diff --git a/.github/workflows/bench.yml b/.github/workflows/bench.yml
index 50ef1b91..13a2d104 100644
--- a/.github/workflows/bench.yml
+++ b/.github/workflows/bench.yml
@@ -42,7 +42,11 @@ jobs:
       - name: Set up the build environment
         uses: ./.github/actions/setup
         with:
-          cache-suffix: benchmark
+          # A cache filled by a non-MPI build would restore a monoprop without MPI, which every
+          # rung below then shares.
+          cache-suffix: benchmark-mpi
+          # The build below requires MPI::MPI_CXX, and the multi-rank rungs need mpiexec.
+          mpi: "on"
 
       - name: Install package
         env: