diff --git a/frontend/app.py b/frontend/app.py
index eca4f7d..33ba1a6 100644
--- a/frontend/app.py
+++ b/frontend/app.py
@@ -456,7 +456,7 @@ def scp_start(
     dest = sdir / filename
 
     # Check disk space
-    usage = shutil.disk_usage(STAGING)
+    usage = shutil.disk_usage(_staging(session_id))
     free_gb = usage.free / (1024**3)
     if free_gb < 10:
         logger.warning("Low disk: %.1f GB free — SCP pull may fail", free_gb)