diff --git a/scripts/test-windows.ps1 b/scripts/test-windows.ps1
index 8c1a8358a..0022560f3 100644
--- a/scripts/test-windows.ps1
+++ b/scripts/test-windows.ps1
@@ -166,6 +166,7 @@ $guards = @(
     "tests\windows\test_hook_augment.py",
     "tests\windows\test_ui_drive_listing.py",
     "tests\windows\test_cli_non_ascii_arg.py",
+    "tests\windows\test_mcp_stdio.py",
     "tests\windows\test_windows_update_handoff.py"
 )
 
diff --git a/tests/windows/mcp_stdio.py b/tests/windows/mcp_stdio.py
index 218e9a27f..12487237d 100644
--- a/tests/windows/mcp_stdio.py
+++ b/tests/windows/mcp_stdio.py
@@ -148,6 +148,7 @@ def close(self):
                 self.proc.kill()
             except Exception:
                 pass
+        self._stderr_thread.join(timeout=10)
 
 
 def wait_projects_with_stats(server, timeout=90.0, poll=1.0):
diff --git a/tests/windows/test_mcp_stdio.py b/tests/windows/test_mcp_stdio.py
new file mode 100644
index 000000000..ced02842a
--- /dev/null
+++ b/tests/windows/test_mcp_stdio.py
@@ -0,0 +1,59 @@
+"""Regression guard for clean Windows MCP stdio startup."""
+
+import os
+import subprocess
+import sys
+import tempfile
+
+from mcp_stdio import McpServer
+
+
+def main():
+    if os.name != "nt":
+        print("SKIP: Windows-only MCP stdio guard")
+        return 2
+
+    binary = sys.argv[1] if len(sys.argv) > 1 else os.environ.get("CBM_TEST_BINARY")
+    if not binary:
+        print("SETUP FAIL: CBM_TEST_BINARY is required", file=sys.stderr)
+        return 2
+
+    with tempfile.TemporaryDirectory(prefix="cbm-mcp-stdio-") as cache:
+        runtime = cache + "-runtime"
+        os.makedirs(runtime)
+        server = McpServer(binary, cache_dir=cache, extra_env={"CBM_RUNTIME_DIR": runtime})
+        try:
+            with server:
+                server.initialize(timeout=30)
+                server.tools_list(timeout=30)
+            stderr = server.stderr_text()
+        finally:
+            stop = subprocess.run(
+                [binary, "daemon", "stop"],
+                env=server.env,
+                capture_output=True,
+                timeout=30,
+            )
+            if stop.returncode != 0:
+                detail = (stop.stdout + stop.stderr).decode("utf-8", "replace")
+                raise RuntimeError(
+                    "daemon cleanup failed (%d): %s" % (stop.returncode, detail.strip())
+                )
+
+    forbidden = (
+        "The system cannot find the path specified.",
+        "El sistema no puede encontrar la ruta especificada.",
+    )
+    leaked = [message for message in forbidden if message in stderr]
+    if leaked:
+        print(
+            "FAIL: startup leaked an OS path error to stderr: " + ", ".join(leaked),
+            file=sys.stderr,
+        )
+        return 1
+    print("PASS: successful MCP startup emitted no OS path error")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())