PYTHON, Azure Functions Python (Core Tools v4) fails with Microsoft.Extensions.Options, Version=8.0.0.0 when running locally

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PYTHON, Azure Functions Python (Core Tools v4) fails with Microsoft.Extensions.Options, Version=8.0.0.0 when running locally

Mayra Lara 0

Problem description I am developing an Azure Functions project in Python (no .NET code at all).

When I run my project locally with Azure Functions Core Tools (4.0.5198) and with Azure Functions Core Tools (4.0.5348) , the host fails to start with the following error:

Azure Functions Core Tools

Core Tools Version: 4.0.5198 Commit hash: N/A (64-bit)

Function Runtime Version: 4.21.1.20667

Skipping 'AzureWebJobsStorage' from local settings as it's already defined in current environment variables.

[2025-09-29T15:14:35.258Z] Error configuring services in an external startup class.

[2025-09-29T15:14:35.259Z] Error configuring services in an external startup class. Microsoft.Extensions.Azure: Could not load file or assembly 'Microsoft.Extensions.Options, Version=8.0.0.0, Culture=neutral, PublicKeyToken=a'. El sistema no puede encontrar el archivo especificado.

[2025-09-29T15:14:35.323Z] A host error has occurred during startup operation '5eb06afb-73f4-4468-b896-e462d844281c'.

[2025-09-29T15:14:35.325Z] Microsoft.Azure.WebJobs.Script: Error configuring services in an external startup class. Microsoft.Extensions.Azure: Could not load file or assembly 'Microsoft.Extensions.Options, Version=8.0.0.0, Culture=neutral, PublicKeyToken=a'. El sistema no puede encontrar el archivo especificado.

Value cannot be null. (Parameter 'provider')

[2025-09-29T15:14:35.342Z] Host startup operation has been canceled

What I have tried

At fisrt I had Azure Functions Core Tools (4.0.5348) and only the .NET 8.0 runtime installed → the error occurred.
Then I downgraded to Azure Functions Core Tools (4.0.5198) and installed the .NET 6.0 runtime (so now both 6.0 and 8.0 are installed) → the error still occurs.
I deleted the ExtensionBundles folder so Core Tools could re-download the latest bundle (currently Microsoft.Azure.Functions.ExtensionBundle 4.26.2) → same error.
Confirmed my project structure is correct (host.json, local.settings.json, requirements.txt, function folders with __init__.py and function.json).

My project was working correctly and suddenly this error appears in local while production is running fine.

Is it a documented problem with a solution?

Praveen Kumar Gudipudi 2,290 Reputation points Microsoft External Staff Moderator

2025-09-29T19:09:58.2+00:00
Hello Mayra Lara,

Above error indicates that your Azure Function runtime cannot find the required assembly Microsoft.Extensions.Options version 8.0.0.0. This usually occurs due to mismatched library versions between the Azure Functions runtime and your project dependencies.

Here are a few things you could try to resolve this issue:

Verify .NET SDK Installation: Make sure that you have the appropriate versions of the .NET SDK installed (both 6.0 and 8.0 in your case seem to be fine). You may need to ensure that your local environment is set correctly for the Azure Functions Core Tools.

Check Extensions and Dependencies: Since you've deleted the ExtensionBundles folder, it might be worth checking your requirements.txt to ensure all necessary packages are installed. You can also run pip install -r requirements.txt to make sure everything is up-to-date.

Local.settings.json File: Make sure your local.settings.json file is correctly configured and that there are no missing or incorrect settings that could interfere with the function startup.

Clear Cache: Sometimes local caches might cause issues. Clearing the Azure Functions cache might help, which you can do by deleting any temporary files or using commands specific to your development environment.

Recreate Project: If the problem persists, consider creating a new Azure Functions project and copying your code files over. This can help eliminate issues caused by misconfigurations in the project's setup.

If these suggestions don't resolve your issue, I have a few questions that might help narrow down the problem:

Can you confirm the exact Python version and the Python packages you have installed?

Are you using any specific libraries or dependencies in your Azure Functions that might have compatibility issues?

Have you made any recent changes to your project or updated any packages right before this error started occurring?

Could you provide the contents of your local.settings.json file (without any sensitive data)?

Please check above steps and let us know with an update.
Mayra Lara 0 Reputation points

2025-09-30T16:10:20.78+00:00

Hi,

I updated the core tools to a newesr version 4.x
https://learn.microsoft.com/en-us/azure/azure-functions/functions-run-local?tabs=windows%2Cisolated…Azure Functions Core ToolsAzure Functions Core Tools

Previously, I was using Azure Functions Core Tools 4.0.5198, and now I’ve updated to Azure Functions Core Tools 4.0.5348.

I also updated the host.json file to set the version as: "version": "[4.0.0, 5.0.0]"

Thanks for your help
Praveen Kumar Gudipudi 2,290 Reputation points Microsoft External Staff Moderator

2025-09-30T16:56:51.41+00:00

Hello Mayra Lara,

Could you please share me the current status to proceed further?
Praveen Kumar Gudipudi 2,290 Reputation points Microsoft External Staff Moderator

2025-10-22T10:03:00.79+00:00

Hello Mayra Lara,

We haven’t heard from you on the last response and was just checking back to see if you have a resolution yet. In case if you have any resolution please do share that same with the community as it can be helpful to others. Otherwise, will respond with more details and we will try to help.

1 answer

Your answer

Mayra Lara 0 Reputation points

2025-09-30T16:10:20.78+00:00

Hi,

I updated the core tools to a newesr version 4.x
https://learn.microsoft.com/en-us/azure/azure-functions/functions-run-local?tabs=windows%2Cisolated…Azure Functions Core ToolsAzure Functions Core Tools

Previously, I was using Azure Functions Core Tools 4.0.5198, and now I’ve updated to Azure Functions Core Tools 4.0.5348.

I also updated the host.json file to set the version as: "version": "[4.0.0, 5.0.0]"

Thanks for your help
Praveen Kumar Gudipudi 2,290 Reputation points Microsoft External Staff Moderator

2025-09-30T16:56:51.41+00:00

Hello Mayra Lara,

Could you please share me the current status to proceed further?
Praveen Kumar Gudipudi 2,290 Reputation points Microsoft External Staff Moderator

2025-10-22T10:03:00.79+00:00

Hello Mayra Lara,

We haven’t heard from you on the last response and was just checking back to see if you have a resolution yet. In case if you have any resolution please do share that same with the community as it can be helpful to others. Otherwise, will respond with more details and we will try to help.

Answer 1

Praveen Kumar Gudipudi 2,290 Microsoft External Staff Moderator

Hello Mayra Lara,

Please ensure you have the following app settings. Most importantly FUNCTIONS_INPROC_NET8_ENABLED

JSONCopy

  {
    "name": "FUNCTIONS_EXTENSION_VERSION",
    "value": "~4",
    "slotSetting": false
  },
  {
    "name": "FUNCTIONS_INPROC_NET8_ENABLED",
    "value": "1",
    "slotSetting": false
  },
  {
    "name": "FUNCTIONS_WORKER_RUNTIME",
    "value": "dotnet",
    "slotSetting": false
  }

Reference: https://learn.microsoft.com/en-us/azure/azure-functions/functions-dotnet-class-library?tabs=v4%2Ccmd#updating-to-target-net-8

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PYTHON, Azure Functions Python (Core Tools v4) fails with Microsoft.Extensions.Options, Version=8.0.0.0 when running locally

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