How to send physical mail from an AI agent

Your agent can already send an email, call an API, open a browser, and write code. The one channel it couldn't reach was the oldest one — a stamped envelope in someone's mailbox. Mailsnail closes that gap. It's an MCP server, so any agent that speaks the protocol — Claude Code, Claude Desktop, Cursor, Codex, the OpenAI Agents SDK — can send a real, postmarked USPS letter, postcard, or certified piece with a single tool call.

Nothing to install. No API keys. You sign in, and mail draws against a prepaid balance.

The 30-second version

claude mcp add -t http mailsnail https://api.mailsnail.dev/mcp

That's it — a URL, not a package. Then open /mcp, pick mailsnail, and choose Authenticate; adding a server doesn't open the browser on its own. Sign in once and your agent has nine tools:

• verify_address — check an address is deliverable (USPS) before you spend a cent
• preview_letter — render the exact piece and return the exact price, nothing charged
• send_letter — mail a first-class or certified letter
• send_postcard — mail a 4×6 postcard
• get_letter / get_postcard — check delivery status
• get_balance — what's left, so the agent knows it can afford the job before it starts
• add_payment_method / top_up — fund the balance without leaving the conversation

What it costs

Flat and public, so an agent can reason about the price before it commits:

• First-class letter — $1.50
• Certified mail — $9.75
• Postcard — $1.00

preview_letter returns that number with nothing charged. The agent decides whether the send is worth it and — if you've wired it that way — checks with you first.

How "nothing to install" actually works

This is the part people don't expect. Most mail APIs make you create an account, add a card, and provision keys before the first send — and most MCP servers make you install a package and keep it updated. Mailsnail does neither. The server runs on our infrastructure; you add its URL and sign in through your client, the same way you'd connect any other account. There is no key to paste, and a fix on our side reaches you without you upgrading anything.

Payment works the same way it does for a transit card: you fund a balance once, and each piece draws from it. That's deliberate — card processing carries a fixed fee per charge, which on a $1.50 letter would be roughly a fifth of the price. Stripe holds the card; we never see the number. If a piece fails to mail, the charge is refunded automatically.

There's also an account-free path for agents that can pay for themselves: post a letter, get an HTTP 402 with the quoted price, and retry with a Stripe Shared Payment Token. It needs a client that can mint one, which most coding agents can't do today. (USDC/x402 settlement is planned, not shipped.)

The upshot is simple: the gap between "add the server" and "a letter is in the mail" is minutes, not a procurement cycle.

What you'd actually use it for

Not bulk marketing blasts. The pieces that fit are low-volume and high-intent — the ones never worth standing up a print vendor for:

• A thank-you note mailed after a support ticket is resolved well
• A welcome kit when someone signs a contract
• Preliminary lien notices and other compliance mail that legally has to be physical and postmarked
• Condolence or birthday cards a personal-assistant agent sends without being reminded
• One personalized letter to one account when a trigger fires, written by the agent that has the context

Send your first piece

1. Add the server — claude mcp add -t http mailsnail https://api.mailsnail.dev/mcp
2. Sign in — open /mcp, pick mailsnail, choose Authenticate, approve it in the browser
3. Ask — tell your agent to mail a postcard to a specific address
4. It verifies — verify_address confirms the address is deliverable
5. It previews — preview_letter returns the rendered piece and the exact price, still $0 charged
6. It sends — on your go-ahead, send_letter / send_postcard mails it and returns a tracking id. The first send hands back a link to add a card if you haven't funded a balance yet
7. You check — get_letter shows status

Related guides

• Send a letter from Claude, Cursor, or Codex — the exact per-client setup.
• A Lob / PostGrid alternative built for agents — if you're moving off an incumbent mail API.

claude mcp add -t http mailsnail https://api.mailsnail.dev/mcp

{
  "mcpServers": {
    "mailsnail": {
      "url": "https://api.mailsnail.dev/mcp"
    }
  }
}