[BUG] Claude Code HTTP MCP OAuth login drops path base

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?

Summary

Claude Code drops the base path during OAuth login for HTTP MCP servers mounted under a non-root path.

For a server configured as:

http://localhost:5210/doc-mcp

Claude Code opens:

http://localhost:5210/login?returnUrl=...

but it should open:

http://localhost:5210/doc-mcp/login?returnUrl=...

## Environment

• Claude Code: 2.1.76
• Platform: Windows
• MCP server URL: http://localhost:5210/doc-mcp
• Client ID: netmicro.middle.mcp.doc
• Callback URL: http://localhost:17899/callback

## Reproduction

1. Add an HTTP MCP server mounted under /doc-mcp
2. Trigger OAuth authentication in Claude Code
3. Observe the opened login URL

## Actual

Claude Code opens:

```text
http://localhost:5210/login?returnUrl=%2Fdoc-mcp%2Fauthorize%3Fresponse_type%3Dcode%26client_id%3Dnet.mcp.doc%26
redirect_uri%3Dhttp%253A%252F%252Flocalhost%253A17899%252Fcallback%26scope%3Dmcp.read%26state%3D...%26code_challenge%3D...%2
6code_challenge_method%3DS256

Expected

Claude Code should open:

http://localhost:5210/doc-mcp/login?returnUrl=%2Fdoc-mcp%2Fauthorize%3Fresponse_type%3Dcode%26client_id%3Dnet.mc
p.doc%26redirect_uri%3Dhttp%253A%252F%252Flocalhost%253A17899%252Fcallback%26scope%3Dmcp.read%26state%3D...%26code_challenge
%3D...%26code_challenge_method%3DS256

Why this looks wrong

The embedded returnUrl still correctly contains /doc-mcp/authorize?..., but the outer login URL loses /doc-mcp.

That suggests Claude Code is dropping the configured MCP base path when constructing the OAuth login redirect.

What Should Happen?

The embedded returnUrl still correctly contains /doc-mcp/authorize?..., but the outer login URL loses /doc-mcp.

Error Messages/Logs

Steps to Reproduce

1

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.76

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

_No response_