[BUG] MCP stream uses POST instead of GET for specific user (405 Method Not Allowed)

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?

We are experiencing an issue connecting Microsoft 365 via MCP for a specific user.

Key findings:

• Azure authentication completes successfully
• Admin user can connect without issues
• This specific user consistently fails during MCP connection

HAR analysis shows incorrect HTTP method usage:

Expected:
GET /v1/toolbox/shttp/mcp/{id}

Actual:
POST /v1/toolbox/shttp/mcp/{id}
→ 405 Method Not Allowed

Additional observations:

• CORS preflight allows only GET, OPTIONS
• Same behavior across Chrome, Safari, and Atlas browser
• Service Worker cleared/unregistered
• QUIC disabled during testing (no impact)
• No VPN, proxy, or network restrictions

Additionally, the UI displays the following error when attempting the connection:

"Authorization with the MCP server failed. You can check your credentials and permissions. If this persists, share this reference with support: ofid_e6e3dd93df70085b"

This occurs consistently for the affected user, even though:

• Credentials are valid
• Azure authentication succeeds
• Admin users can connect successfully

Screenshot:

!MCP Authorization Error
<img width="480" height="89" alt="Image" src="https://github.com/user-attachments/assets/ba177e03-fd4c-4fde-8a21-fd561f3c3059" />

What Should Happen?

The MCP stream should be initiated using a GET request, as expected by the backend.

The user should be able to successfully establish the MCP connection, similar to the admin user.

Error Messages/Logs

POST /v1/toolbox/shttp/mcp/{id} → 405 Method Not Allowed

CORS preflight:
Access-Control-Allow-Methods: GET, OPTIONS

Steps to Reproduce

1. Log in with affected user account
2. Attempt to connect Microsoft 365 integration via MCP
3. Observe network requests in DevTools
4. MCP request is sent as POST instead of GET
5. Backend responds with 405 Method Not Allowed
6. Connection fails

Control test:

• Repeat same steps with admin user → works correctly (uses GET)

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

Unknown (issue happens before model interaction)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

• Issue seems user-specific (likely feature flag or backend routing inconsistency)
• Admin account works correctly under same environment
• Issue persists across multiple browsers and clean environments
• Suspected root cause: incorrect MCP client behavior or feature flag mismatch

Reference ID:
ofid_e6e3dd93df70085b