Using Pydantic Settings library causes intermittent 404s in Flex Consumption plan

[derekmcloughlin]

Hi,

My Python Azure Function App uses the Pydantic Settings library (https://github.com/pydantic/pydantic-settings). We have a config module that has a Settings class that looks like this:

from pydantic_settings import BaseSettings, SettingsConfigDict

class Settings(BaseSettings):

    model_config = SettingsConfigDict(env_file='.env', extra='ignore')

    SOME_CONFIG_SETTING: str

settings = Settings()

We import the settings variable in our function_app.py file:

from config import settings

This works fine locally.

The problem is that when we deploy this (Flex Consumption plan), it sometimes works, and sometimes we get back a 404 from the function app endpoint.

curl -i https://********.swedencentral-01.azurewebsites.net/api/test
HTTP/1.1 200 OK
Content-Type: text/plain; charset=utf-8
Date: Fri, 12 Sep 2025 15:40:15 GMT
Server: Kestrel
Transfer-Encoding: chunked

v6 - This is a terrible secret

... and immediately after:

curl -i https://********.swedencentral-01.azurewebsites.net/api/test
HTTP/1.1 404 Not Found
Content-Length: 0
Date: Fri, 12 Sep 2025 15:40:16 GMT
Server: Kestrel
Request-Context: appId=cid-v1:ecdd68ef-42b2-48f5-8e23-4abb541b8212

If instead I remove the last line in the config module above, and instead create the settings variable directly in the endpoint function, it works, no 404s.

Any ideas why this might be so? We normally import our config initialisation at the top in function_app.py so that it only happens once per Python worker startup. We don't want to have to put it in every function (our real one reads from an Azure Key Vault)

I've created a minimal example that shows the issue here: https://github.com/derekmcloughlin/azure-functions-pydantic-settings-test

Python 3.12.10
V2
Core Tools 4.2.2
Function Runtime Version: 4.1041.200.25360
Manual deployment from VS Code

[derekmcloughlin]

I've just tested this on a function app in the "Functions Premium" plan and it works fine. Is it only the Flex Consumption or Consumption plans that this sort of stuff happens in?