diff --git a/tools/lock/with-lock.sh b/tools/lock/with-lock.sh
index 55b931251..5e5615742 100755
--- a/tools/lock/with-lock.sh
+++ b/tools/lock/with-lock.sh
@@ -59,7 +59,11 @@ def wait_lock(fd, name):
         time.sleep(5)
 held = []
 if mode == 'build':
-    slots = [opened(f"build-{k}") for k in range(3)]
+    # the number of build slots open to NEW builds comes from ~/.config/igneum/build-slots (1 to 3, default 3): 1 when the
+    # owner needs the Mac responsive (5 October 2026), builds already running keep their slot
+    try: nslots = max(1, min(3, int(open(os.path.expanduser('~/.config/igneum/build-slots')).read().strip())))
+    except Exception: nslots = 3
+    slots = [opened(f"build-{k}") for k in range(nslots)]
     got = None
     while got is None:
         for k, fd in enumerate(slots):