fix(matrix): add text batching to merge split long messages

Ports the adaptive batching pattern from the Telegram adapter.
Matrix clients split messages around 4000 chars. Adaptive delay waits
2.0s when a chunk is near the limit, 0.6s otherwise. Only text messages
are batched; commands dispatch immediately.

Ref #6892

gateway/platforms/matrix.py

@@ -120,6 +120,11 @@ def check_matrix_requirements() -> bool:
 class MatrixAdapter(BasePlatformAdapter):
     """Gateway adapter for Matrix (any homeserver)."""
 
+    # Threshold for detecting Matrix client-side message splits.
+    # When a chunk is near the ~4000-char practical limit, a continuation
+    # is almost certain.
+    _SPLIT_THRESHOLD = 3900
+
     def __init__(self, config: PlatformConfig):
         super().__init__(config, Platform.MATRIX)
 
@@ -172,6 +177,13 @@ class MatrixAdapter(BasePlatformAdapter):
             "MATRIX_REACTIONS", "true"
         ).lower() not in ("false", "0", "no")
 
+        # Text batching: merge rapid successive messages (Telegram-style).
+        # Matrix clients split long messages around 4000 chars.
+        self._text_batch_delay_seconds = float(os.getenv("HERMES_MATRIX_TEXT_BATCH_DELAY_SECONDS", "0.6"))
+        self._text_batch_split_delay_seconds = float(os.getenv("HERMES_MATRIX_TEXT_BATCH_SPLIT_DELAY_SECONDS", "2.0"))
+        self._pending_text_batches: Dict[str, MessageEvent] = {}
+        self._pending_text_batch_tasks: Dict[str, asyncio.Task] = {}
+
     def _is_duplicate_event(self, event_id) -> bool:
         """Return True if this event was already processed. Tracks the ID otherwise."""
         if not event_id:
@@ -1088,7 +1100,81 @@ class MatrixAdapter(BasePlatformAdapter):
         # Acknowledge receipt so the room shows as read (fire-and-forget).
         self._background_read_receipt(room.room_id, event.event_id)
 
-        await self.handle_message(msg_event)
+        # Only batch plain text messages — commands dispatch immediately.
+        if msg_type == MessageType.TEXT:
+            self._enqueue_text_event(msg_event)
+        else:
+            await self.handle_message(msg_event)
+
+    # ------------------------------------------------------------------
+    # Text message aggregation (handles Matrix client-side splits)
+    # ------------------------------------------------------------------
+
+    def _text_batch_key(self, event: MessageEvent) -> str:
+        """Session-scoped key for text message batching."""
+        from gateway.session import build_session_key
+        return build_session_key(
+            event.source,
+            group_sessions_per_user=self.config.extra.get("group_sessions_per_user", True),
+            thread_sessions_per_user=self.config.extra.get("thread_sessions_per_user", False),
+        )
+
+    def _enqueue_text_event(self, event: MessageEvent) -> None:
+        """Buffer a text event and reset the flush timer.
+
+        When a Matrix client splits a long message, the chunks arrive within
+        a few hundred milliseconds.  This merges them into a single event
+        before dispatching.
+        """
+        key = self._text_batch_key(event)
+        existing = self._pending_text_batches.get(key)
+        chunk_len = len(event.text or "")
+        if existing is None:
+            event._last_chunk_len = chunk_len  # type: ignore[attr-defined]
+            self._pending_text_batches[key] = event
+        else:
+            if event.text:
+                existing.text = f"{existing.text}\n{event.text}" if existing.text else event.text
+            existing._last_chunk_len = chunk_len  # type: ignore[attr-defined]
+            # Merge any media that might be attached
+            if event.media_urls:
+                existing.media_urls.extend(event.media_urls)
+                existing.media_types.extend(event.media_types)
+
+        # Cancel any pending flush and restart the timer
+        prior_task = self._pending_text_batch_tasks.get(key)
+        if prior_task and not prior_task.done():
+            prior_task.cancel()
+        self._pending_text_batch_tasks[key] = asyncio.create_task(
+            self._flush_text_batch(key)
+        )
+
+    async def _flush_text_batch(self, key: str) -> None:
+        """Wait for the quiet period then dispatch the aggregated text.
+
+        Uses a longer delay when the latest chunk is near Matrix's ~4000-char
+        split point, since a continuation chunk is almost certain.
+        """
+        current_task = asyncio.current_task()
+        try:
+            pending = self._pending_text_batches.get(key)
+            last_len = getattr(pending, "_last_chunk_len", 0) if pending else 0
+            if last_len >= self._SPLIT_THRESHOLD:
+                delay = self._text_batch_split_delay_seconds
+            else:
+                delay = self._text_batch_delay_seconds
+            await asyncio.sleep(delay)
+            event = self._pending_text_batches.pop(key, None)
+            if not event:
+                return
+            logger.info(
+                "[Matrix] Flushing text batch %s (%d chars)",
+                key, len(event.text or ""),
+            )
+            await self.handle_message(event)
+        finally:
+            if self._pending_text_batch_tasks.get(key) is current_task:
+                self._pending_text_batch_tasks.pop(key, None)
 
     async def _on_room_message_media(self, room: Any, event: Any) -> None:
         """Handle incoming media messages (images, audio, video, files)."""