[BUG] VS Code extension cannot connect to SSE MCP servers that require POST method

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?

Some SSE-based MCP servers require POST requests instead of GET
These servers return HTTP 405 (Method Not Allowed) with Allow: POST header
The same SSE MCP server configuration works fine in:
Claude CLI (when used by skills)
Cline VS Code extension
Claude Desktop app
But fails in Claude Code VS Code extension with "SSE error: Non-200 status code (405)"
Claude Code's VS Code extension should support POST-based SSE connections like other MCP clients do

Actual behavior: Connection fails with 405 error, making these enterprise SSE MCP endpoints unusable in the VS Code extension.

This affects any organization deploying SSE-based MCP servers that follow the POST pattern.

What Should Happen?

Expected behavior: SSE MCP servers that use POST should connect successfully, as they do in other Claude clients.

Error Messages/Logs

Claude Code VS Code extension with "SSE error: Non-200 status code (405)"

Steps to Reproduce

Configure an SSE MCP server in ~/.claude.json that requires POST method:

{
"mcpServers": {
"test-sse-server": {
"type": "sse",
"url": "https://example.com/mcp",
"headers": {
"Authorization": "Bearer <token>"
}
}
}
}
Ensure the SSE endpoint at https://example.com/mcp:

Returns HTTP 401/405 on GET requests
Returns HTTP 405 with Allow: POST header when using GET with auth
Accepts POST requests for SSE connection
Reload VS Code window ("Developer: Reload Window")

Check MCP server status in Claude Code chat interface

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.120

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_