diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 7c52f36..d81d06c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -18,6 +18,8 @@ jobs:
       - uses: Swatinem/rust-cache@v2
         with:
           workspaces: "expander_compiler -> expander_compiler/target"
+          # The prefix cache key, this can be changed to start a new cache manually.
+          prefix-key: "mpi-v5.0.5" # update me if brew formula changes to a new version
       - if: matrix.os == 'macos-latest'
         run: brew install openmpi
       - if: matrix.os == 'ubuntu-latest'