Azure Function MCP trigger fails to respond to initialize request from VS Code MCP client

Question

Azure Function MCP trigger fails to respond to initialize request from VS Code MCP client

Aaron Ching 10 Microsoft Employee

We have a vanilla Azure Function created with a vanilla hello world McpToolTrigger. It works great when called locally from VS Code MCP Client but it keeps failing with "Waiting for server to respond to initialize request..." when calling with the exact same Azure Function published to Azure. It doesn't seem to be networking issue because if we modify the mcp_extension key a little bit, it will immediately return 401 error.

Our VS Code version is 1.99.3. Function app is version 4, just the basic vanilla project created from Visual Studio 2022 and published directly from Visual Studio. We have also confirmed that the Function app is running as expected in Azure and we can ping the endpoint as expected.

Loknathsatyasaivarma Mahali 1,875 Reputation points Microsoft External Staff Moderator

2025-05-06T07:41:27.87+00:00

Hello Aaron Ching,

Thanks for reaching out, you're encountering the issue “Waiting for server to respond to initialize request...” because your deployed Azure Function using an McpToolTrigger likely isn't initializing properly in the cloud environment. According to the official Microsoft blog on building MCP remote servers with Azure Functions, there are a few key configurations you need to ensure. First, set the AzureWebJobsFeatureFlags to EnableWorkerIndexing in your Azure Function App's application settings—this is required for custom triggers like MCP to work properly in Azure.

Next, enable the “Always On” option in your App Service settings to avoid cold starts, which can interfere with custom extension initialization. Also, make sure the authLevel in your function.json is correctly set (e.g., anonymous for open access or function if you're using function keys). Additionally, confirm that the MCP extension is correctly referenced, either through extensions.csproj or an extension bundle in host.json.

Finally, check Azure's Log Stream or Application Insights for any errors during the function startup to pinpoint what’s going wrong. Following these steps should help your remote MCP function initialize and respond correctly.
Loknathsatyasaivarma Mahali 1,875 Reputation points Microsoft External Staff Moderator

2025-05-07T08:49:26.3966667+00:00

Hello Aaron Ching,

Just checking in to see if the information provided helps you better understand. If you have any further concerns or queries, please feel free to drop them here.
Aaron Ching 10 Reputation points Microsoft Employee

2025-05-07T16:40:33.8366667+00:00

Loknathsatyasaivarma, thanks for your suggestions. The issue got resolved when we change our host to Linus instead of Windows. We also got confirmation from some Azure folks that it's a known issue with Windows host at the moment.

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Loknathsatyasaivarma Mahali 1,875 Reputation points Microsoft External Staff Moderator

2025-05-06T07:41:27.87+00:00

Hello Aaron Ching,

Thanks for reaching out, you're encountering the issue “Waiting for server to respond to initialize request...” because your deployed Azure Function using an McpToolTrigger likely isn't initializing properly in the cloud environment. According to the official Microsoft blog on building MCP remote servers with Azure Functions, there are a few key configurations you need to ensure. First, set the AzureWebJobsFeatureFlags to EnableWorkerIndexing in your Azure Function App's application settings—this is required for custom triggers like MCP to work properly in Azure.

Next, enable the “Always On” option in your App Service settings to avoid cold starts, which can interfere with custom extension initialization. Also, make sure the authLevel in your function.json is correctly set (e.g., anonymous for open access or function if you're using function keys). Additionally, confirm that the MCP extension is correctly referenced, either through extensions.csproj or an extension bundle in host.json.

Finally, check Azure's Log Stream or Application Insights for any errors during the function startup to pinpoint what’s going wrong. Following these steps should help your remote MCP function initialize and respond correctly.
Loknathsatyasaivarma Mahali 1,875 Reputation points Microsoft External Staff Moderator

2025-05-07T08:49:26.3966667+00:00

Hello Aaron Ching,

Just checking in to see if the information provided helps you better understand. If you have any further concerns or queries, please feel free to drop them here.
Aaron Ching 10 Reputation points Microsoft Employee

2025-05-07T16:40:33.8366667+00:00

Loknathsatyasaivarma, thanks for your suggestions. The issue got resolved when we change our host to Linus instead of Windows. We also got confirmation from some Azure folks that it's a known issue with Windows host at the moment.

Answer 1

Hello Aaron Ching,Thanks for you patience on this.

I have reached out to Azure Functions engineering team on this issue and sharing the below.

Upon investigating further, team has confirmed that this is a know issue and a fix related to this issue has been published in preview version 4.*. NuGet Gallery | Microsoft.Azure.Functions.Worker.Extensions.Mcp 1.0.0-preview.4

Also engineer team has suggested to the latest version preview5 which helps in avoiding hanging and opening a bunch of connections. NuGet Gallery | Microsoft.Azure.Functions.Worker.Extensions.Mcp 1.0.0-preview.5

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Azure Function MCP trigger fails to respond to initialize request from VS Code MCP client

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