Vercel MCP connector fails during handshake after successful OAuth

### What happened?

## Description
Connecting the Vercel MCP server (https://mcp.vercel.com) fails
consistently. OAuth authorisation completes successfully, but the
connection then fails and the connector loads with no tools available.

## Error message
"Your account was authorised, but Vercel returned an error when
connecting. You can try again or check whether the server is working.
If the problem persists, share this reference with support: [ID]"

## Reference IDs
- ofid_d699c20c11607a5e
- ofid_ec50b1e9c1106f66

## Environment
- Server URL: https://mcp.vercel.com
- Reproduced on: claude.ai (web) and Claude desktop app
- Both attempts: OAuth flow completes, connector appears, zero tools

## Steps already taken
- Unlinked and relinked the connector
- Fresh conversations after reconnecting
- Added the server via claude_desktop_config.json using mcp-remote
- Incognito window
- Cleared Vercel session cookies and logged out before reconnecting

None of the above changed the outcome.

## Note
Vercel support confirmed these ofid_ reference IDs are generated on
the Claude side during the connection handshake, and directed me here.

### What did you expect to happen?

The connector should complete the connection after OAuth and expose
the Vercel MCP tools (documentation search, project and deployment
management, log analysis).

### Steps to reproduce

1. Go to Settings > Connectors and add the Vercel connector
2. Complete the OAuth authorisation flow (this succeeds)
3. Connection then fails with the error below
4. Connector appears in the list but shows "This connector has no
tools available"

Reproduced identically on claude.ai (web) and the Claude desktop app.
Also attempted via claude_desktop_config.json using mcp-remote, with
the same outcome.

### Area

MCP Connector (adding/managing servers)

### MCP Server (if applicable)

Vercel (https://mcp.vercel.com)

### Error messages or logs

```shell
Your account was authorised, but Vercel returned an error when
connecting. You can try again or check whether the server is working.
If the problem persists, share this reference with support:
ofid_d699c20c11607a5e

(second attempt returned: ofid_ec50b1e9c1106f66)
```

### Additional context

Vercel support confirmed that the ofid_ reference IDs are generated
on the Claude side during the connection handshake, and directed me
here as Anthropic has visibility into the connector logs.

Already tried without success:
- Unlinking and relinking the connector
- Fresh conversations after reconnecting
- Incognito window
- Logging out of Vercel and clearing its session cookies
- claude_desktop_config.json with mcp-remote

Vercel MCP is currently in Beta.