fix(telegram): preserve new DM topic lanes

gateway/run.py

@@ -2272,13 +2272,14 @@ class GatewayRunner:
     ) -> Optional[str]:
         """Pin DM-topic routing to the user's last-active topic.
 
-        Telegram fragments topic-mode DMs two ways: a Reply on a message
-        in another topic delivers ``message_thread_id`` for *that* topic,
-        and ``_build_message_event`` strips the thread_id on plain replies
-        (#3206 — needed for non-topic users). Both route the user to the
-        wrong session. When topic mode is on, rewrite the thread_id to the
-        user's most-recent binding if the inbound id is missing/General or
-        not a known topic for this chat. Returns None to leave it alone.
+        Telegram can omit ``message_thread_id`` or surface General (``1``)
+        for some topic-mode DM replies. In those lobby-shaped cases, keep the
+        conversation attached to the user's most-recent bound topic.
+
+        Do not rewrite a non-lobby, previously-unbound thread id: a newly
+        created Telegram DM topic is also "unknown" until the first inbound
+        message is recorded, and rewriting it would send that brand-new topic's
+        answer into an older lane. Returns None to leave the source alone.
         """
         if (
             source.platform != Platform.TELEGRAM
@@ -2288,6 +2289,14 @@ class GatewayRunner:
             or not self._telegram_topic_mode_enabled(source)
         ):
             return None
+        inbound = str(source.thread_id or "")
+        is_lobby = not inbound or inbound in self._TELEGRAM_GENERAL_TOPIC_IDS
+        if not is_lobby:
+            # A non-lobby, unknown thread_id is most likely the first message in
+            # a brand-new Telegram DM topic. Preserve it so it can be recorded
+            # as a new independent lane below instead of hijacking the latest
+            # existing topic binding.
+            return None
         session_db = getattr(self, "_session_db", None)
         if session_db is None:
             return None
@@ -2300,11 +2309,6 @@ class GatewayRunner:
             return None
         if not bindings:
             return None
-        inbound = str(source.thread_id or "")
-        is_lobby = not inbound or inbound in self._TELEGRAM_GENERAL_TOPIC_IDS
-        known = {str(b.get("thread_id") or "") for b in bindings}
-        if not is_lobby and inbound in known:
-            return None
         user_id = str(source.user_id)
         for b in bindings:  # newest-first
             if str(b.get("user_id") or "") == user_id:

tests/gateway/test_telegram_thread_fallback.py

@@ -98,6 +98,7 @@ _fake_telegram_ext.Application = object
 _fake_telegram_ext.CommandHandler = object
 _fake_telegram_ext.CallbackQueryHandler = object
 _fake_telegram_ext.MessageHandler = object
+_fake_telegram_ext.TypeHandler = object
 _fake_telegram_ext.ContextTypes = SimpleNamespace(DEFAULT_TYPE=object)
 _fake_telegram_ext.filters = object
 _fake_telegram_request = types.ModuleType("telegram.request")

tests/gateway/test_telegram_topic_mode.py

@@ -1175,13 +1175,15 @@ def test_recover_returns_none_for_known_topic(tmp_path):
     assert runner._recover_telegram_topic_thread_id(_make_source(thread_id="222")) is None
 
 
-def test_recover_rewrites_unknown_thread_id_to_most_recent(tmp_path):
-    # Cross-topic Reply leak: inbound thread_id is a Telegram-only id we never bound.
+def test_recover_preserves_unknown_thread_id_for_new_topic(tmp_path):
+    # A newly-created Telegram DM topic arrives with a real, previously-unbound
+    # message_thread_id. It must become its own session lane rather than being
+    # rewritten to whichever older topic was most recently active.
     db = SessionDB(db_path=tmp_path / "state.db")
     _seed_two_topic_bindings(db)
     runner = _make_runner(session_db=db)
 
-    assert runner._recover_telegram_topic_thread_id(_make_source(thread_id="9999")) == "222"
+    assert runner._recover_telegram_topic_thread_id(_make_source(thread_id="9999")) is None
 
 
 def test_recover_rewrites_lobby_thread_id_to_most_recent(tmp_path):