How to Buy an eSIM with ChatGPT
You can already ask ChatGPT which eSIM to get for Japan. It'll search the web, summarise a few blog posts and give you a confident answer built on prices that may be six months old. If you want to buy an eSIM with ChatGPT properly, with live plans, the real total and a link that ends in a QR code, you need to connect a store to it. Here's how to do that with Siima. It takes about two minutes and one URL.
With and without the connection
The difference is bigger than it sounds. Plain ChatGPT is doing research on your behalf. Connected, it's working the shop counter.
| ChatGPT on its own | ChatGPT + Siima | |
|---|---|---|
| Where prices come from | Web search: blogs, review sites, cached product pages | Live catalogue, read at the moment you ask |
| Fees | Often left out, or guessed | Included; the number quoted is the checkout total |
| Multi-country trips | Tends to assume a region covers what its name says | Reads the actual country list for each regional plan |
| Ends with | A recommendation and a link to go shopping yourself | A checkout link for the exact plan |
Connecting Siima to ChatGPT
ChatGPT talks to outside services through MCP, the same protocol Claude and most agent tools use. A custom MCP server goes in as an “app” (older builds called them connectors), and adding your own needs developer mode switched on. Do this on chatgpt.com in a browser; the mobile app doesn't have the setting.
| Step | What to do |
|---|---|
| 1 | Open Settings, go to Apps (Connectors on some accounts), then Advanced settings, and turn on Developer mode |
| 2 | Back in Apps, choose Create |
| 3 | Name it Siima. For the MCP server URL, paste https://api.siima.online/mcp |
| 4 | Set authentication to No authentication. There is no key or login to add |
| 5 | Tick the box acknowledging it's a custom app and save. ChatGPT reads the tool list straight away |
| 6 | In a new chat, pick Siima from the + menu (under developer mode) so it's active for that conversation |
OpenAI moves these menus around every few months. If the labels don't match, the thing you're looking for is always the same: developer mode, then a form that asks for a server URL. OpenAI's own help article on developer mode and MCP apps has the current path.
ChatGPT shows a fairly alarming notice for any custom app: it isn't verified by OpenAI, and a malicious one could misuse your data. That's a reasonable warning in general. For this server specifically, it has no login, only ever sees the tool inputs (a country, a plan id, a currency) rather than your conversation, and its five tools either read the public catalogue or create a checkout link. It never receives payment details. ChatGPT may also ask you to confirm before it calls a tool. That's fine to allow.
What to ask
Talk to it like you'd talk to someone at a travel shop. The more it knows about the trip, the less it has to guess. These all work:
The overland one is where the connection earns its keep. ChatGPT will check which regional plans actually include all three countries rather than trusting the region's name, which matters because they don't always match what you'd expect. And on the last question it should tell you the truth: Siima's unlimited plans give 1 GB a day at full speed, then 512 kbps until the day resets. If it tells you anything else, it's answering from memory. Check that Siima is switched on for the chat.
What happens when you say “buy it”
ChatGPT doesn't buy anything. It creates a checkout link for the plan you picked and hands it to you. From there:
That extra step is deliberate. An assistant that misheard “Peru” as “Portugal” shouldn't be able to spend your money on it, and an eSIM can't really be returned once it's installed. You see the plan, the country and the price before anything is charged. If you've not installed a travel eSIM before, the install guide walks through the QR step. Do it before you fly, and turn the line on when you land.
If you can't use developer mode
Custom apps aren't on ChatGPT's free tier, and some work accounts have them switched off by an admin. You still have two decent options.
Building your own assistant or GPT on top of the API instead? The eSIM for AI agents guide covers the tools and the REST endpoints, and the Siima for AI agents page has setup lines for other clients.