diff --git a/.gitea/workflows/ci.yml b/.gitea/workflows/ci.yml
index 2e35e8f..0e15d4a 100644
--- a/.gitea/workflows/ci.yml
+++ b/.gitea/workflows/ci.yml
@@ -75,6 +75,8 @@ jobs:
                   grep -E 'hostedtoolcache|workspace|overlay' /proc/mounts || true
                   ls -la /opt/hostedtoolcache || true
                   ls -la /opt/hostedtoolcache/node || true
+                  ls -la "/opt/hostedtoolcache/node/$(tr -d '[:space:]' < .node-version)/" || true
+                  test -f "/opt/hostedtoolcache/node/$(tr -d '[:space:]' < .node-version)/x64.complete" && echo MARKER-PRESENT || echo MARKER-MISSING
                   ls -la "/opt/hostedtoolcache/node/$(tr -d '[:space:]' < .node-version)/x64/bin" || true
                   "/opt/hostedtoolcache/node/$(tr -d '[:space:]' < .node-version)/x64/bin/node" -v || true
                   env | grep -iE 'RUNNER|TOOL|CACHE' || true
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index ee23cfd..786164a 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -102,6 +102,11 @@ Bumping Node is one coordinated change, committed as a unit:
 
 Skipping step 2 fails CI at image pull; skipping step 3 silently reverts to the per-job download.
 
+Two invariants the image must satisfy for the probe to hit, both easy to break:
+
+- **The `x64.complete` marker.** `actions/tool-cache` accepts a cached tool only when `<version>/<arch>.complete` exists next to the directory (`tc.find()` checks it); a plausible-looking `node/<version>/x64/` alone is ignored and the download happens anyway. See the comment in [docker/Dockerfile](./docker/Dockerfile).
+- **Tag freshness.** The tag encodes only the Node version, so a Dockerfile change (like the marker above) produces _new content under an unchanged tag_. `act_runner` skips the pull when a tag of that name already exists locally (`forcePull=false` in the job log), so the runner must either force-pull (`force_pull` under `container:` in its `config.yaml`, if the installed version has it) or have the tag removed on the runner host (`docker rmi gitea.e1nsnull.de/tmu/act-ci:<version>`) after any image change. Symptom of getting this wrong: CI keeps running the previous image while the registry shows the new digest.
+
 ## Publishing workflow
 
 Publishing is CI-only by policy. Local `npm publish` is not supported. The maintainer triggers releases from `main`:
diff --git a/docker/Dockerfile b/docker/Dockerfile
index babcb5a..8cb0475 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -39,6 +39,13 @@ ARG NODE_VERSION=26.8.2
 # node image: bin/ + lib/ under /usr/local → tool cache: bin/ + lib/ under <ver>/x64.
 COPY --from=nodebase /usr/local /opt/hostedtoolcache/node/${NODE_VERSION}/x64
 
+# actions/tool-cache only accepts a cached tool when the sibling marker file
+# "<version>/<arch>.complete" exists — tc.find() checks it and falls back to
+# downloading otherwise, however complete the directory is. The marker is what
+# tc.cacheDir() writes after *it* installs a tool, so a pre-baked entry has to
+# reproduce it explicitly.
+RUN touch "/opt/hostedtoolcache/node/${NODE_VERSION}/x64.complete"
+
 # Fail the build (not CI) if the overlay or the version arg were wrong.
 # Shell form on purpose: exec form (`RUN [...]`) does not expand ARG values.
 RUN "/opt/hostedtoolcache/node/${NODE_VERSION}/x64/bin/node" --version