diff --git a/.gitea/workflows/probe-concurrency.yml b/.gitea/workflows/probe-concurrency.yml
new file mode 100644
index 0000000..fa8dcd9
--- /dev/null
+++ b/.gitea/workflows/probe-concurrency.yml
@@ -0,0 +1,38 @@
+# Throwaway probe, deleted once it has answered. weblib-viewer#15.
+#
+# Question: does Gitea 1.25.2 honour `concurrency:` with cancel-in-progress?
+# bit's versionCode design would rest on it — a later commit in the same
+# minute cancels the earlier build, so the two never both publish.
+#
+# This instance has already contradicted four reasonable assumptions (no
+# environments, owner-gated /actions/runs, artifacts invisible in both
+# listings, job logs 500), so this is measured rather than read off the docs.
+#
+# How to read the result, given job logs are unreadable here: push twice in
+# quick succession. Then in /actions/tasks,
+#
+#   both runs "success"            -> concurrency is IGNORED, both ran
+#   first "failure"/"cancelled"    -> cancel-in-progress WORKS
+#   first finishes, second waits   -> honoured, but queues instead of cancels
+#
+# The sleep is long enough that the second push lands while the first is
+# still running; without it both would finish before they could overlap.
+name: probe concurrency
+
+on:
+  push:
+    branches: [probe/concurrency]
+
+concurrency:
+  group: probe-concurrency
+  cancel-in-progress: true
+
+jobs:
+  slow:
+    runs-on: nix
+    steps:
+      - name: occupy the group for a while
+        run: |
+          echo "start $(date -u +%H:%M:%S) sha=$GITHUB_SHA"
+          sleep 120
+          echo "end   $(date -u +%H:%M:%S) sha=$GITHUB_SHA"
diff --git a/marker.txt b/marker.txt
new file mode 100644
index 0000000..740bca8
--- /dev/null
+++ b/marker.txt
@@ -0,0 +1 @@
+run 1