diff --git a/rugnux/Rugnux.cpp b/rugnux/Rugnux.cpp
index 9fdd59ca9..0585ac3e5 100644
--- a/rugnux/Rugnux.cpp
+++ b/rugnux/Rugnux.cpp
@@ -883,7 +883,10 @@ ProcessResult Rugnux::RunPipeline(RugnuxObserver *observer, bool write_output, b
     // at the half-integer nodes of crystals whose cell is right, so the run carries the doubled cell as a
     // second hypothesis where the class stands clear out to the limit (DoubledCellHypothesis), and merges
     // both.
-    if (full && indexer && experiment_.IsRotationIndexing()) {
+    //
+    // Only an output pass probes: the answer the run returns is always one (RunAllPasses), and no other
+    // pass's probe is read - the pre-pass, the beam-centre arm and the probe passes leave theirs unread.
+    if (full && indexer && experiment_.IsRotationIndexing() && write_output && !geometry_prepass) {
         std::vector<int64_t> frames;
         for (const int o : select_equally_spaced_image_ordinals(images_to_process, 60))
             frames.push_back(o);