fix(code): run stdio MCP server pre-flight check off the event loop (#4434)

Fixes #4433

---

The stdio MCP server pre-flight check calls `shutil.which`, which makes
blocking `os.access` calls. When run on the event loop, this triggers a
`BlockingError` from blockbuster, preventing the agent from starting.

This wraps the `_check_stdio_server` call in `asyncio.to_thread` to
execute it off the event loop, resolving the blocking call issue.

---------

Co-authored-by: Dhruv <heyparth@Dhruvs-MacBook-Air.local>
Co-authored-by: Mason Daugherty <mason@langchain.dev>
Co-authored-by: Mason Daugherty <github@mdrxy.com>

libs/code/deepagents_code/mcp_tools.py

@@ -1521,7 +1521,9 @@ async def _load_tools_from_config(
             if server_type in _SUPPORTED_REMOTE_TYPES:
                 await _check_remote_server(server_name, server_config)
             elif server_type == "stdio":
-                _check_stdio_server(server_name, server_config)
+                # `shutil.which` makes blocking `os.access` calls; run it
+                # off the event loop so blockbuster doesn't reject it.
+                await asyncio.to_thread(_check_stdio_server, server_name, server_config)
         except RuntimeError as exc:
             logger.warning(
                 "MCP server '%s' skipped: pre-flight failed: %s",

libs/code/tests/unit_tests/test_mcp_tools.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 import asyncio
 import json
 import logging
+import threading
 from contextlib import asynccontextmanager
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, cast
@@ -2075,6 +2076,38 @@ class TestHealthChecks:
         ):
             _check_stdio_server("srv", {"command": "missing"})
 
+    async def test_check_stdio_server_runs_off_event_loop(
+        self,
+        write_config: Callable[..., str],
+    ) -> None:
+        """The stdio pre-flight's `shutil.which` runs off the event loop."""
+        path = write_config({"mcpServers": {"srv": {"command": "missing"}}})
+        event_loop_thread = threading.current_thread()
+        which_threads: list[threading.Thread] = []
+
+        def _record_missing_command(_command: str) -> str | None:
+            which_threads.append(threading.current_thread())
+            return None
+
+        with patch(
+            "deepagents_code.mcp_tools.shutil.which",
+            side_effect=_record_missing_command,
+        ):
+            tools, manager, server_infos = await get_mcp_tools(path)
+
+        try:
+            assert which_threads
+            assert which_threads[0] is not event_loop_thread
+            assert tools == []
+            assert server_infos[0].name == "srv"
+            assert server_infos[0].status == "error"
+            error = server_infos[0].error or ""
+            assert "command 'missing' not found on PATH" in error
+            assert manager is not None
+        finally:
+            if manager is not None:
+                await manager.cleanup()
+
     async def test_check_remote_server_transport_error(self) -> None:
         """Transport errors are wrapped as `RuntimeError`."""
         import httpx