feat(agent): enhance message injection handling and content merging

tests/tools/test_message_tool_suppress.py

@@ -1,5 +1,6 @@
 """Test message tool suppress logic for final replies."""
 
+import asyncio
 from pathlib import Path
 from unittest.mock import AsyncMock, MagicMock
 
@@ -86,6 +87,42 @@ class TestMessageToolSuppressLogic:
         assert result is not None
         assert "Hello" in result.content
 
+    @pytest.mark.asyncio
+    async def test_injected_followup_with_message_tool_does_not_emit_empty_fallback(
+        self, tmp_path: Path
+    ) -> None:
+        loop = _make_loop(tmp_path)
+        tool_call = ToolCallRequest(
+            id="call1", name="message",
+            arguments={"content": "Tool reply", "channel": "feishu", "chat_id": "chat123"},
+        )
+        calls = iter([
+            LLMResponse(content="First answer", tool_calls=[]),
+            LLMResponse(content="", tool_calls=[tool_call]),
+            LLMResponse(content="", tool_calls=[]),
+            LLMResponse(content="", tool_calls=[]),
+            LLMResponse(content="", tool_calls=[]),
+        ])
+        loop.provider.chat_with_retry = AsyncMock(side_effect=lambda *a, **kw: next(calls))
+        loop.tools.get_definitions = MagicMock(return_value=[])
+
+        sent: list[OutboundMessage] = []
+        mt = loop.tools.get("message")
+        if isinstance(mt, MessageTool):
+            mt.set_send_callback(AsyncMock(side_effect=lambda m: sent.append(m)))
+
+        pending_queue = asyncio.Queue()
+        await pending_queue.put(
+            InboundMessage(channel="feishu", sender_id="user1", chat_id="chat123", content="follow-up")
+        )
+
+        msg = InboundMessage(channel="feishu", sender_id="user1", chat_id="chat123", content="Start")
+        result = await loop._process_message(msg, pending_queue=pending_queue)
+
+        assert len(sent) == 1
+        assert sent[0].content == "Tool reply"
+        assert result is None
+
     async def test_progress_hides_internal_reasoning(self, tmp_path: Path) -> None:
         loop = _make_loop(tmp_path)
         tool_call = ToolCallRequest(id="call1", name="read_file", arguments={"path": "foo.txt"})