How to fix Azure Functions Core Tools publish failure in VS Code: The underlying compression routine could not be loaded correctly

Resolve Azure Functions publish failures in VS Code: reinstall Core Tools via npm to fix the compression routine error, or use zip deployment as a fallback.

Publishing an Azure Functions app from VS Code can unexpectedly fail with the message The underlying compression routine could not be loaded correctly. This happens even when the project structure looks fine, the right runtime is selected, and you are signed in with Azure CLI. Below is a practical walkthrough of the symptom, the root cause, and a straight-line fix that unblocks deployment.

What the failure looks like

The publish is kicked off with the standard Core Tools command. Instead of a successful upload and remote build, the process stops as soon as packaging begins.

func azure functionapp publish my-funcapp-prod
Getting site publishing info...
[2025-04-23T12:26:20.365Z] Starting the function app deployment...
Creating archive for current directory...
Performing remote build for functions project.
Deleting the old .python_packages directory
The underlying compression routine could not be loaded correctly.

The surrounding setup in this scenario had already been validated: the working folder pointed at the actual function app code, host.json and local.settings.json were in place, requirements.txt contained azure-functions, both the Azure Function App and the local project were on Python 3.10, environment variables existed in the app configuration, Azure CLI was up to date, Azure Functions Core Tools reported version 4.0.7030, and az login had been run. None of these checks changed the outcome.

Why this happens

The error stems from issues in the Azure Functions Core Tools installation when those tools are installed via the Windows installer. In this state, the packaging step used by func publish cannot load the compression routine it depends on, and the deployment aborts.

Fix: reinstall Core Tools via npm and publish again

Reinstalling Azure Functions Core Tools using npm resolves the compression failure. The command below installs the v4 tools globally and ensures permissions are set appropriately.

npm install -g azure-functions-core-tools@4 --unsafe-perm true

After installation completes, rerun the publish command using your Function App name.

func azure functionapp publish <AppFnName>

In practice, reinstalling with npm fixed the issue where reinstalling through the Windows installer did not.

Alternative: zip deployment as a fallback

If you need a quick workaround or prefer an explicit package upload, you can deploy using zip deployment. First, create a zip archive of your project, then push it to the Function App with Azure CLI.

tar -a -c -f appbundle.zip *
az functionapp deployment source config-zip --resource-group <ResGroupName> --name <FnAppName> --src appbundle.zip

Why this matters

func publish relies on local packaging before a remote build and deployment. When the Core Tools installation is misconfigured, the compression step becomes a single point of failure and blocks delivery. Knowing that the installer source matters helps avoid wasted time checking unrelated settings like host.json, local.settings.json, runtime versions, or app configuration when the root cause is the toolchain itself.

Takeaways

If you encounter The underlying compression routine could not be loaded correctly during Azure Functions deployment, reinstall Azure Functions Core Tools via npm and retry the publish. Keep the publish flow simple: confirm you are in the right project directory, ensure you are signed in with az login, install the tools with npm, and run the publish. When needed, use the zip deployment route as a pragmatic fallback.