Send a letter from an AI agent in a minute, with no account

An AI agent can put a real paper letter in the post through FrankKi with no account, no API key, and nothing to install. The public MCP lane sits at https://mcp.frankki.app/api/mcp/v1/public, it exposes seven tools, and the path from an empty page to a payable checkout link is three calls. Payment happens once per letter, through a Stripe link the agent hands back into the conversation.

I ran the whole thing against production while writing this. One page, Berlin to Munich, 3.49 EUR, checkout link in hand, well under a minute. Nothing asked me for an email address, a password, or a token.

Seven tools, no login

The lane keeps its own hand-written tool list. That sounds like housekeeping, but it's the reason a business-account tool can never leak out here: if a tool isn't on the list, it doesn't exist publicly. All seven carry securitySchemes: [{ "type": "noauth" }].

The seven public tools, read on 2026-08-21

Tool	What it does	Needed for the short path?
letter_quote	Lists shipping methods and prices for a destination	optional
address_check	Checks a recipient address	optional
letter_compose	Builds the draft from sender, recipient, subject, body	yes
letter_preview	Returns page images of the finished letter	yes
checkout_link	Mints the Stripe checkout link for that exact draft	yes
order_track	Shows shipment status for the private tracking token	afterwards
upgrade_info	Explains what an account adds after a guest purchase	no

The short path is three calls

Sender, recipient, text. That's all the first call needs, because everything else has a default: black and white, standard letter, German language.

1. letter_compose renders the letter and returns a draftToken. That token is the thread the next two calls hang on, and it lives 24 hours.
2. letter_preview turns the pages into images. Skip it and there is no checkout link, more on that below.
3. checkout_link returns a Stripe checkout URL and the amount in cents. The agent posts the URL, the human pays by card, Apple Pay, Google Pay, or Link.

One detail that otherwise costs you ten minutes. The endpoint speaks Streamable HTTP and insists on both accept types. Leave out text/event-stream and you get a 406 saying "Client must accept both application/json and text/event-stream". That's the MCP transport rule, not something FrankKi invented.

The actual run, call by call

Everything below is trimmed output from the 2026-08-21 run. Tokens are shortened, nothing else was changed.

curl -X POST https://mcp.frankki.app/api/mcp/v1/public \
  -H 'content-type: application/json' \
  -H 'accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{
    "name":"letter_compose",
    "arguments":{
      "subject":"Kurze Nachricht",
      "content":"Hallo, ich melde mich kurz und wuensche dir eine gute Woche. Viele Gruesse",
      "sender":{"name":"Pascal Lindenau","street":"Musterstrasse","houseNumber":"1","zip":"10115","city":"Berlin","country":"DE"},
      "recipient":{"name":"Maria Beispiel","street":"Beispielweg","houseNumber":"2","zip":"80331","city":"Muenchen","country":"DE"}
    }}}'

The response already carries everything the agent can report back:

{"status":"success","code":"OK","data":{
  "draftToken":"k3nyw8jAeVNBKto4...",
  "trackingToken":"JM36mwCfmnE3xTx7...",
  "version":1,
  "deliveryType":"standard",
  "color":false,
  "method":{"code":"standard","nameEn":"Standard letter","deliveryTimeEn":"1 to 2 business days","tracked":false,"proofOfDelivery":false},
  "previewReady":false,
  "reviewPending":false}}

Then the preview, using that draftToken. It returns one signed image URL per page, valid for half an hour:

"pages":{"tokenExpiresAt":"2026-08-21T10:05:08.073Z",
  "items":[{"page":1,"url":"https://mcp.frankki.app/api/mcp/v1/public/page?token=eyJhbGciOiJIUzI1NiJ9..."}]}

And the till:

{"status":"success","code":"OK","data":{
  "checkoutUrl":"https://checkout.stripe.com/c/pay/cs_live_a120IQWj2PfN...",
  "sessionId":"cs_live_a120IQWj2PfN...",
  "amountCents":349,
  "deliveryType":"standard",
  "shipping":"Versand als Standardbrief, schwarz-weiss gedruckt."}}

Three calls, 349 cents, one link to pass on.

What a guest letter costs

The price is never computed in the agent. It comes from the same shipping policy that serves the iPhone app, and letter_quote hands it out fresh before every send. An agent that hardcodes a price will be wrong eventually, so the call is cheaper than the assumption.

One-page letter to Germany, read live on 2026-08-21

Method	Price	Tracked	Proof of delivery
Standard letter	3.49 EUR	no	no
Registered, drop-in	7.59 EUR	yes	yes
Registered, hand delivery	8.19 EUR	yes	yes

Transit time for the standard letter inside Germany is roughly 1 to 3 business days. If you need registered mail with real posting evidence, the longer workflow lives in how to send registered mail from an AI agent.

Six rules the lane enforces in code

An anonymous lane that prints real paper and collects real money needs edges. These are wired into the handlers, not promised in a doc.

• No preview, no checkout. checkout_link calls the preview again itself and refuses with PREVIEW_REQUIRED when no page image exists. Nobody should pay for a sheet they never saw.
• Paid first, printed second. The only trigger for submission is Stripe's checkout.session.completed. Before the money lands, the print provider sees nothing.
• Sender and recipient worldwide. Printing and posting always happen in Germany; the sender country is only the return address on the letter.
• Drafts live 24 hours. After that the draftToken is worthless and a cleanup cron collects the row.
• Every draft passes a content screen. Impersonation of authorities, banks, and debt collectors gets rejected, borderline cases go to a human, and the checkout link only appears after release. An angry but perfectly legitimate cancellation letter must not trip it, and the list was tuned against exactly that.
• No invented prices. If a destination has no priceable method, the lane refuses instead of estimating.

The account is created at the till, not before it

Stripe asks for an email address because it needs one for the receipt. After the purchase that address becomes a magic link, and the profile it opens is the same one Apple Sign-In would have created. Same address, same person, one profile. Someone who never clicks still sent a letter and keeps the private tracking token for order_track.

Where the public lane stops

It's deliberately narrow. No balance, no client separation, no templates, no letterhead, no archive, no volume tiers. One letter, one recipient, one payment. But once you send regularly, you're overpaying: the guest price is 3.49 EUR, while on the partner tariff a German standard letter runs between 2.49 and 1.49 EUR depending on monthly volume, and the ladder starts at 20 letters a month.

That's what the second lane is for, the one with sign-in, an approval queue, and a wallet. Its tool list is several times longer. Start at the developer documentation, the B2B pricing page, or the MCP overview. And if you first want to see who else ships paper from an agent, eight providers are lined up here with a check date on every row.