User Data Function: 'NotFound' Error (WorkloadException) after Publishing, but Success in Develop

[Izyaan_sc]

I am setting up a monitoring workflow in Microsoft Fabric using Fabric Activator. I have created an alert based on a KQL query that monitors row ingestion. kql query - 

table1

| where ingestion_time() > ago(10m)

| summarize RowsIngested = count()
I am calling a User Data Function (UDF) as the action to push this data to an external Mimir endpoint via Remote Write.

@udf.function()
def testFunction(metricName: str, metricValue: float, labels: dict) -> str:
    logging.info(f"Starting Prometheus Remote Write push for: {metricName}")

    try:
        writer = RemoteWriter(
            url=MIMIR_URL,
            auth={
                "username": MIMIR_USERNAME,
                "password": MIMIR_PASSWORD
            }
        )

        if not isinstance(labels, dict):
            logging.error("Input 'labels' was not provided as a dictionary/object.")
            return "Push failed: labels must be a JSON object"

        timestamp_ms = int(datetime.datetime.now().timestamp() * 1000)

        metric_definition = labels.copy()
        metric_definition["__name__"] = metricName

        series = [
            {
                "metric": metric_definition,
                "values": [float(metricValue)],
                "timestamps": [timestamp_ms]
            }
        ]

        writer.send(series)

        logging.info(f"Successfully pushed {metricName} to Mimir.")
        return f"Metric pushed successfully at {datetime.datetime.now()}"

    except Exception as e:
        logging.error(f"Remote Write failed: {str(e)}")
        return f"Push failed: {str(e)}"

1. Develop Mode (Success)

When testing the function in Develop Mode using the Test Panel with the following parameters:

• metricName: "fabric_rows_ingested_test"

• metricValue: 111

• metricLabels: {"workspace": "dev", "table": "table1"}

Output: "Metric pushed successfully at 2026-03-23 06:53:51" Logs: Successfully pushed fabric_rows_ingested_test to Mimir.

above exposed metric can be accessed in grafana.

2. Run/Published Mode (Failure)

After clicking Publish and ensuring all libraries (prometheus-remote-writer, requests) are listed as "Published" in Library Management, I switch to Run only mode & invoke the function, 

{
  "functionName": "testFunction",
  "invocationId": "00000000-0000-0000-0000-000000000000",
  "status": "Failed",
  "errors": [
    {
      "errorCode": "WorkloadException",
      "subErrorCode": "NotFound",
      "message": "User data function: 'testFunction' invocation failed."
    }
  ]
}

Why does the Fabric backend fail to resolve the endpoint in Published mode (returning NotFound) when the same function is fully functional in the Develop sandbox?