Mailsnail

smoke

Open-source, provider-agnostic physical mail for AI agents.

Agents have every channel except one. They can email, text, call, and post to every feed — but until now, putting a real, postmarked envelope in a mailbox meant a human signing up for a print-provider account, provisioning API keys, and wiring billing. Mailsnail closes that gap:

• No signup in managed mode. Agents pay per piece via Stripe Shared Payment Tokens over the Machine Payments Protocol (HTTP 402). First letter can go out minutes after install. $1.50 first-class, $9.00 certified, $1.00 postcard — flat, no subscription.
• No lock-in, ever. The provider layer is open and swappable: bring your own Click2Mail or Lob account, self-host the whole gateway, or chain providers for failover. The managed service is a convenience, not a cage.
• Built for compliance mail. Certified letters with extra_service: "certified" — the mail that legally must be physical (preliminary lien notices, legal notices) and must not miss its deadline because one print API had a bad day.

claude mcp add mailsnail -- npx -y mailsnail
# then: "Send me a postcard that says hello."

Packages

Package	What it is
mailsnail	MCP server — 7 tools (verify_address, preview_letter, send_letter, send_postcard, get_letter, list_letters, cancel_letter) over any provider below. Works in Claude Code, Claude Desktop, Cursor, Codex CLI, OpenAI Agents SDK.
@mailsnail/core	Provider-agnostic core: adapters (Click2Mail, Lob, any Mailsnail gateway), multi-provider failover router, request validation. Zero dependencies.
@mailsnail/gateway	Self-hostable REST API: BYO provider credentials, body_text→PDF rendering, proofs, invocation logging. The managed service at api.mailsnail.dev runs the same wire protocol.
spec/	The provider-neutral mail-piece schema + the failover-safety contract. Implement it and anything — including a print shop — becomes a Mailsnail-compatible node.

Four ways to run it

Mode	Credentials	Paying	Good for
managed (default)	none	agent, per piece (Stripe SPT)	agents; zero-setup
gateway	your own, on your server	your provider account	teams; PII stays in your infra
click2mail / lob (BYOK)	yours, local env	your provider account	direct control, one provider
failover chain	yours, per provider	your provider accounts	mail that must not miss deadlines

Failover is deliberately conservative: a send moves to the next provider only when the failed provider guarantees nothing entered production and no money moved (safeToRetry — see the spec). A duplicate certified letter is worse than an error.

Safety

• Dry-run by default everywhere; live mail requires an explicit MAIL_MCP_ALLOW_LIVE=1.
• Per-session spend caps in the MCP server; auto-refund in managed mode when a paid piece fails to mail.
• preview_letter returns a proof PDF and exact price before anything is charged — show it to your user first.
• Don't use this for spam, harassment, or fraud. Provider terms and USPS regulations apply; managed mode enforces its own abuse screening.

Why open source

You're letting software put paper in mailboxes and spend money doing it. Read the code. Run it yourself with your own provider account for free. Use managed mode when you'd rather not hold credentials at all. If we ever disappoint you, MAIL_PROVIDER=click2mail and your mail keeps moving — that's the deal, and it's enforceable because this repo exists.

Status

Early. US-only today. Letters, certified letters, and postcards. The managed API, MCP server, self-host gateway, and Click2Mail/Lob adapters are live; additional adapters (PostGrid, Stannp) are next — CONTRIBUTING.md has the adapter guide if you want one sooner.

License

MIT.