diff --git a/rugnux/Rugnux.cpp b/rugnux/Rugnux.cpp
index 31b89117..fff06f75 100644
--- a/rugnux/Rugnux.cpp
+++ b/rugnux/Rugnux.cpp
@@ -2130,8 +2130,18 @@ ProcessResult Rugnux::RunPipeline(RugnuxObserver *observer, bool write_output, b
                 const auto want_system = fixed_sg->crystal_system() == gemmi::CrystalSystem::Trigonal
                                          && fixed_sg->centring_type() == 'P'
                                        ? gemmi::CrystalSystem::Hexagonal : fixed_sg->crystal_system();
-                const auto cand = LatticeSearchForClass(*end_msg.rotation_lattice, want_system,
-                                                        fixed_sg->centring_type());
+                // Search the PRIMITIVE cell. rotation_lattice is the conventional one and its centring
+                // is carried beside it, but the character table is reached through a Niggli reduction
+                // that takes whatever it is handed to be primitive - so a centred conventional cell
+                // reduces as a lattice with the wrong point set. This arm is entered exactly when the
+                // centrings differ, which includes every centred indexed lattice, so the distinction is
+                // not academic: a C-centred orthorhombic conventional cell read as primitive is itself a
+                // perfectly good oP metric, and -S P212121 on one would match, reindex by the identity
+                // and merge in a group whose absence rule deletes half the reflections that exist.
+                // The other lattice arithmetic in this file goes through ToPrimitive for the same reason.
+                const auto cand = LatticeSearchForClass(
+                        end_msg.rotation_lattice->ToPrimitive(indexed_centering), want_system,
+                        fixed_sg->centring_type());
                 if (cand && reindex_into(*cand)) {
                     const auto &uc = *result.consensus_cell;
                     logger.Info("Reindexed the {}-centred indexed lattice into the {}-centred setting the "
diff --git a/rugnux_vs_xds.py b/rugnux_vs_xds.py
index 854d67b7..9f994020 100755
--- a/rugnux_vs_xds.py
+++ b/rugnux_vs_xds.py
@@ -250,7 +250,7 @@ def find_rugnux(explicit):
     return None
 
 
-def run_rugnux(master, workdir, name, xds, rugnux_bin, threads, timeout, reuse, extra_args=""):
+def run_rugnux(master, workdir, name, xds, rugnux_bin, threads, timeout, reuse, extra_args="", force_sg=False):
     """Run rugnux; return (cif_path, error). Resolution + anomalous matched to XDS."""
     workdir.mkdir(parents=True, exist_ok=True)
     cif = workdir / f"{name}.cif"
@@ -267,6 +267,8 @@ def run_rugnux(master, workdir, name, xds, rugnux_bin, threads, timeout, reuse,
         cmd += ["--scaling-low-resolution", f"{xds['dmax']:.3f}"]
     if threads:
         cmd += ["-N", str(threads)]
+    if force_sg and xds.get("sg"):
+        cmd += ["-S", str(xds["sg"])]
     if extra_args:
         import shlex
         cmd += shlex.split(extra_args)
@@ -391,6 +393,9 @@ def main():
     ap.add_argument("--reuse", action="store_true", help="skip rugnux where its .cif already exists")
     ap.add_argument("--xds-only", action="store_true", help="only parse+print the XDS side (no rugnux run)")
     ap.add_argument("--progress", action="store_true", help="print per-crystal progress to stderr")
+    ap.add_argument("--force-xds-sg", action="store_true",
+                    help="pin each crystal to the XDS reference space group with -S, instead of "
+                         "letting rugnux determine it de novo (exercises the fixed-group path)")
     ap.add_argument("--extra-args", default="", help="extra args appended to every rugnux command (e.g. '--rotation-refine-geometry')")
     args = ap.parse_args()
 
@@ -432,7 +437,7 @@ def main():
                       file=sys.stderr, flush=True)
             cif, err, elapsed = run_rugnux(master, workdir / name, name, xds,
                                            rugnux_bin, args.threads, args.timeout, args.reuse,
-                                           args.extra_args)
+                                           args.extra_args, args.force_xds_sg)
             if cif:
                 try:
                     rug = parse_rugnux_cif(cif)