diff --git a/build.sh b/build.sh
index 607dd17..22d2097 100755
--- a/build.sh
+++ b/build.sh
@@ -47,7 +47,13 @@ CACHE_DIR="${REPO_ROOT}/cache"
 
 [[ -n "${BUILD_MODE}" ]] || { echo "BUILD_MODE not set in ${CONF}" >&2; exit 1; }
 
-buildid="$(date '+%Y%m%d.0')"
+# Build IDs must be unique per build (release tags are derived from them), so
+# include the CI run number when available, otherwise a timestamp.
+if [[ -n "${GITHUB_RUN_NUMBER:-}" ]]; then
+  buildid="$(date '+%Y%m%d').${GITHUB_RUN_NUMBER}"
+else
+  buildid="$(date '+%Y%m%d.%H%M%S')"
+fi
 echo "${buildid}" > "${REPO_ROOT}/.buildid"
 
 rm -rf "${BUILD_DIR}"