Trying to connect to remote MCP server stuck at "Waiting for server to respond to `initialize` request..."

[SinOverCos]

Does this issue occur when all extensions are disabled?: Yes

• VS Code Version:

~> code-insiders --version
1.100.0-insider
7dbf25d8427b59f519d04587e71a9d3dfb77224c
x64

• my VSCode is ssh'd to my AWS EC2 instance

• OS Version:

ubuntu 20.04

Steps to Reproduce:

1. add remote MCP server

{
  "servers": {
    "joke-staging": {
      "type": "sse",
      "url": "http://localhost:19193/sse",
      "headers": {
        "mcp-server": "joke-staging",
      }
    }
  }
}

2. connect via the "start" button in mcp.json

VSCode debug console says:

2025-04-16 14:13:05.469 [info] 405 status sending message to http://localhost:19193/sse, will attempt to fall back to legacy SSE
2025-04-16 14:13:05.470 [warning] Failed to parse message: "/messages/?session_id=7f7ad4150c504c3bb9b9741f1930403f"
2025-04-16 14:13:10.437 [info] Waiting for server to respond to `initialize` request...
2025-04-16 14:13:15.436 [info] Waiting for server to respond to `initialize` request...
2025-04-16 14:13:20.437 [info] Waiting for server to respond to `initialize` request...

It seems that this only comes up when connecting to an MCP server that's behind an envoy proxy, but I'm looking for anything that can help me debug what's happening. On the remote MCP server side, it all looks normal

2025-04-16T18:06:55.369234266Z - stdout: INFO:     127.0.0.1:46178 - "GET /sse HTTP/1.1" 200 OK

I can connect to this server just fine via the langgraph MCP adapter and via curl:

~> curl localhost:19193/sse -H 'mcp-server: joke-staging'
event: endpoint
data: /messages/?session_id=650f5491d6914bd989d646b7fcd327b0

[SinOverCos]

Update: v1.99 doesn't have support for headers but it at least can connect via SSE to an MCP server that doesn't need headers.

v1.100 cannot connect to an MCP server via SSE at all

[connor4312]

The 405 is expected as the Python SDK uses the SSE transport (now removed from the spec) instead of streamable HTTP. We fall back gracefully when this is encountered.