feat(feishu): add reply_in_thread for visual topic grouping

When reply_to_message config is enabled, the bot's first reply now
uses reply_in_thread=True to create a visual topic/thread in the
Feishu client. Subsequent chunks fall back to regular create.

The reply_to_message default remains False for backward compatibility.
Failed replies still fall back to regular send — messages are never
silently dropped.

nanobot/channels/feishu.py

@@ -1101,17 +1101,23 @@ class FeishuChannel(BaseChannel):
             logger.debug("Feishu: error fetching parent message {}: {}", message_id, e)
             return None
 
-    def _reply_message_sync(self, parent_message_id: str, msg_type: str, content: str) -> bool:
+    def _reply_message_sync(self, parent_message_id: str, msg_type: str, content: str, *, reply_in_thread: bool = False) -> bool:
-        """Reply to an existing Feishu message using the Reply API (synchronous)."""
+        """Reply to an existing Feishu message using the Reply API (synchronous).
+
+        Args:
+            reply_in_thread: If True, reply as a thread/topic message
+                in the Feishu client.
+        """
         from lark_oapi.api.im.v1 import ReplyMessageRequest, ReplyMessageRequestBody
 
         try:
+            body_builder = ReplyMessageRequestBody.builder().msg_type(msg_type).content(content)
+            if reply_in_thread:
+                body_builder = body_builder.reply_in_thread(True)
             request = (
                 ReplyMessageRequest.builder()
                 .message_id(parent_message_id)
-                .request_body(
+                .request_body(body_builder.build())
-                    ReplyMessageRequestBody.builder().msg_type(msg_type).content(content).build()
-                )
                 .build()
             )
             response = self._client.im.v1.message.reply(request)
@@ -1430,11 +1436,18 @@ class FeishuChannel(BaseChannel):
             first_send = True  # tracks whether the reply has already been used
 
             def _do_send(m_type: str, content: str) -> None:
-                """Send via reply (first message) or create (subsequent)."""
+                """Send via reply (first message) or create (subsequent).
+
+                When reply_to_message is enabled, the first message uses
+                reply_in_thread=True to create a visual topic thread.
+                """
                 nonlocal first_send
                 if reply_message_id and first_send:
                     first_send = False
-                    ok = self._reply_message_sync(reply_message_id, m_type, content)
+                    ok = self._reply_message_sync(
+                        reply_message_id, m_type, content,
+                        reply_in_thread=self.config.reply_to_message,
+                    )
                     if ok:
                         return
                     # Fall back to regular send if reply fails

tests/channels/test_feishu_reply.py

@@ -534,3 +534,75 @@ async def test_session_key_private_chat_no_override() -> None:
 
     assert len(bus_spy) == 1
     assert bus_spy[0].session_key_override is None
+
+
+# ---------------------------------------------------------------------------
+# reply_in_thread tests
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_reply_uses_reply_in_thread_when_enabled() -> None:
+    """When reply_to_message is True, reply includes reply_in_thread=True."""
+    channel = _make_feishu_channel(reply_to_message=True)
+
+    reply_resp = MagicMock()
+    reply_resp.success.return_value = True
+    channel._client.im.v1.message.reply.return_value = reply_resp
+
+    await channel.send(OutboundMessage(
+        channel="feishu",
+        chat_id="oc_abc",
+        content="hello",
+        metadata={"message_id": "om_001"},
+    ))
+
+    channel._client.im.v1.message.reply.assert_called_once()
+    call_args = channel._client.im.v1.message.reply.call_args
+    request = call_args[0][0]
+    assert request.request_body.reply_in_thread is True
+
+
+@pytest.mark.asyncio
+async def test_reply_without_reply_in_thread_when_disabled() -> None:
+    """When reply_to_message is False, reply does NOT use reply_in_thread."""
+    channel = _make_feishu_channel(reply_to_message=False)
+
+    create_resp = MagicMock()
+    create_resp.success.return_value = True
+    channel._client.im.v1.message.create.return_value = create_resp
+
+    await channel.send(OutboundMessage(
+        channel="feishu",
+        chat_id="oc_abc",
+        content="hello",
+    ))
+
+    # No message_id in metadata → no reply attempt, direct create
+    channel._client.im.v1.message.create.assert_called_once()
+
+
+@pytest.mark.asyncio
+async def test_reply_keeps_fallback_when_reply_fails() -> None:
+    """Even with reply_to_message=True, fallback to create on reply failure."""
+    channel = _make_feishu_channel(reply_to_message=True)
+
+    reply_resp = MagicMock()
+    reply_resp.success.return_value = False
+    reply_resp.code = 99991400
+    reply_resp.msg = "rate limited"
+    channel._client.im.v1.message.reply.return_value = reply_resp
+
+    create_resp = MagicMock()
+    create_resp.success.return_value = True
+    channel._client.im.v1.message.create.return_value = create_resp
+
+    await channel.send(OutboundMessage(
+        channel="feishu",
+        chat_id="oc_abc",
+        content="hello",
+        metadata={"message_id": "om_001"},
+    ))
+
+    channel._client.im.v1.message.reply.assert_called()
+    channel._client.im.v1.message.create.assert_called()