diff --git a/rugnux/rugnux_cli.cpp b/rugnux/rugnux_cli.cpp
index 5abd5ab3..93cc6914 100644
--- a/rugnux/rugnux_cli.cpp
+++ b/rugnux/rugnux_cli.cpp
@@ -849,7 +849,8 @@ static int RunRugnux(int argc, char **argv) {
             scale_fulls_arg = false;
             break;
         case OPT_DETECT_ICE_RINGS:
-            spot_finding_options_given = true;
+            // Deliberately NOT a spot-finding option for the purpose of re-finding the first-pass
+            // rotation spots - see where reuse_rotation_spots is decided below.
             if (optarg == nullptr || strcmp(optarg, "on") == 0)
                 detect_ice_rings = true;
             else if (strcmp(optarg, "off") == 0)
@@ -1591,6 +1592,27 @@ static int RunRugnux(int argc, char **argv) {
     else if (!dataset->file_detect_ice_rings.has_value())
         experiment.DetectIceRings(rotation_indexing);
 
+    // First-pass rotation indexing reuses the spots stored in the file, and does NOT re-mark them -
+    // their ice flags are the ones the acquisition wrote. So --detect-ice-rings can only invalidate
+    // them when it asks for something the file did not do; matching the file is a no-op for that pass.
+    // Re-finding whenever the flag is merely present would swap the acquisition's spots for this
+    // program's own, which moves the first-pass lattice by itself: measured over the rotation battery,
+    // passing the semantically null --detect-ice-rings=on to files that already carry it changed the
+    // merged data on every crystal and lost one to indexing failure. That also made the flag impossible
+    // to test, since both arms of an A/B moved for a reason unrelated to ice.
+    // A file with no key at all is treated as "did not mark", which is what its stored spots show:
+    // such a dataset carries no per-spot ice flags to reuse. That is a statement about the SPOTS, not
+    // about the setting - which is why it stays value_or(false) even though a keyless rotation file
+    // now defaults to detecting ice.
+    if (detect_ice_rings.has_value()
+        && detect_ice_rings.value() != dataset->file_detect_ice_rings.value_or(false)) {
+        if (reuse_rotation_spots)
+            logger.Info("--detect-ice-rings={} differs from the spots stored in the file: re-finding "
+                        "them for first-pass rotation indexing too",
+                        detect_ice_rings.value() ? "on" : "off");
+        reuse_rotation_spots = false;
+    }
+
     // Scale-fulls refits the per-frame scale on the rotation combined fulls; on by default for rotation
     // data (where it lifts ISa substantially) and off for stills. --no-scale-fulls overrides.
     const bool scale_fulls = scale_fulls_arg.value_or(rotation_indexing);