diff --git a/.gitea/workflows/publish-docs.yml b/.gitea/workflows/publish-docs.yml
index f9ba3f6d..6aa1b400 100644
--- a/.gitea/workflows/publish-docs.yml
+++ b/.gitea/workflows/publish-docs.yml
@@ -117,6 +117,8 @@ jobs:
           git config user.email "actions@gitea.tourolle.paris"
           git checkout -q -b gitea-pages
           git add -A
-          git commit -q -m "docs: publish site from ${GITHUB_SHA::8}"
+          # POSIX sh has no ${VAR::N} substring expansion — cut instead.
+          SHORT_SHA="$(printf '%s' "$GITHUB_SHA" | cut -c1-8)"
+          git commit -q -m "docs: publish site from ${SHORT_SHA}"
           echo "🚀 Force-pushing to gitea-pages"
           git push -f "$REMOTE" gitea-pages