fix(mcp): isolate tool discovery failures (#3772)

backend/packages/harness/deerflow/mcp/tools.py

@@ -615,9 +615,20 @@ async def get_mcp_tools() -> list[BaseTool]:
             tool_name_prefix=True,
         )
 
-        # Get all tools from all servers (discovers tool definitions via
+        async def load_server_tools(server_name: str) -> list[BaseTool]:
-        # temporary sessions – the persistent-session wrapping is applied below).
+            try:
-        tools = await client.get_tools()
+                return await client.get_tools(server_name=server_name)
+            except Exception as e:
+                logger.warning(
+                    f"Skipping MCP server '{server_name}' after tool discovery failed: {e}",
+                    exc_info=True,
+                )
+                return []
+
+        # Get tools from each server independently so one broken MCP server does
+        # not prevent healthy servers from contributing their tools.
+        tools_by_server = await asyncio.gather(*(load_server_tools(name) for name in servers_config))
+        tools = [tool for server_tools in tools_by_server for tool in server_tools]
         logger.info(f"Successfully loaded {len(tools)} tool(s) from MCP servers")
 
         # Wrap each tool with persistent-session logic.

backend/tests/test_mcp_session_pool.py

@@ -889,7 +889,15 @@ async def test_http_transport_tools_not_pooled():
         patch("langchain_mcp_adapters.sessions.create_session", return_value=mock_cm),
     ):
         mock_client_instance = MockClient.return_value
-        mock_client_instance.get_tools = AsyncMock(return_value=[http_tool, stdio_tool])
+
+        async def get_tools_for_server(*, server_name: str | None = None):
+            if server_name == "myserver":
+                return [http_tool]
+            if server_name == "playwright":
+                return [stdio_tool]
+            raise AssertionError(f"unexpected server_name: {server_name}")
+
+        mock_client_instance.get_tools = AsyncMock(side_effect=get_tools_for_server)
 
         tools = await get_mcp_tools()
 

backend/tests/test_mcp_sync_wrapper.py

@@ -53,6 +53,46 @@ def test_mcp_tool_sync_wrapper_generation():
         assert result == "result: 42"
 
 
+def test_mcp_tool_loading_skips_failed_server():
+    """A broken MCP server should not drop tools from healthy servers."""
+
+    async def mock_coro(x: int):
+        return f"result: {x}"
+
+    good_tool = StructuredTool(
+        name="good-server_search",
+        description="search from healthy server",
+        args_schema=MockArgs,
+        func=None,
+        coroutine=mock_coro,
+    )
+
+    async def get_tools_for_server(*, server_name: str | None = None):
+        if server_name == "good-server":
+            return [good_tool]
+        if server_name == "bad-server":
+            raise RuntimeError("SSE endpoint returned text/html")
+        raise AssertionError(f"unexpected server_name: {server_name}")
+
+    mock_client_instance = MagicMock()
+    mock_client_instance.get_tools = AsyncMock(side_effect=get_tools_for_server)
+
+    with (
+        patch("langchain_mcp_adapters.client.MultiServerMCPClient", return_value=mock_client_instance),
+        patch("deerflow.config.extensions_config.ExtensionsConfig.from_file", return_value=MagicMock(model_extra={})),
+        patch("deerflow.mcp.tools.build_servers_config", return_value={"good-server": {}, "bad-server": {}}),
+        patch("deerflow.mcp.tools.get_initial_oauth_headers", new_callable=AsyncMock, return_value={}),
+        patch("deerflow.mcp.tools.build_oauth_tool_interceptor", return_value=None),
+        patch("deerflow.mcp.tools.logger.warning") as mock_warning,
+    ):
+        tools = asyncio.run(get_mcp_tools())
+
+    assert [tool.name for tool in tools] == ["good-server_search"]
+    assert tools[0].func is not None
+    mock_warning.assert_called_once()
+    assert "bad-server" in mock_warning.call_args[0][0]
+
+
 def test_mcp_tool_sync_wrapper_in_running_loop():
     """Test the shared sync wrapper from production code."""