Claude.ai web/mobile connector fails OAuth for remote MCP server despite valid server config

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.ai web connector and Claude mobile app fail to complete OAuth for a remote
MCP server using Clerk as the auth provider. User completes Clerk login (including
2FA) but returns to connections page without connection established.
Error: "Authorization with the MCP server failed."

MCP Inspector connects successfully to the same endpoint — confirming the
server-side implementation is correct.

MCP Endpoint

https://chat.stack.vc/api/mcp-proxy/lucern (Streamable HTTP)

Auth Provider

Clerk (clerk.stack.vc) using @clerk/mcp-tools@0.3.1

Clerk Config

• DCR: enabled
• JWT access tokens: enabled
• Public clients: enabled
• openid scope: enabled
• Callback URIs: https://claude.ai/api/mcp/auth_callback,

https://claude.com/api/mcp/auth_callback

What Works

• MCP Inspector connects (DCR → authorize → token exchange → valid JWT)
• All .well-known endpoints return correct metadata
• WWW-Authenticate header present and CORS-exposed
• Clerk login loads, user completes 2FA

What Fails

• Claude.ai web connector: login completes, redirect back fails
• Claude mobile: same
• Consistent across incognito, multiple attempts, both endpoint URLs
• ofid refs: ofid_f0231790743de464, ofid_bf48a3ed06e7da59

Environment

• Next.js 15, Vercel
• @clerk/nextjs@^6.35.5, @clerk/mcp-tools@^0.3.1

Possibly Related

• #4540 (Missing scope in DCR/authorization requests)

What Should Happen?

Should authenticate the MCP server after Clerk login, and be accessible in Claude web and Claude mobile.

Desktop works fine because I'm going through STDIO locally.

Error Messages/Logs

Steps to Reproduce

You'd need Clerk ID to reproduce it.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

Opus 4.6

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_