diff --git a/hack/jenkins/upload-to-gcs.sh b/hack/jenkins/upload-to-gcs.sh
index 13c5fafb39b..5c099dbde28 100755
--- a/hack/jenkins/upload-to-gcs.sh
+++ b/hack/jenkins/upload-to-gcs.sh
@@ -31,6 +31,8 @@
 # Note: for magicfile support to work correctly, the "file" utility must be
 # installed.
 
+# TODO(rmmh): rewrite this script in Python so we can actually test it!
+
 set -o errexit
 set -o nounset
 set -o pipefail
@@ -102,6 +104,35 @@ function find_version() {
   )
 }
 
+# Output started.json. Use test function below!
+function print_started() {
+  local metadata_keys=$(compgen -e | grep ^BUILD_METADATA_)
+  echo "{"
+  echo "    \"version\": \"${version}\","
+  echo "    \"timestamp\": ${timestamp},"
+  if [[ -n "${metadata_keys}" ]]; then
+    # Any exported variables of the form BUILD_METADATA_KEY=VALUE
+    # will be available as started["metadata"][KEY.lower()].
+    echo "    \"metadata\": {"
+    local sep=""  # leading commas are easy to track
+    for env_var in $metadata_keys; do
+      local var_upper="${env_var#BUILD_METADATA_}"
+      echo "        $sep\"${var_upper,,}\": \"${!env_var}\""
+      sep=","
+    done
+    echo "    },"
+  fi
+  echo "    \"jenkins-node\": \"${NODE_NAME:-}\""
+  echo "}"
+}
+
+# Use this to test changes to print_started.
+if [[ -n "${TEST_STARTED_JSON:-}" ]]; then
+  version=$(find_version)
+  cat <(print_started) | jq .
+  exit
+fi
+
 function upload_version() {
   local -r version=$(find_version)
   local upload_attempt
@@ -117,13 +148,7 @@ function upload_version() {
   local -r json_file="${gcs_build_path}/started.json"
   for upload_attempt in {1..3}; do
     echo "Uploading version to: ${json_file} (attempt ${upload_attempt})"
-    gsutil -q -h "Content-Type:application/json" cp -a "${gcs_acl}" <(
-      echo "{"
-      echo "    \"version\": \"${version}\","
-      echo "    \"timestamp\": ${timestamp},"
-      echo "    \"jenkins-node\": \"${NODE_NAME:-}\""
-      echo "}"
-    ) "${json_file}" || continue
+    gsutil -q -h "Content-Type:application/json" cp -a "${gcs_acl}" <(print_started) "${json_file}" || continue
     break
   done
 }