fix(gateway): stop internal bookkeeping writes from advancing the session activity clock

set_session_metadata() and advance_compression_session()'s repoint both
stamped entry.updated_at = now. updated_at is the user-activity clock that
drives idle/daily reset policy and the restart-resume freshness gate
(suspend_recently_active, #85709), so a background metadata write (e.g.
Slack thread watermark) or a background compression repoint on a long-idle
session could make it look freshly active and get it falsely resume_pending
after a gateway restart.

These are the last internal stamp sites after 784f733cf (recover) and
5462f689b (touch_activity gating): drop the stamps, keep the durable save.

Follow-up to #85895 (closed) — credit @GodsBoy for the report-side push and
@chelsealong for the analysis on #85709.

gateway/session.py

@@ -2895,6 +2895,12 @@ class SessionStore:
         Values must be small and JSON-serializable — they are written into
         the routing index (state.db gateway_routing table + the legacy
         sessions.json mirror) so they survive gateway restarts.
+
+        Metadata writes are internal bookkeeping and deliberately do NOT
+        advance ``updated_at``: it is the user-activity clock that drives
+        idle/daily reset policy and the restart-resume freshness gate
+        (#85709), and a background write must not make an idle session look
+        fresh.
         """
         with self._lock:
             self._ensure_loaded_locked()
@@ -2902,7 +2908,6 @@ class SessionStore:
             if entry is None:
                 return False
             entry.metadata[key] = value
-            entry.updated_at = _now()
             self._save()
             return True
 
@@ -3349,7 +3354,10 @@ class SessionStore:
                 target_session_id,
             ):
                 return None
-            entry.updated_at = _now()
+            # Compression repoint is store bookkeeping, not user activity —
+            # leave ``updated_at`` alone so a background compression on an
+            # idle session cannot make it look fresh to reset policy or the
+            # restart-resume freshness gate (#85709).
             self._save()
             return entry
 

tests/gateway/test_session.py

@@ -2,7 +2,7 @@
 import json
 import pytest
 from dataclasses import replace
-from datetime import datetime
+from datetime import datetime, timedelta
 from pathlib import Path
 from unittest.mock import patch, MagicMock
 from hermes_state import SessionDB
@@ -1291,6 +1291,34 @@ class TestSessionMetadata:
             == "123.456"
         )
 
+    def test_metadata_write_does_not_touch_activity_clock(self, tmp_path):
+        """set_session_metadata is bookkeeping — it must not bump updated_at.
+
+        updated_at drives idle/daily reset policy and the restart-resume
+        freshness gate (#85709); a background metadata write on an idle
+        session must not make it look recently active.
+        """
+        config = GatewayConfig()
+        store = SessionStore(sessions_dir=tmp_path, config=config)
+        store._db = None
+        source = SessionSource(
+            platform=Platform.SLACK,
+            chat_id="C123",
+            chat_type="group",
+            user_id="U123",
+            thread_id="123.000",
+        )
+
+        entry = store.get_or_create_session(source)
+        idle = datetime.now() - timedelta(days=21)
+        with store._lock:
+            entry.updated_at = idle
+
+        assert store.set_session_metadata(entry.session_key, "k", "v")
+        assert entry.updated_at == idle
+        # And the restart freshness gate must still see it as idle.
+        assert store.suspend_recently_active(max_age_seconds=120) == 0
+
 
 class TestRewriteTranscriptPreservesReasoning:
     """rewrite_transcript must not drop reasoning fields from SQLite."""

tests/gateway/test_session_store_runtime_stale_guard.py

@@ -292,4 +292,25 @@ class TestAdvanceCompressionSession:
         db.end_session.assert_not_called()
         db.reopen_session.assert_not_called()
 
+    def test_repoint_does_not_touch_activity_clock(self, tmp_path):
+        """Compression repoint is bookkeeping — it must not bump updated_at.
+
+        A background compression on an idle session must not make it look
+        fresh to reset policy or the restart-resume freshness gate (#85709).
+        """
+        db = _db_returning({})
+        store = _make_store_with_db(tmp_path, db)
+        source = _source()
+        key = store._generate_session_key(source)
+        original = _make_entry(key, "sid_parent")
+        idle = datetime.now() - timedelta(days=21)
+        original.updated_at = idle
+        store._entries[key] = original
+
+        result = store.advance_compression_session(key, "sid_parent", "sid_tip")
+
+        assert result is not None
+        assert result.updated_at == idle
+        assert store.suspend_recently_active(max_age_seconds=120) == 0
+