MCP client authenticate using Kerberos/NTLM if the server replies with WWW-Authenticate: Negotiate

[dagirard]

Does this issue occur when all extensions are disabled?: Yes/No

• VS Code Version: 1.108.2
• OS Version: 25HW 26200.7634

Some server software runs locally for development. This is typical for ERP software for example.
Those servers may offer a remote Http Streamable MCP implementation out of the box.

For example:

"MyServer": {
  "url": "http://localhost:7047/mcp",
  "type": "http",
  "headers": {
      "ServerHeader": "Sample"
  }
}

Those on-prem servers usually uses Windows authentication for APIs.

Repro step:

1. Set-up MCP connection string in VS Code
2. VS Code tries to authenticate to the server, it fails and it receives an HTTP 401 with WWW-Authenticate set to "Negotiate"

Expected:
VS Code see the WWW-Authenticate header set to "Negotiate", and it tries to use current user Windows authentication to connect to the MCP server. Which, succeed.

Actual:
VS Code initiate the MCP authorization flow. Trying to request all the .well-known etc... which are failing with HTTP 404. Then it falls back, and still ask the user to provide a client ID.
At the end, authentication failed and VSCode cannot connect.

Current workaround:
Using Fiddler as proxy, with the "Automatically Authenticate" feature enabling. So Fiddler automatically authenticate with the local MCP server and everything works fine.
But, if anything goes wrong, VS Code must be restarted. It is not a nice experience.
Remark: You must change localhost with your hostname, so the call goes through Fiddler proxy.

[TylerLeonhardt]

I don't know much about Kerberos/NTLM so I'd really love it if you give me a sample MCP server to test with so I can have a live example of how this should work.

Is that something you can help me with?

@chrmarti any chance you have any advice for me from your ownership of proxy support?

[dagirard]

I don't know much about Kerberos/NTLM so I'd really love it if you give me a sample MCP server to test with so I can have a live example of how this should work.

Is that something you can help me with?

@chrmarti any chance you have any advice for me from your ownership of proxy support?

Sure, I updated the SDK AspNetCore sample to use ASP.NET Kerberos/NTLM default implementation, and the sample app.
modelcontextprotocol/csharp-sdk@main...dagirard:csharp-sdk:dagirard/KerberosSample

You need to update the http launchSettings with your hostname, eg:
http://localhost:3001;http://**YOURHOSTNAME**:3001

Then VSCode should target http://YOURHOSTNAME:3001.

With Fiddler, I managed to see the HTTP 401 returned by the server with the WWW-Authenticate: Negotiate, then with the "Automatically connect" Fiddler negotiated the authentication for me and VSCode managed to connect.

[TylerLeonhardt]

Thanks @dagirard, as soon as I am able to understand how we do Kerberos/NTLM in VS Code I can try out your sample.

[chrmarti]

@TylerLeonhardt When we get an 407 response from the proxy with a proxy-authenticate header we look up the credentials here:

vscode/src/vs/workbench/api/node/proxyResolver.ts

Line 414 in a591433

async function lookupProxyAuthorization(

We could look into reusing that for regular connections (not just proxy connections).

[dagirard]

@TylerLeonhardt, Any progress on this project?