[BUG] SSE MCP connection fails with "Could not convert argument of type symbol to string"

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?

SSE-based MCP servers that were working in recent previous versions (confirmed working in 2.0.76 and regressed in 2.0.77 and 2.1.1) now fail to connect with the error:

SSE error: Could not convert argument of type symbol to string.

This error occurs immediately upon connection attempt, before any OAuth flow is attempted.

What Should Happen?

SSE MCP should continue to function as 2.0.67 and prior

Error Messages/Logs

2026-01-08T05:40:05.437Z [DEBUG] MCP server "sectool": Starting connection with timeout of 30000ms
2026-01-08T05:40:05.458Z [DEBUG] MCP server "sectool": SSE Connection failed after 47ms: {"url":"http://127.0.0.1:9119/sse","error":"SSE error: Could not convert argument of type symbol to string.","errorType":"CQ2","stack":"Error: SSE error: Could not convert argument of type symbol to string.\n    at _eventSource.onerror (file:///home/jent/.nvm/versions/node/v20.13.1/lib/node_modules/@anthropic-ai/claude-code/cli.js:1695:26035)\n    at CKA.jZ0 (file:///home/jent/.nvm/versions/node/v20.13.1/lib/node_modules/@anthropic-ai/claude-code/cli.js:1695:5571)\n    at file:///home/jent/.nvm/versions/node/v20.13.1/lib/node_modules/@anthropic-ai/claude-code/cli.js:1695:2987\n    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)"}
2026-01-08T05:40:05.459Z [ERROR] MCP server "sectool" Error: SSE error: Could not convert argument of type symbol to string.

Steps to Reproduce

1. Configure an SSE MCP server
2. Start the MCP server (any SSE-based MCP server)
3. Start Claude Code in the configured project directory
4. Connection fails immediately (can be verified via /mcp or via claude mcp list)

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.0.67

Claude Code Version

2.1.1

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

• This error pattern matches Node.js undici issue: https://github.com/nodejs/node/issues/53069
• The error occurs when HTTP headers are set with Symbol values instead of strings
• Adding X-API-Key header to config did not resolve the issue
• The same MCP server configuration worked correctly in version 2.0.76 and regressed in 2.0.77 and 2.1.1