diff --git a/docker/owlbot/nodejs_mono_repo/build-dev.sh b/docker/owlbot/nodejs_mono_repo/build-dev.sh
new file mode 100755
index 000000000..5e5700eac
--- /dev/null
+++ b/docker/owlbot/nodejs_mono_repo/build-dev.sh
@@ -0,0 +1,40 @@
+#!/bin/bash
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# File for local development of the Docker image. This is not intended to be used in CI, but can be used to test changes to the Dockerfile and entrypoint script.
+set -e
+
+# Get the directory of the script
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
+# Navigate to the root of the synthtool repo
+cd "$DIR/../../.."
+
+echo "Preparing post-processor-changes.txt..."
+# Generate post-processor-changes.txt as expected by some Dockerfiles/CI
+git log -1 --format="%B%n%nSource-Link: https://github.com/googleapis/synthtool/commit/%H" > post-processor-changes.txt && 
+sed -i "s/([^()]*)$//g" post-processor-changes.txt && 
+sed -i "s/^\(feat\|fix\)/chore/g" post-processor-changes.txt && 
+sed -i "s/\!:/:/g" post-processor-changes.txt
+
+echo "Building owlbot-nodejs-mono-repo..."
+docker build -t owlbot-nodejs-mono-repo -f docker/owlbot/nodejs_mono_repo/Dockerfile .
+
+# Cleanup temporary file
+rm post-processor-changes.txt
+
+echo ""
+echo "Successfully built owlbot-nodejs-mono-repo"
+echo "To run it against a local repository, use:"
+echo "  docker run --rm -v \$(pwd):/workspace -w /workspace owlbot-nodejs-mono-repo [RELATIVE_DIRS]"
diff --git a/synthtool/languages/node_mono_repo.py b/synthtool/languages/node_mono_repo.py
index 4232a4bb0..a545addf6 100644
--- a/synthtool/languages/node_mono_repo.py
+++ b/synthtool/languages/node_mono_repo.py
@@ -27,7 +27,6 @@
 from synthtool.languages import common
 from datetime import date
 import logging
-from os import system
 from synthtool import _tracked_paths
 from synthtool import gcp
 
@@ -641,7 +640,7 @@ def owlbot_entrypoint(
         for dir in specified_owlbot_dirs:
             owlbot_py_file_path = hasOwlBotPy(dir)
             if owlbot_py_file_path:
-                system(f"python3 {owlbot_py_file_path}")
+                subprocess.run(["python3", str(owlbot_py_file_path)], check=True)
             else:
                 owlbot_main(
                     dir,
@@ -657,7 +656,7 @@ def owlbot_entrypoint(
         for dir in owlbot_dirs:
             owlbot_py_file_path = hasOwlBotPy(dir)
             if owlbot_py_file_path:
-                system(f"python3 {owlbot_py_file_path}")
+                subprocess.run(["python3", str(owlbot_py_file_path)], check=True)
             else:
                 owlbot_main(
                     dir,