._mutation_lock:
             try:
                 await self.underlying_session.clear_session()
-            except (Exception, asyncio.CancelledError):
+            except asyncio.CancelledError:
+                # Some session backends shield mutations so a cancellation can be
+                # observed only after the clear has committed. Treat cancellation as
+                # potentially destructive and forget the response chain as well as
+                # cached history, so cleared history cannot be reconstructed later.
+                self._compaction_candidate_items = None
+                self._session_items = None
+                self._response_id = None
+                self._deferred_response_id = None
+                self._last_unstored_response_id = None
+                self._mutation_generation += 1
+                raise
+            except Exception:
                 self._compaction_candidate_items = None
                 self._session_items = None
                 self._deferred_response_id = None
@@ -516,7 +528,9 @@ async def clear_session(self) -> None:
                 raise
             self._compaction_candidate_items = []
             self._session_items = []
+            self._response_id = None
             self._deferred_response_id = None
+            self._last_unstored_response_id = None
             self._mutation_generation += 1
 
     async def _ensure_compaction_candidates(
diff --git a/tests/memory/test_openai_responses_compaction_session.py b/tests/memory/test_openai_responses_compaction_session.py
index 7f8895a31c..cf13aad57c 100644
--- a/tests/memory/test_openai_responses_compaction_session.py
+++ b/tests/memory/test_openai_responses_compaction_session.py
@@ -100,6 +100,51 @@ def test_excludes_easy_user_messages_without_type(self) -> None:
 
 
 class TestOpenAIResponsesCompactionSession:
+    @pytest.mark.asyncio
+    async def test_clear_session_forgets_response_chain_state(self) -> None:
+        mock_session = self.create_mock_session()
+        mock_client = MagicMock()
+        mock_client.responses.compact = AsyncMock()
+        session = OpenAIResponsesCompactionSession(
+            session_id="test",
+            underlying_session=mock_session,
+            client=mock_client,
+            should_trigger_compaction=lambda _context: False,
+        )
+
+        await session.run_compaction({"response_id": "resp-old", "store": True})
+        await session.clear_session()
+
+        with pytest.raises(ValueError, match="requires a response_id"):
+            await session.run_compaction({"force": True, "compaction_mode": "previous_response_id"})
+        mock_client.responses.compact.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_cancelled_clear_forgets_response_chain_state(self) -> None:
+        class CancelAfterCommittedClearSession(SimpleListSession):
+            async def clear_session(self) -> None:
+                await super().clear_session()
+                raise asyncio.CancelledError()
+
+        underlying = CancelAfterCommittedClearSession()
+        mock_client = MagicMock()
+        mock_client.responses.compact = AsyncMock()
+        session = OpenAIResponsesCompactionSession(
+            session_id="test",
+            underlying_session=underlying,
+            client=mock_client,
+            should_trigger_compaction=lambda _context: False,
+        )
+
+        await session.run_compaction({"response_id": "resp-old", "store": True})
+        with pytest.raises(asyncio.CancelledError):
+            await session.clear_session()
+
+        assert await underlying.get_items() == []
+        with pytest.raises(ValueError, match="requires a response_id"):
+            await session.run_compaction({"force": True, "compaction_mode": "previous_response_id"})
+        mock_client.responses.compact.assert_not_called()
+
     def test_client_preserves_falsy_default_client(self) -> None:
         mock_client = MagicMock()
         mock_client.__bool__.return_value = False