diff --git a/nanobot/cli/commands.py b/nanobot/cli/commands.py
index aab27242..d11b5b67 100644
--- a/nanobot/cli/commands.py
+++ b/nanobot/cli/commands.py
@@ -1065,8 +1065,15 @@ def _run_gateway(
     def _pick_heartbeat_target() -> tuple[str, str]:
         """Pick a routable channel/chat target for heartbeat-triggered messages."""
         enabled = set(channels.enabled_channels)
+
+        from nanobot.webui.sidebar_state import read_webui_sidebar_state
+        sidebar_state = read_webui_sidebar_state()
+        archived_keys = set(sidebar_state.get("archived_keys", []))
+
         for item in session_manager.list_sessions():
             key = item.get("key") or ""
+            if key in archived_keys:
+                continue
             if ":" not in key:
                 continue
             channel, chat_id = key.split(":", 1)
diff --git a/nanobot/session/manager.py b/nanobot/session/manager.py
index c951c651..da62f0f0 100644
--- a/nanobot/session/manager.py
+++ b/nanobot/session/manager.py
@@ -843,11 +843,13 @@ class SessionManager:
                                 if not fallback_preview and item.get("role") == "assistant":
                                     fallback_preview = text
                             preview = preview or fallback_preview
+                            from datetime import datetime
+                            fallback_time = datetime.fromtimestamp(path.stat().st_mtime).isoformat()
                             sessions.append(
                                 {
                                     "key": key,
-                                    "created_at": data.get("created_at"),
-                                    "updated_at": data.get("updated_at"),
+                                    "created_at": data.get("created_at") or fallback_time,
+                                    "updated_at": data.get("updated_at") or fallback_time,
                                     "title": title,
                                     "preview": preview,
                                     "path": str(path),
diff --git a/nanobot/webui/session_list_index.py b/nanobot/webui/session_list_index.py
index aaadc97a..fdd64575 100644
--- a/nanobot/webui/session_list_index.py
+++ b/nanobot/webui/session_list_index.py
@@ -269,13 +269,20 @@ def _scan_session_row(session_manager: SessionManager, path: Path) -> dict[str,
                 if not fallback_preview and item.get("role") == "assistant":
                     fallback_preview = text
             signature = _file_signature(path)
+            created_at_s = data.get("created_at")
+            updated_at_s = data.get("updated_at")
+            if not created_at_s or not updated_at_s:
+                from datetime import datetime
+                fallback_time = datetime.fromtimestamp(signature["mtime_ns"] / 1e9).isoformat()
+                created_at_s = created_at_s or fallback_time
+                updated_at_s = updated_at_s or fallback_time
             key = data.get("key") or fallback_key
             activity_signature = _webui_activity_signature(key)
             activity_updated_at = _webui_activity_updated_at(activity_signature)
             return {
                 "key": key,
-                "created_at": data.get("created_at"),
-                "updated_at": _latest_updated_at(data.get("updated_at"), activity_updated_at),
+                "created_at": created_at_s,
+                "updated_at": _latest_updated_at(updated_at_s, activity_updated_at),
                 "title": _metadata_title(data.get("metadata", {})),
                 "preview": preview or fallback_preview,
                 "file": path.name,
diff --git a/tests/webui/test_session_list_index.py b/tests/webui/test_session_list_index.py
index cc10f8c1..e68f9eeb 100644
--- a/tests/webui/test_session_list_index.py
+++ b/tests/webui/test_session_list_index.py
@@ -171,3 +171,40 @@ def test_webui_session_list_rescans_when_transcript_changes(
 
 def list_webui_sessions(manager: SessionManager) -> list[dict]:
     return session_list_index.list_webui_sessions(manager)
+
+
+def test_webui_session_list_fallback_time_when_missing(tmp_path: Path) -> None:
+    manager = SessionManager(tmp_path)
+    path = manager._get_session_path("websocket:missing-time")
+    path.write_text(
+        '{"_type": "metadata", "key": "websocket:missing-time"}\n'
+        '{"_type": "message", "role": "user", "content": "hello"}\n',
+        encoding="utf-8",
+    )
+
+    rows = list_webui_sessions(manager)
+    assert len(rows) == 1
+    assert rows[0]["key"] == "websocket:missing-time"
+    assert rows[0]["created_at"] is not None
+    assert rows[0]["updated_at"] is not None
+    datetime.fromisoformat(rows[0]["created_at"])
+    datetime.fromisoformat(rows[0]["updated_at"])
+
+
+def test_session_manager_list_sessions_fallback_time_when_missing(tmp_path: Path) -> None:
+    manager = SessionManager(tmp_path)
+    path = manager._get_session_path("websocket:missing-time2")
+    path.write_text(
+        '{"_type": "metadata", "key": "websocket:missing-time2"}\n'
+        '{"_type": "message", "role": "user", "content": "hello"}\n',
+        encoding="utf-8",
+    )
+
+    sessions = manager.list_sessions()
+    assert len(sessions) == 1
+    assert sessions[0]["key"] == "websocket:missing-time2"
+    assert sessions[0]["created_at"] is not None
+    assert sessions[0]["updated_at"] is not None
+    datetime.fromisoformat(sessions[0]["created_at"])
+    datetime.fromisoformat(sessions[0]["updated_at"])
+