fix(gateway): pin Telegram DM-topic routing to user's current topic

Topic-mode DM replies were fragmenting one conversation across many sessions: a Reply on a message in another topic delivered Telegram's message_thread_id for *that* topic, and #3206's strip routed plain replies to the lobby. Both pulled the user away from their current session. Fix: when topic mode is on, rewrite source.thread_id to the user's most-recent binding if the inbound id is missing/General or not a known topic. Non-topic-mode users unchanged.

gateway/run.py

@@ -1814,6 +1814,54 @@ class GatewayRunner:
             session_id=session_entry.session_id,
         )
 
+    def _recover_telegram_topic_thread_id(
+        self,
+        source: SessionSource,
+    ) -> 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.
+        """
+        if (
+            source.platform != Platform.TELEGRAM
+            or source.chat_type != "dm"
+            or not source.chat_id
+            or not source.user_id
+            or not self._telegram_topic_mode_enabled(source)
+        ):
+            return None
+        session_db = getattr(self, "_session_db", None)
+        if session_db is None:
+            return None
+        try:
+            bindings = session_db.list_telegram_topic_bindings_for_chat(
+                chat_id=str(source.chat_id),
+            )
+        except Exception:
+            logger.debug("topic-recover: read failed", exc_info=True)
+            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:
+                recovered = str(b.get("thread_id") or "")
+                if recovered and recovered != inbound:
+                    return recovered
+                return None
+        return None
+
     def _resolve_session_agent_runtime(
         self,
         *,
@@ -7498,6 +7546,21 @@ class GatewayRunner:
         )
 
         # Get or create session
+        # Topic-mode DMs: rewrite a stale/foreign thread_id to the user's
+        # last-active topic so a cross-topic Reply or stripped plain reply
+        # doesn't fragment the conversation across sessions.
+        recovered = self._recover_telegram_topic_thread_id(source)
+        if recovered is not None:
+            logger.info(
+                "telegram topic recovery: chat=%s user=%s %r -> %s",
+                source.chat_id, source.user_id, source.thread_id, recovered,
+            )
+            source = dataclasses.replace(source, thread_id=recovered)
+            try:
+                event.source = source
+            except Exception:
+                pass
+
         session_entry = self.session_store.get_or_create_session(source)
         session_key = session_entry.session_key
         self._cache_session_source(session_key, source)