[BUG] MCP Oauth - using wrong redirect_uri for remote mcp with oauth2.1

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

I am using a remote mcp server that support oauth2.1 to enable multiple users accessing the same MCP server instance

When I add an MCP server with "type": "http", that uses oauth, it generates a call for the authorize endpoint with wrong redirect_url.

The mcp server have:
/.well-known/oauth-protected-resource
/.well-known/oauth-authorization-server
/oauth2/register
They are called, and the register returns the correct redirect_url:

{
   "redirect_uris": [
        "https://my-mcp-server/oauth2callback"
    ],
    ...
}

In the claude code (and in the remote mcp server) I see that those endpoints are called.
Regardless, the authorize endpoint that is built is:
https://my-mcp-server/oauth2/authorize?response_type=code&client_id=<CLIENT_ID>&code_challenge=<CODE_CHALLENGE>&code_challenge_method=<METHOD>&redirect_uri=http%3A%2F%2Flocalhost%3A56648%2Fcallback&state=<STATE>&scope=<SCOPES>&resource=https%3A%2F%2Fmy-mcp-server%2Fmcp
as can see - the redirect_uri is http://localhost:<port>/callback

This auth request is rejected, as this redirect_url is not authorized by the client.

What Should Happen?

The generated authorize endpoint address should have the redirect_url parameter set to the redirect_uris as set in the response of the /oauth2/register.

i.e.
https://my-mcp-server/oauth2/authorize?response_type=code&client_id=<CLIENT_ID>&code_challenge=<CODE_CHALLENGE>&code_challenge_method=<METHOD>&redirect_uri=https%3A%2F%2Fmy-mcp-server%3A56648%2Foauth2callback&state=<STATE>&scope=<SCOPES>&resource=https%3A%2F%2Fmy-mcp-server%2Fmcp
as can see - the redirect_uri is http://localhost:<port>/callback

Error Messages/Logs

Steps to Reproduce

1. Create a local .mcp.json file with the following:

{
    "mcpServers": {
        "my_oauth_mcp": {
            "type": "http",
            "url": "https://my_oauth_mcp/mcp"
        }
    }
}

1. Run claude
2. call /mcp command
3. choose the my_oauth_mcp
4. choose authenticate

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.27

Platform

Google Vertex AI

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

example mcp like that can be https://github.com/taylorwilsdon/google_workspace_mcp