diff --git a/.gitea/workflows/cache-probe.yml b/.gitea/workflows/cache-probe.yml
index 066404d..036102c 100644
--- a/.gitea/workflows/cache-probe.yml
+++ b/.gitea/workflows/cache-probe.yml
@@ -51,3 +51,41 @@ jobs:
         run: |
           echo "runner.arch = ${{ runner.arch }}"
           echo "runner.os   = ${{ runner.os }}"
+
+  # The first probe showed ACTIONS_CACHE_URL unset but ACTIONS_RESULTS_URL set,
+  # i.e. the runner serves the v2 cache service and not v1. actions/cache v4.3
+  # can speak v2, but only when ACTIONS_CACHE_SERVICE_V2 is set, and it is not.
+  # These three variants test the ways round that.
+  variant-v4-flag:
+    name: cache@v4 + ACTIONS_CACHE_SERVICE_V2
+    runs-on: ubuntu-latest
+    env:
+      ACTIONS_CACHE_SERVICE_V2: "true"
+    steps:
+      - run: mkdir -p /tmp/probe && head -c 1048576 /dev/urandom > /tmp/probe/blob
+      - uses: actions/cache@v4
+        with:
+          path: /tmp/probe
+          key: probe-v4flag-${{ github.run_id }}
+
+  variant-v6:
+    name: cache@v6
+    runs-on: ubuntu-latest
+    steps:
+      - run: mkdir -p /tmp/probe && head -c 1048576 /dev/urandom > /tmp/probe/blob
+      - uses: actions/cache@v6
+        with:
+          path: /tmp/probe
+          key: probe-v6-${{ github.run_id }}
+
+  variant-v6-flag:
+    name: cache@v6 + ACTIONS_CACHE_SERVICE_V2
+    runs-on: ubuntu-latest
+    env:
+      ACTIONS_CACHE_SERVICE_V2: "true"
+    steps:
+      - run: mkdir -p /tmp/probe && head -c 1048576 /dev/urandom > /tmp/probe/blob
+      - uses: actions/cache@v6
+        with:
+          path: /tmp/probe
+          key: probe-v6flag-${{ github.run_id }}