Azure Function App function key validation and consistency

Question

I have a Azure Function App with a function that has "authLevel" set to "anonymous" in the function.json. All works fine.

The function will not be called from anywhere other than Azure services - namely EventGrid. I still wish to secure it so I have set the "authLevel" to "function" and to get the necessary function key I have gone to the function in Azure, clicked on "Function Keys" and copied the value from the "default" function key.
To test if this will work, I have used a CURL like this:

curl -v -X POST "https://my-end-function-app.azurewebsites.net/api/my-end-point?code=my-function-key" \ -H "Content-Type: application/json" \ -d '[ { "id": "abc", "eventType": "Microsoft.EventGrid.SubscriptionValidationEvent", "subject": "test", "eventTime": "2025-05-06T00:00:00Z", "data": { "validationCode": "1234567890" }, "dataVersion": "1.0" } ]'

Initially this worked and returned a HTTP 200 but on subsequent tries, without any code or infrastructure changes, it returns HTTP 401.

Sometime later when I retried this, without any code changes, it worked, then stopped working again with a HTTP 401.

The function key on the function itself hasn't changed during these attempts.

I'm presuming the HTTP 401 is preventing me from getting EventGrid to verify this endpoint as a webhook URL - although that also seems to intermittently pass, although actual calls to the function don't work without any useful logging.

The function app is using the Consumption Hosting Plan and is using Python on Linux.

To redeploy, I'm using ZIP deploy for now and not recreating the Function app.Any ideas on why the function keys aren't working consistently?

I have also tried to add a new function key within the function, however the new one does not show up after creation even though I get a "Key ... created" message confirming it worked.

Adding a new host key initially worked then started failing with 401 again.

Regardless of the 401 failures, I tried to use this as the webhook URL for an EventGrid subscription but get the following error (oddly enough the URL it shows in the message does not contain the "code" parameter:
Deployment has failed with the following error: {"code":"URL validation","message":"Webhook validation handshake failed for [my url]. Http POST request failed with response code Unknown

Answer 1

Hello Simon Stewart,

Thank you for posting your question in the Microsoft Q&A forum. 

The intermittent HTTP 401 errors and vanishing function keys in your Azure Function App suggest underlying key management instability, potentially tied to the Consumption Plan's dynamic scaling or Linux/Python runtime quirks. While your authLevel="function" configuration is correct, the inconsistent key behavior points to: 

• Key Synchronization Delays: The Linux Consumption Plan’s ephemeral containers may fail to propagate key updates immediately, causing temporary 401s until the new key syncs across instances. 
• EventGrid Validation Quirks: EventGrid’s webhook validation ignores the code parameter in its initial handshake, requiring a publicly accessible endpoint (anonymous) for validation, followed by function-level auth for actual events. 
• ZIP Deploy Cache Issues: Python functions on Linux may retain stale key references after deployment. Try restarting the Function App post-deployment. 

Solutions you may try - 

• Use Host Keys Instead: Replace code=my-function-key with code=my-host-key (found under App Keys), as host keys are more stable. 
• Split Authentication: Temporarily set authLevel="anonymous" for EventGrid validation, then switch to function post-validation (despite the security trade-off). 
• Check Logs: Enable Application Insights to trace 401s and key-related errors (Host.Function.APIKey). 
• Migrate to Premium Plan: For production, avoid Consumption Plan key sync issues by upgrading. 

If keys still disappear, recreate the Function App or raise a support ticket this may indicate a platform bug. 

Some reference documentation may help: 

https://learn.microsoft.com/en-us/azure/azure-functions/functions-bindings-http-webhook-trigger?tabs=python-v2%2Cisolated-process%2Cnodejs-v4%2Cfunctionsv2&pivots=programming-language-csharp#api-key-authorization 

https://learn.microsoft.com/en-us/azure/event-grid/end-point-validation-cloud-events-schema  

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