[BUG] Claude for Excel — "Work across files" MCP proxy returns 401 authentication error

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?

Subject: Bug Report: Claude for Excel — "Work across files" MCP proxy returns 401 authentication error

I can't find any helps from your support team. THerefore I have to report here. You may mark Invalid but I do need help with Claude and NO ONE is supporting me. I'll keep posting until you take notice. I'm a consistent Max user for the last 2 years already.

Hi Anthropic Support,

I'm reporting a persistent authentication failure with the "Work across files" feature in Claude for Excel. The cross-file functionality does not work — agents register locally but cannot communicate across open files.

Error details:

1. MCP Proxy Authentication Failure

• Endpoint: mcp-proxy.anthropic.com/v1/mcp/mcpsrv_01SbRTZUdryzQkHZtXLgoJwi
• HTTP Status: 401
• Response: {"type":"error","error":{"type":"authentication_error","message":"Invalid or missing authentication"},"request_id":"req_011CZwp4WYpSG5tRgJ3cxt1p"}
• Request ID: req_011CZwp4WYpSG5tRgJ3cxt1p

1. Skills Download Server Error

• Endpoint: api.anthropic.com/api/oauth/organizations/1f4540a4-75a8-49b8-aed8-7c62a25779cf/skills/skill_01PydUFvsdVS5kRk96Sa1Sew/download
• HTTP Status: 500 (Internal Server Error)

1. Agent Registry Instability

• DevTools console shows repeated: [agentRegistry] Agent registered → Agent unregistered → Agent registered (loop)
• Also logged: [DefenseInDepthBox] AsyncLocalStorage not available, defense-in-depth disabled: t is not a function

Troubleshooting already performed:

• Signed out and signed back in to the add-in
• Toggled "Let Claude work across files" on in all open files
• Set Edge Tracking Prevention to Off (resolved storage-blocking errors but 401 persists)
• Cleared Office add-in cache (%LOCALAPPDATA%\Microsoft\Office\16.0\Wef\)
• Cleared WebServiceCache (%LOCALAPPDATA%\Microsoft\Office\16.0\WebServiceCache\)
• Uninstalled antivirus software (McAfee)
• Confirmed not using LLM gateway — signed in directly with Claude account
• Confirmed paid plan with cross-file feature toggled on

Environment:

• Organization ID: 1f4540a4-75a8-49b8-aed8-7c62a25779cf
• Region: Vietnam (Hanoi)
• OS: Windows, Microsoft 365 subscription
• Add-in: Claude by Anthropic for Excel (latest version from Microsoft Marketplace)
• Sign-in method: Direct Claude account (not LLM gateway)

The MCP proxy consistently rejects authentication on every attempt. Since the request_id and org ID are included above, your backend team should be able to trace the exact failure. Please advise if this is a known issue or if there is a regional/account-level restriction affecting the MCP proxy endpoint.

Thank you for your help.

Best regards,
Hai

What Should Happen?

Work across files function should work

Error Messages/Logs

Not working

Steps to Reproduce

Try to open 2 Excel files on my machine

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

1.2.086

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_