[BUG] Plugin OAuth with different account silently fails - no error message indicates account mismatch

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?

When authenticating a plugin (e.g., Notion MCP) via OAuth using a different account than the one logged into Claude Code CLI, the authentication silently fails with a misleading error message.

• Claude Code is logged in with Account A
• /plugin → Notion MCP → "Connect" opens OAuth in browser
• User authenticates with Account B (different from Account A)
• Web Claude (claude.ai) shows Notion as connected
• Claude Code displays: "Authentication successful, but server still requires authentication. You may need to manually restart Claude Code."
• Even after restarting Claude Code, the plugin remains disconnected
• There is no indication that the account mismatch is the root cause

What Should Happen?

Either:

1. OAuth should succeed regardless of browser account, OR
2. Claude Code should display a clear error message like: "Account mismatch: please authenticate with the same account used to log into Claude Code (Account A)"

The current error message sends users into a dead-end loop of restarting and re-authenticating with no resolution.

Error Messages/Logs

Authentication successful, but server still requires authentication. You may need to manually restart Claude Code.

Steps to Reproduce

1. Log into Claude Code CLI with Account A (claude auth login)
2. Run /plugin → select "Notion" (or any OAuth-based MCP plugin)
3. Click "Connect" — browser opens OAuth authorization page
4. In the browser, log in with Account B (a different account from Account A) and authorize
5. Web Claude (claude.ai) on Account B shows Notion as connected
6. Return to Claude Code — see error: "Authentication successful, but server still requires authentication"
7. Restart Claude Code — plugin still shows as disconnected
8. Now repeat steps 2-4 but authenticate with Account A (same as CLI login)
9. Result: "Authentication successful. Connected to claude.ai Notion." — works immediately

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.78 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

• The workaround is to ensure the browser is logged into the same account as Claude Code CLI before OAuth
• Related issue: #10250 (OAuth succeeds but MCP reconnection fails — similar error message, different root cause)
• The core UX problem is that the error message is identical whether the issue is an account mismatch or a genuine reconnection failure, making it impossible for users to self-diagnose