diff --git a/gateway/session.py b/gateway/session.py
index b964adddb7aa..5b71e1581f26 100644
--- a/gateway/session.py
+++ b/gateway/session.py
@@ -1582,6 +1582,19 @@ class SessionStore:
                         "Session DB expiry_finalized write failed for %s: %s",
                         entry.session_id, exc,
                     )
+            try:
+                # Expiry finalization is a real conversation boundary. Without
+                # a durable ``session_reset`` end_reason, later agent cleanup can
+                # close the row as ``agent_close``; stale-route recovery treats
+                # that as resumable and resurrects the expired full history.
+                # Reopen first because SessionDB.end_session is first-writer-wins.
+                self._db.reopen_session(entry.session_id)
+                self._db.end_session(entry.session_id, "session_reset")
+            except Exception as exc:
+                logger.debug(
+                    "Session DB end_session(session_reset) failed for %s: %s",
+                    entry.session_id, exc,
+                )
     
     def _is_session_expired(self, entry: SessionEntry) -> bool:
         """Check if a session has expired based on its reset policy.
diff --git a/tests/gateway/test_session_store_expiry_finalized.py b/tests/gateway/test_session_store_expiry_finalized.py
new file mode 100644
index 000000000000..f777eca2ebb5
--- /dev/null
+++ b/tests/gateway/test_session_store_expiry_finalized.py
@@ -0,0 +1,67 @@
+"""Session expiry finalization closes sessions as session_reset.
+
+Regression coverage for #61220: the expiry watcher marks a session expired,
+then agent cleanup can close it as ``agent_close``. Stale routing recovery treats
+``agent_close`` as recoverable, so expired sessions were reopened with full
+history unless expiry finalization also persisted the real conversation boundary
+as ``end_reason='session_reset'``.
+"""
+
+from datetime import datetime, timedelta
+from unittest.mock import MagicMock, patch
+
+from gateway.config import GatewayConfig, Platform, SessionResetPolicy
+from gateway.session import SessionEntry, SessionStore
+
+
+def _make_store_with_db(tmp_path, db_mock) -> SessionStore:
+    config = GatewayConfig(default_reset_policy=SessionResetPolicy(mode="daily"))
+    with patch("gateway.session.SessionStore._ensure_loaded"):
+        store = SessionStore(sessions_dir=tmp_path, config=config)
+    store._db = db_mock
+    store._loaded = True
+    return store
+
+
+def _entry(session_id: str = "sid-expired") -> SessionEntry:
+    now = datetime.now()
+    return SessionEntry(
+        session_key="agent:main:telegram:dm:8494508720",
+        session_id=session_id,
+        created_at=now - timedelta(days=1),
+        updated_at=now - timedelta(days=1),
+        platform=Platform.TELEGRAM,
+        chat_type="dm",
+        model_override={"provider": "openrouter", "model": "test/model"},
+    )
+
+
+def test_set_expiry_finalized_persists_session_reset_boundary(tmp_path):
+    db = MagicMock()
+    db.set_expiry_finalized.return_value = None
+    db.reopen_session.return_value = None
+    db.end_session.return_value = None
+    store = _make_store_with_db(tmp_path, db)
+    entry = _entry()
+
+    store.set_expiry_finalized(entry)
+
+    assert entry.expiry_finalized is True
+    assert entry.model_override is None
+    db.set_expiry_finalized.assert_called_once_with("sid-expired", True)
+    db.reopen_session.assert_called_once_with("sid-expired")
+    db.end_session.assert_called_once_with("sid-expired", "session_reset")
+
+
+def test_set_expiry_finalized_still_sets_flag_if_end_session_fails(tmp_path):
+    db = MagicMock()
+    db.set_expiry_finalized.return_value = None
+    db.reopen_session.side_effect = RuntimeError("database locked")
+    store = _make_store_with_db(tmp_path, db)
+    entry = _entry()
+
+    store.set_expiry_finalized(entry)
+
+    assert entry.expiry_finalized is True
+    db.set_expiry_finalized.assert_called_once_with("sid-expired", True)
+    db.end_session.assert_not_called()