Testing MCP Server with NGINX + Postman - Session ID Error

[Rihab8]

Setup Details:

• MCP server running in Docker
• Exposed via NGINX reverse proxy:
• Command used for running : fastmcp run server.py --transport http --port 9000
• Trying to call MCP methods (e.g., tools/list) from Postman

When I POST a typical JSON-RPC request to the MCP endpoint (e.g., via Postman), I receive this error:

{
    "jsonrpc": "2.0",
    "id": "server-error",
    "error": {
        "code": -32600,
        "message": "Bad Request: Missing session ID"
    }
}

please suggest ways to test the MCP server and tools

[Felix-hash9898]

Hi! Did you solve the problem? I'm also facing the same issue

[Rihab8]

yes , we need to call initialize and initialized before calling the get tools.

In Initialize respnse we will get the mcp session header which can be used for further calls

SESSION_ID=$(curl -s -v -X POST http://localhost:8080/mcp/ \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc": "2.0", "method": "initialize", "id": 1, "params": {"protocolVersion": "2025-06-18", "capabilities": {}, "clientInfo": {"name": "curl-client", "version": "1.0.0"}}}' \
  2>&1 | grep -i "mcp-session-id" | sed 's/< mcp-session-id: //' | tr -d '\r')

curl -v -X POST http://localhost:8080/mcp/ \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -H "mcp-session-id: your-session-id“ \
  -d '{
        "jsonrpc": "2.0",
        "method": "notifications/initialized"
      }'

curl -X POST http://localhost:8080/mcp/ \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -H "mcp-session-id: $SESSION_ID" \
  -d '{"jsonrpc": "2.0", "method": "tools/list", "id": 2, "params": {}}'