diff --git a/tests/parallel/serial_reference.py b/tests/parallel/serial_reference.py
index ae147c3a2..90c366a1b 100644
--- a/tests/parallel/serial_reference.py
+++ b/tests/parallel/serial_reference.py
@@ -42,7 +42,13 @@
 import underworld3 as uw
 
 # Launcher variables that would make a spawned singleton try to join the parent job.
-_MPI_ENV_PREFIXES = ("OMPI_", "PMIX_", "PMI_", "MPI_", "HYDRA_", "I_MPI_", "SLURM_")
+# Every implementation we run under is named here, because a list that covers only
+# the one on your desk passes locally and fails on the other one: a copy of this
+# naming just the Open MPI family passed under Open MPI and aborted the child's
+# MPI_Init under MPICH, whose variables are PMI_* (#675). PRTE_/PRTERUN_/OPAL_ are
+# Open MPI 5's runtime, which is a separate family again from OMPI_.
+_MPI_ENV_PREFIXES = ("OMPI_", "PMIX_", "PMI_", "MPI_", "HYDRA_", "I_MPI_", "SLURM_",
+                     "PRTE_", "PRTERUN_", "OPAL_")
 
 _CACHE = {}
 
diff --git a/tests/parallel/test_0855_mesh_smoothing_parallel.py b/tests/parallel/test_0855_mesh_smoothing_parallel.py
index 6f98f22c8..2f11ca5cf 100644
--- a/tests/parallel/test_0855_mesh_smoothing_parallel.py
+++ b/tests/parallel/test_0855_mesh_smoothing_parallel.py
@@ -36,6 +36,8 @@
 import underworld3 as uw
 from underworld3.meshing import smooth_mesh_interior
 
+from serial_reference import _MPI_ENV_PREFIXES
+
 
 pytestmark = [pytest.mark.mpi(min_size=2), pytest.mark.timeout(120)]
 
@@ -248,21 +250,21 @@ def test_parallel_matches_serial_bit_identical():
     comm = MPI.COMM_WORLD
     rank = comm.rank
 
-    # 1. Compute the serial reference (rank 0 only) in a clean
-    # subprocess. We strip MPI/PMIX/PRTE env vars first so the
-    # subprocess's PETSc doesn't try to attach to this mpirun's
-    # MPI world (which would deadlock both processes).
+    # 1. Compute the serial reference (rank 0 only) in a clean subprocess. The
+    # launcher's variables are stripped first, or the child's MPI_Init tries to
+    # join THIS mpirun's job and aborts on a descriptor it does not own.
+    #
+    # The prefix list is shared with serial_reference rather than restated here:
+    # a local copy that named only the Open MPI family passed on Open MPI and
+    # failed under MPICH, whose variables are PMI_* (#675). One list, so a
+    # launcher that is missing from it is missing everywhere and gets noticed.
     ref_path = None
     if rank == 0:
         tmpdir = tempfile.mkdtemp(prefix="winslow_ref_")
         ref_path = os.path.join(tmpdir, "ref.npz")
         clean_env = {
             k: v for k, v in os.environ.items()
-            if not (k.startswith("OMPI_")
-                    or k.startswith("PMIX_")
-                    or k.startswith("PRTE_")
-                    or k.startswith("PRTERUN_")
-                    or k == "OPAL_PREFIX")
+            if not k.startswith(_MPI_ENV_PREFIXES)
         }
         proc = subprocess.run(
             [sys.executable, "-c",