Send a letter from Claude, Cursor, or Codex

The exact per-client setup for the Mailsnail MCP server, plus a 60-second walkthrough to mail yourself a postcard from your agent.

Here's something your coding agent can do that it probably hasn't tried: put a physical letter in the mail. One MCP server — mailsnail — adds seven tools for real USPS mail, with no account and pay-per-piece billing. Below is the exact setup for the clients most people run, then a 60-second "mail me a postcard" walkthrough.

The source of truth for every client config is mailsnail.dev/setup — copy from there if a snippet here ever drifts.

Claude Code

One command:

claude mcp add mailsnail -- npx -y mailsnail

No restart needed — the tools show up on the next turn. Managed mode is the default, so there's nothing else to configure.

Claude Desktop

Add Mailsnail to claude_desktop_config.json (Settings → Developer → Edit Config):

{
  "mcpServers": {
    "mailsnail": {
      "command": "npx",
      "args": ["-y", "mailsnail"]
    }
  }
}

Quit and reopen Claude Desktop. The mail tools appear under the MCP icon.

Cursor

Add the same server block to .cursor/mcp.json (project) or ~/.cursor/mcp.json (global):

{
  "mcpServers": {
    "mailsnail": {
      "command": "npx",
      "args": ["-y", "mailsnail"]
    }
  }
}

Codex CLI

Codex reads MCP servers from ~/.codex/config.toml:

[mcp_servers.mailsnail]
command = "npx"
args = ["-y", "mailsnail"]

The magic moment: mail yourself a postcard

Once it's wired in, try this prompt verbatim:

Use mailsnail to send a postcard to [your address] that says "my agent mailed this."

Watch what the agent does:

  1. verify_address confirms your address is deliverable before spending anything.
  2. preview_letter (postcard variant) renders the piece and returns the exact price — $1.00, still nothing charged.
  3. It shows you the preview and the price, and — if you've set it up that way — waits for your go-ahead.
  4. send_postcard mails it and hands back a tracking id.

A first-class letter is the same flow at $1.50; certified mail is one flag at $9.00 with tracking. Payment is per piece via Stripe over HTTP 402, inside the wallet you authorized, and it's auto-refunded if the piece doesn't mail. (Stablecoin/x402 settlement is on the way.)

Related guides

FAQ

Do I need a Mailsnail account or API key to send a letter from Claude?

No. There is no signup and no keys. The agent pays per piece within the wallet you authorized.

The client didn't pick up the mail tools — how do I fix it?

Fully restart the client after editing its config and confirm the JSON or TOML is valid. Exact per-client steps are at https://mailsnail.dev/setup.

Can I stop the agent from sending mail without asking me?

Yes. Keep send_letter and send_postcard behind your client's tool-approval prompt. The agent can call preview_letter freely because it charges nothing; only the send actually spends.

Wire Mailsnail into your agent

Drop this into your client's MCP config (or use /setup for one-line installs).

mcp.json
{
  "mcpServers": {
    "mailsnail": {
      "command": "npx",
      "args": [
        "-y",
        "mailsnail"
      ],
      "env": {
        "MAIL_PROVIDER": "managed",
        "MAIL_API_BASE_URL": "https://api.mailsnail.dev"
      }
    }
  }
}
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