GPT action + ngrok - how to setup?

I’m trying to setup a GPT action (using custom GPT and not the API) with local server, and use ngrok as the mediator But the GPT action request does not reach my server

Here’s my setup :

  1. I called the command line ‘ngrok http 8080’, which gave me
  2. I create an action in custom GPT with the following schema and no authentication
    "openapi": "3.0.0",
    "info": {
        "title": "Test Connectivity API",
        "version": "1.0.0",
        "description": "API to test connectivity by calling a specified URL and expecting a 200   status"
    "servers": [
      "url": "",
      "description": "Ngrok server for testing connectivity"
  "paths": {
    "/": {
      "get": {
        "summary": "Test Connectivity",
        "operationId": "testConnectivity",
        "parameters": [],
        "responses": {
          "200": {
            "description": "Successful response with status 200 and 'hello' message",
            "content": {
              "application/json": {
                "schema": {
                  "type": "object",
                  "properties": {
                    "message": {
                      "type": "string",
                      "example": "Hello, HTTP!"
          "400": {
            "description": "Bad request response"
  1. From my local server, I started some process with C# HttpListener on port *:8080

When I try to connect with a browser to, everything is working and I can see it reaches my server, and I get the response in my browser

When I try to use it from the GPT action it does not reach my server at all

Same schema works for me:

but response is unexpected, it returns a HTML text.

When I open url in browser, and received a screen prompt:

I’m not sure if your problem caused by this problem.