diff --git a/evalharness/sandbox/prefetch.py b/evalharness/sandbox/prefetch.py
index c3aa8e7..9a9baed 100644
--- a/evalharness/sandbox/prefetch.py
+++ b/evalharness/sandbox/prefetch.py
@@ -146,7 +146,13 @@ def _pull_one(image: str, verbose: bool = True) -> None:
         buf = ''
         _lines_iter = iter(lambda: None, 1)  # placeholder, replaced below
 
-        _IDLE_S = 300   # no output for 5 minutes -> dead mirror
+        # no output at all -> dead mirror, switch source. ANY output resets
+        # the timer (manifest/layer announcements/DOWNLOADING refreshes), so
+        # this only fires on sources that print nothing. 10s default;
+        # override with EVALHARNESS_PULL_IDLE_S for slow-negotiating mirrors.
+        import os as _os2
+
+        _IDLE_S = float(_os2.environ.get('EVALHARNESS_PULL_IDLE_S', '10'))
 
         def _iter_lines():
             nonlocal buf
@@ -154,8 +160,8 @@ def _pull_one(image: str, verbose: bool = True) -> None:
                 if not _selr.select(timeout=_IDLE_S):
                     proc.kill()
                     if verbose:
-                        print(f'· no data for {_IDLE_S // 60}min -- dead '
-                              'mirror, trying next source ...',
+                        print(f'· no data for {_IDLE_S:.0f}s -- dead mirror, '
+                              'trying next source ...',
                               file=_sys.stderr, flush=True)
                     return
                 chunk = _os.read(_fd, 4096)