Letters API Live · UK

Let your agent post a real letter

Printed, enveloped and handed to Royal Mail from one call. Quoted before it sends, and on record once it has gone.

Join the previewRead the quickstart

Letters from £1.50 (15 credits). Test mode is free and posts nothing.

How a letter goes out

  1. Quote

    Send an address and content. Get back a rendered proof, a page count and a price.

  2. Approve

    Sends automatically inside your budget, or waits for approval by email.

  3. Post

    Printed, enveloped and handed to Royal Mail. A retry never sends a second copy.

  4. Prove

    The exact file, its hash and a timestamp for each stage.

The API

Three tools

The same three calls work over MCP, REST and the CLI.

  • quote_letterValidate the address, render the proof, return the price
  • send_letterCommit a quote, with a required idempotency key
  • get_letterStatus, timeline and the evidence record
quote_letterrequest and response
{
  "to": {
    "name": "Ms A. Sample",
    "line1": "12 Specimen Road",
    "postcode": "XX1 1XX"
  },
  "content_markdown": "Dear Ms Sample, ...",
  "class": "second"
}

// nothing is charged yet
{
  "quote": "qt_7f3k92qd",
  "address": "valid",
  "pages": 1,
  "price": { "credits": 15, "gbp": "1.50" },
  "proof": "proof.pdf",
  "expires": "15 minutes"
}
Field names are illustrative.

Included with every letter

Address checking

UK addresses are validated before you are charged.

Markdown or PDF

We lay out Markdown to fit the address window. PDFs are checked against the layout rules.

Fixed sender

Your verified name and return address go on every letter.

Per-recipient limit

A loop cannot send the same person letter after letter.

Status webhooks

Signed events at each stage from print to dispatch.

Refund on failure

If a letter cannot be sent, the credits go back to your balance.

Proof

A record for every letter

Every letter is kept on record against your account, with the file that was printed and when each stage happened.

  • The exact PDF that was printed, and its hash
  • Timestamps for quote, approval, print and dispatch
  • [Tracking detail, once confirmed with the printer]
On record 14 Nov 2026 16:02
ltr_7f3k92qd
quoted
15:48:02
approved
15:51:40
printed
15:58:09
dispatched
16:02:17
sha256
3f9a 61d0 8b27 e4c5 … c2e1

Sample record.

What a letter is for

Three jobs where paper still does the work.

  • Property

    Rent reminders and inspection notices

    Sent to the tenant’s address, with a record kept against the tenancy.

  • Accounts receivable

    Overdue invoice and final reminder letters

    For when the emails have gone unanswered.

  • Legal and compliance

    Notices that have to go on paper

    With the exact file and its timestamps kept on record.

Sample letters.

Letters questions

Where can you send to?

Any UK address.

How long does a letter take?

[Print cut-off and delivery estimate by class]

What happens to returned post?

Returns go to your verified return address.

Is Atomside’s name on my letter?

Only as a small “Posted via Atomside” line at the foot of the page. The recipient sees your sender name and return address.

What can’t I send?

Collecting debts for someone else, unsolicited consumer marketing, impersonation, and threats or harassment. Chasing your own invoices is fine.

Send your first test letter in the preview

Join the preview

No card. Test mode is free and posts nothing.