SPNEGO MCP Proxy

This local HTTP proxy binds exclusively to 127.0.0.1 and forwards requests to the configured MCP endpoint. It uses the current macOS user's Kerberos ticket cache to authenticate MCP Streamable HTTP requests with SPNEGO. TLS validation uses the macOS system trust store, including private certificate authorities.

The upstream is set at startup through MCP_UPSTREAM_URL. The proxy does not store passwords or disable certificate validation. All components use Python, and VS Code starts them automatically as a stdio MCP server.

Prerequisites

• A valid Kerberos ticket: klist -s

• Python 3.11 or newer

• Xcode Command Line Tools and Kerberos development files if requests-gssapi must be compiled locally

Related MCP server: macOS Companion MCP Server

Installation

cd /Users/cwbr/Tools/git/spnego-mcp-proxy
python3 -m venv .venv
.venv/bin/pip install -r requirements.txt

If the required private CA is not installed in the macOS Keychain, ask your IT team to install it. Certificate validation remains enabled.

VS Code Configuration and Startup

VS Code starts the adapter automatically when the MCP server starts. The local HTTP port is only used inside the adapter process. Set the upstream for each server through env in the user mcp.json file:

"spnegoMcpProxy": {
  "type": "stdio",
  "command": "/Users/cwbr/Tools/git/spnego-mcp-proxy/.venv/bin/python",
  "args": [
    "/Users/cwbr/Tools/git/spnego-mcp-proxy/stdio_adapter.py"
  ],
  "env": {
    "MCP_UPSTREAM_URL": "https://mcp.example.internal/mcp"
  }
}

The URL is not a secret, so a .env file is not needed. Additional MCP servers can each use their own name and MCP_UPSTREAM_URL value. The dynamically chosen local port allows them to run concurrently.

Then run MCP: List Servers and start or restart the server. Use MCP: List Servers > Show Output to inspect connection issues.

The process starts the SPNEGO proxy automatically. It is stopped when the MCP server is stopped or restarted.

Manual HTTP Check

For a separate HTTP test, you can still start the proxy manually:

MCP_UPSTREAM_URL=https://mcp.example.internal/mcp \
  .venv/bin/waitress-serve --host=127.0.0.1 --port=8765 spnego_mcp_proxy:app

In a second terminal:

curl --fail-with-body --silent --show-error \
  --request POST http://127.0.0.1:8765/mcp \
  --header 'Content-Type: application/json' \
  --header 'Accept: application/json, text/event-stream' \
  --data '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-03-26","capabilities":{},"clientInfo":{"name":"manual-test","version":"1.0"}}}'

Verify that the MCP response is successful, then stop the proxy with Ctrl+C.