diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a1013c0..875fe85 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -127,8 +127,7 @@ jobs:
 
   argus:
     name: Argus
-    # Souffle's PPA package needs libffi7, which Ubuntu 24.04 no longer ships.
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4
 
@@ -140,6 +139,11 @@ jobs:
 
       - name: Install Souffle
         run: |
+          # Souffle's PPA package links against libffi7, which Ubuntu 24.04
+          # dropped; install the Ubuntu 20.04 build (checksum-pinned) first.
+          curl -fsSLo /tmp/libffi7.deb http://archive.ubuntu.com/ubuntu/pool/main/libf/libffi/libffi7_3.3-4_amd64.deb
+          echo "4584aa8fef1bf5086168ce2f7078cd2ebd78fdc4cc0d86d958d795d4e0b0f50d  /tmp/libffi7.deb" | sha256sum -c -
+          sudo apt-get install -y /tmp/libffi7.deb
           sudo wget https://souffle-lang.github.io/ppa/souffle-key.public -O /usr/share/keyrings/souffle-archive-keyring.gpg
           echo "deb [signed-by=/usr/share/keyrings/souffle-archive-keyring.gpg] https://souffle-lang.github.io/ppa/ubuntu/ stable main" | sudo tee /etc/apt/sources.list.d/souffle.list
           sudo apt-get update
@@ -151,9 +155,9 @@ jobs:
           path: |
             deps
             _build
-          key: mix-argus-${{ steps.beam.outputs.otp-version }}-${{ steps.beam.outputs.elixir-version }}-${{ hashFiles('**/mix.lock') }}
+          key: mix-argus-u2404-${{ steps.beam.outputs.otp-version }}-${{ steps.beam.outputs.elixir-version }}-${{ hashFiles('**/mix.lock') }}
           restore-keys: |
-            mix-argus-${{ steps.beam.outputs.otp-version }}-${{ steps.beam.outputs.elixir-version }}-
+            mix-argus-u2404-${{ steps.beam.outputs.otp-version }}-${{ steps.beam.outputs.elixir-version }}-
 
       - run: mix deps.get --check-locked
       - run: mix argus --fail-above 0