diff --git a/api/models.py b/api/models.py
index fd6557ec..d7210ae9 100644
--- a/api/models.py
+++ b/api/models.py
@@ -436,7 +436,14 @@ class Session:
         self.read_only = bool(kwargs.get('read_only', False))
         self.enabled_toolsets = enabled_toolsets  # List[str] or None — per-session toolset override
         self.composer_draft = composer_draft if isinstance(composer_draft, dict) else {}
-        self._metadata_message_count = None
+        raw_message_count = kwargs.get('message_count')
+        parsed_message_count = None
+        if raw_message_count is not None:
+            try:
+                parsed_message_count = int(raw_message_count)
+            except (TypeError, ValueError):
+                parsed_message_count = None
+        self._metadata_message_count = parsed_message_count if parsed_message_count is not None and parsed_message_count >= 0 else None
 
     @property
     def path(self):
@@ -601,7 +608,19 @@ class Session:
             parsed['messages'] = []
             parsed['tool_calls'] = []
             session = cls(**parsed)
-            session._metadata_message_count = _lookup_index_message_count(sid)
+            metadata_message_count = _lookup_index_message_count(sid)
+            if metadata_message_count is None:
+                raw_count = parsed.get('message_count')
+                if isinstance(raw_count, int) and raw_count >= 0:
+                    metadata_message_count = raw_count
+                else:
+                    try:
+                        parsed_count = int(raw_count)
+                    except (TypeError, ValueError):
+                        parsed_count = None
+                    if parsed_count is not None and parsed_count >= 0:
+                        metadata_message_count = parsed_count
+            session._metadata_message_count = metadata_message_count
             # Mark this session as a metadata-only stub. save() refuses to write
             # such a session because doing so would atomically replace the
             # on-disk JSON with messages=[], wiping the conversation. Any
diff --git a/api/routes.py b/api/routes.py
index ee01a3e3..81dda7cb 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -3721,6 +3721,18 @@ def handle_get(handler, parsed) -> bool:
                     _all_msgs = merge_session_messages_append_only(_metadata_sidecar, state_db_messages)
             if not load_messages:
                 _summary_message_count = len(_all_msgs)
+                if _summary_message_count == 0:
+                    # Legacy session with no loaded sidecar and no state.db summary —
+                    # fall back to the persisted metadata count from session JSON.
+                    # See PR #2605 (LumenYoung): without this, the metadata poll
+                    # returns 0 and the active-session external-refresh signal
+                    # never trips on legacy sessions.
+                    try:
+                        metadata_count = getattr(s, "_metadata_message_count", None)
+                        if metadata_count is not None:
+                            _summary_message_count = max(0, int(metadata_count))
+                    except (TypeError, ValueError):
+                        pass
                 try:
                     _summary_last_message_at = max(
                         float((m or {}).get("timestamp") or 0)
diff --git a/tests/test_webui_state_db_reconciliation.py b/tests/test_webui_state_db_reconciliation.py
index 5e0e17b2..a9748bab 100644
--- a/tests/test_webui_state_db_reconciliation.py
+++ b/tests/test_webui_state_db_reconciliation.py
@@ -135,6 +135,32 @@ def test_api_session_includes_state_db_messages_newer_than_webui_sidecar(monkeyp
     assert payload["session"]["message_count"] == 4
 
 
+def test_metadata_poll_uses_sidecar_message_count_for_external_updates(monkeypatch, tmp_path):
+    """Active-session external refresh relies on metadata-only counts.
+
+    When no session index exists, metadata-only loads may fall back to
+    _metadata_message_count=None. The refresh poll must still report the real
+    sidecar message count; otherwise an external session JSON update can be
+    invisible until a full reload.
+    """
+    import api.routes as routes
+
+    sid = "webui_reconcile_metadata_sidecar"
+    sidecar_messages = [
+        {"role": "user", "content": "before external update", "timestamp": 1000.0},
+        {"role": "assistant", "content": "externally appended", "timestamp": 1001.0},
+    ]
+    _install_test_session(monkeypatch, tmp_path, sid, sidecar_messages)
+
+    handler = _GetHandler(f"/api/session?session_id={sid}&messages=0&resolve_model=0")
+    routes.handle_get(handler, urlparse(handler.path))
+
+    assert handler.status == 200
+    session = handler.response_json["session"]
+    assert session["message_count"] == 2
+    assert session["last_message_at"] == 1001.0
+
+
 def test_state_db_reconciliation_preserves_sidecar_only_messages(monkeypatch, tmp_path):
     import api.routes as routes