diff --git a/backend/packages/harness/deerflow/mcp/tools.py b/backend/packages/harness/deerflow/mcp/tools.py
index 0d24aa563a..e04e473ee6 100644
--- a/backend/packages/harness/deerflow/mcp/tools.py
+++ b/backend/packages/harness/deerflow/mcp/tools.py
@@ -1086,9 +1086,14 @@ async def _discover_server_tools(
     except (asyncio.CancelledError, StaleMCPBindingError, McpTaskConfigurationError):
         raise
     except Exception as e:
+        # Never interpolate ``e`` nor attach its traceback: ``ExtensionsConfig``
+        # resolves ``$VAR`` before validation, so a discovery error message can
+        # embed a resolved-credential request URL. Keep the server name and the
+        # exception type for diagnosis.
         logger.warning(
-            f"Skipping MCP server '{server_name}' after tool discovery failed: {e}",
-            exc_info=True,
+            "Skipping MCP server '%s' after tool discovery failed (%s)",
+            server_name,
+            type(e).__name__,
         )
         return server_name, None
 
@@ -1432,5 +1437,7 @@ async def get_mcp_tools(
     except (McpTaskConfigurationError, StaleMCPBindingError):
         raise
     except Exception as e:
-        logger.error(f"Failed to load MCP tools: {e}", exc_info=True)
+        # Only the exception type is logged: the message/traceback can carry a
+        # resolved-credential request URL (see ``_discover_server_tools``).
+        logger.error("Failed to load MCP tools (%s)", type(e).__name__)
         return []
diff --git a/backend/tests/test_mcp_per_server_discovery.py b/backend/tests/test_mcp_per_server_discovery.py
index 1f0ed69e28..edefa792bb 100644
--- a/backend/tests/test_mcp_per_server_discovery.py
+++ b/backend/tests/test_mcp_per_server_discovery.py
@@ -7,6 +7,7 @@ import builtins
 import logging
 from unittest.mock import AsyncMock, MagicMock
 
+import httpx
 import pytest
 from langchain_core.tools import StructuredTool
 
@@ -711,3 +712,37 @@ async def test_discovery_and_wrapper_use_the_same_session_init_timeout(monkeypat
     assert len(groups["A"].tools) == 1
     assert observed["discovery"] == 5.0
     assert observed["wrapper"] == observed["discovery"]
+
+
+@pytest.mark.asyncio
+async def test_per_server_discovery_failure_log_redacts_resolved_credentials(monkeypatch, caplog):
+    """A failed per-server discovery must not log a credential-bearing URL."""
+    from langchain_mcp_adapters import client as adapter_client
+
+    secret = "s3cr3t-pass-123"
+    config = _config({"A": {"enabled": True, "type": "http", "url": "https://alice@remote.test/mcp"}})
+    _patch_common(monkeypatch)
+
+    class FakeClient:
+        def __init__(self, connections, **kwargs) -> None:
+            self.callbacks = kwargs.get("callbacks") or []
+            self.tool_interceptors = kwargs.get("tool_interceptors") or []
+            self.tool_name_prefix = kwargs.get("tool_name_prefix", True)
+
+        async def get_tools(self, *, server_name=None):
+            request = httpx.Request("GET", f"https://alice:{secret}@remote.test/mcp")
+            raise httpx.HTTPStatusError(
+                f"Client error for url '{request.url}'",
+                request=request,
+                response=httpx.Response(401, request=request),
+            )
+
+    monkeypatch.setattr(adapter_client, "MultiServerMCPClient", FakeClient)
+
+    with caplog.at_level("WARNING", logger="deerflow.mcp.tools"):
+        result = await get_mcp_tools_by_server(config, server_names={"A"})
+
+    assert result == {}
+    assert secret not in caplog.text  # the resolved credential never reaches the log
+    assert "server 'A'" in caplog.text  # the offending server name is retained
+    assert "HTTPStatusError" in caplog.text  # exception type retained
diff --git a/backend/tests/test_mcp_sync_wrapper.py b/backend/tests/test_mcp_sync_wrapper.py
index 30197b8219..e729f962bf 100644
--- a/backend/tests/test_mcp_sync_wrapper.py
+++ b/backend/tests/test_mcp_sync_wrapper.py
@@ -130,7 +130,11 @@ def test_mcp_tool_loading_skips_failed_server():
     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]
+    args, kwargs = mock_warning.call_args
+    assert args[1] == "bad-server"
+    assert args[2] == "RuntimeError"
+    assert "server 'bad-server'" in args[0] % args[1:]
+    assert kwargs.get("exc_info") is None
 
 
 def test_mcp_tool_sync_wrapper_in_running_loop():