Open Claude Open ChatGPT
Setup Guide

Connect to ChatGPT

Add Chubby as a ChatGPT App to run Monte Carlo simulations in conversation with an interactive widget.

Requires ChatGPT Plus/Pro/Team with Developer Mode enabled in Advanced Settings.

One-Time Setup

1

Enable Developer Mode

Go to Settings → Advanced and toggle Developer Mode on. This lets you create custom Apps.

2

Create the App

Go to Settings → Apps → Create App

3

Enter the MCP Server URL

https://api.chubby.fyi/mcp

Authentication: None (no API key or OAuth required)

4

Name it

Call it chubby.fyi (or whatever you like). Save the App.

Using Chubby in a Chat

1

Start a new chat

Open chatgpt.com and start a new conversation.

2

Attach the App

Click the + button (add files and more) → More → select chubby.fyi

3

Ask your question

Type a question like "Can I retire at 50?" and ChatGPT will run the simulation and show an interactive widget inline.

Test Your Connection

Try this prompt to verify the App is working:

I'm 35, make $150k, spend $50k, saved $800k. Can I retire at 50? Seed 12345.

ChatGPT should invoke the run_simulation_packet tool and display an interactive simulation widget with trajectory bands.