How to send a postcard with Muse, Meta’s AI agent

1. Ask Muse to write the note

Tell Muse who the card is for and what you’d like to say, in the Muse app or wherever you chat with it. For example: “Write a postcard note to my aunt Rosa thanking her for the week at the lake. Keep it under 350 characters and make it sound like me.” Ask for three versions and keep the one you’d actually say out loud.

The 350-character limit is what fits on the back of the card. You can also ask Muse to help you choose between a couple of photos, or to set out an address the way the destination country expects.

2. Make the card at Paper Plane Post

Open https://paperplanepost.com/make in any browser, on your phone or a computer. It’s three steps on one page: add your photo, paste the note and enter the address, then review the card and pay $7 through Stripe. There’s no app to install and no account to create.

Nothing is charged or mailed until you pay. After that, the card is printed and handed to USPS within 2 business days.

What arrives in the mailbox

Your photo is printed edge to edge on a 4 × 6 inch postcard with heavy gloss stock, and your note is printed on the back in a handwriting-style typeface. The price covers printing and first-class postage, and it’s the same to every country. There’s no subscription.

Connecting Muse directly

Paper Plane Post has an MCP server at https://api.paperplanepost.com/v1/mcp. It needs no sign-in or API key, and it offers one tool, start_postcard, which takes a photo, a note and an address and returns a link where you check the card and pay. Assistants that accept custom MCP servers, ChatGPT and Claude among them, can use it today.

Muse adds outside services through Connectors. Meta’s help page says you can ask Muse to create a Custom Connector for a service that isn’t on its list, and that this can involve gathering API details from the service. The page doesn’t mention MCP, and we haven’t confirmed that a Custom Connector can reach our server, so we’d rather you didn’t count on it. If Muse does come to accept custom MCP servers, ours is ready as it stands.

For the assistant: building the start_postcard request

Call start_postcard with exactly one photo. Prefer photo_base64: read the photo file with your file or code tool and send its bytes as base64, either bare or as a data: URL. JPEG, PNG, WebP and HEIC work, up to about 10 MB. The base64 has to come from the file itself; never write it from memory or from looking at an image. If the photo is already online and you don’t have the file, send photo_url, a public https:// link, instead.

note is optional and holds up to 350 characters. recipient is optional, and so is every field in it: name, line1, line2, city, state, zip, and country as an ISO 3166-1 alpha-2 code such as US or FR. Send what the person gave you and leave the rest out; they check and complete everything before paying.

If you can’t call MCP tools but can make HTTP requests, POST the same call as JSON-RPC to the server URL. The server is stateless, so this one request is enough, with no initialize step and no session. The Accept header must list both types, as in the example below.

The reply is a single server-sent event whose data line is the JSON-RPC result. The link to give the person is result.structuredContent.url, and result.content repeats it as text. A refused photo comes back with isError set and a message saying what to change. On a busy day the reply may instead be a challenge: four questions and an id, with isError set. Answer them in order (the last one is a SHA-256 proof of work, so run code for it) and send the same request again with challenge: { "id": ..., "answers": [...] } added to the arguments. Requests larger than 16 MB are answered 413.

curl -X POST https://api.paperplanepost.com/v1/mcp \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "tools/call",
    "params": {
      "name": "start_postcard",
      "arguments": {
        "photo_base64": "<base64 of the photo file>",
        "note": "Happy birthday, Grandma. See you in June.",
        "recipient": {
          "name": "Rosa Alvarez",
          "line1": "12 Elm Street",
          "city": "Portland",
          "state": "OR",
          "zip": "97201",
          "country": "US"
        }
      }
    }
  }'