x402-sms

x402-sms-mcp

A paid MCP server that lets AI agents send SMS messages to US phone numbers.

Each send_sms tool call:

1. Sends a transactional SMS via the seller's toll-free number (Twilio verification in progress)
2. Auto-appends Reply STOP to opt out if the body doesn't include opt-out language
3. Costs $0.03 USDC per message, paid automatically from the configured wallet via x402

No API keys. No Twilio account. The agent pays the toll, the message goes out.

Status (2026-05-22): Public beta on Base Sepolia (testnet). The seller's toll-free number is undergoing Twilio TFV approval (submitted 2026-05-14, still in queue). During this window /send calls short-circuit to a 503 delivery_pending_tfv_approval response before the buyer wallet signs anything, so no USDC is moved. The day Twilio approves the number, delivery flips on; pricing flips to Base mainnet shortly after.

Install in Claude Desktop / Cursor / Windsurf

Add this to your MCP config (~/Library/Application Support/Claude/claude_desktop_config.json on macOS):

{
  "mcpServers": {
    "x402-sms": {
      "command": "npx",
      "args": ["-y", "x402-sms-mcp"],
      "env": {
        "BUYER_PRIVATE_KEY": "0xYOUR_PRIVATE_KEY"
      }
    }
  }
}

Restart your MCP client. A send_sms tool will appear.

What you need

A wallet on Base Sepolia (testnet) funded with:

• A small amount of ETH for gas (free from https://www.coinbase.com/faucets/base-ethereum-sepolia-faucet)
• USDC for payments (free from https://faucet.circle.com, select Base Sepolia)

Generate a throwaway key:

node -e "const {generatePrivateKey,privateKeyToAccount}=require('viem/accounts');const k=generatePrivateKey();console.log('PRIVATE_KEY=',k);console.log('ADDRESS=',privateKeyToAccount(k).address)"

Use the printed address to claim from faucets, then put the printed key into your MCP config.

Try it

In Claude Desktop, ask:

Text my cell at +15551234567 when this long-running task finishes. The recipient (me) consents to receive this message.

Claude will call send_sms with opt_in_attestation: true. You'll get back a Twilio SID + the on-chain settlement hash. The transfer is visible at https://sepolia.basescan.org.

Compliance — please read

This MCP wraps a regulated SMS gateway. The operator (you) is responsible for ensuring every recipient has consented to receive messages before invoking send_sms. The opt_in_attestation: true argument is your legal attestation under TCPA and CTIA short-code/long-code rules.

Do NOT use this MCP to:

• Send marketing or promotional content
• Text strangers, scraped lists, or anyone who hasn't opted in
• Send to non-US numbers (the seller currently rejects non-+1 E.164)
• Send anything related to S.H.A.F.T.-C (Sex, Hate, Alcohol, Firearms, Tobacco, Cannabis)

The seller-side automatically:

• Appends Reply STOP to opt out to every message
• Honors carrier-level STOP/HELP keyword handling
• Logs each send for audit purposes

If you have any doubt about consent, do not call this tool.

Configuration

Env var	Required	Default
BUYER_PRIVATE_KEY	yes	—
SMS_URL	no	https://x402-sms-server-production.up.railway.app/send

To point the MCP server at your own seller deployment, override SMS_URL.

Errors you might see

The MCP surfaces three distinct error shapes so the model can explain what happened to the user and decide whether to retry.

Cause	What you'll see in chat	Was payment taken?
Seller's toll-free number still in TFV review (HTTP 503)	"SMS not sent — service is in pre-launch wait" + the seller's submitted date and retry guidance	no
Buyer wallet exceeded per-minute/hour/day rate limit (HTTP 429)	"SMS not sent — this buyer wallet hit the per-X rate limit" + retry_after_seconds	no
Twilio-side rejection (e.g. invalid number 21211, opted-out 21610)	Friendly hint mapped from the Twilio error code + the full response body	yes (the send tried)

For the full mapping of Twilio error codes the MCP recognizes, see TWILIO_HINTS in src/index.ts.

How it works

Claude Desktop ──tool call──> MCP server (this package, on your machine)
                              │
                              │ x402 payment (USDC, Base Sepolia)
                              │ + to/message/opt_in_attestation
                              ▼
                              Public seller (Hono + Twilio on Railway)
                              │
                              │ Twilio dispatch
                              ▼
                              SMS lands on recipient's phone
                              │
                              │ Twilio SID + status
                              ▼
                              MCP server ──tool result──> Claude Desktop

The MCP server doesn't talk to Twilio directly. It signs an x402 payment with the buyer's wallet, sends the payment + message details to the seller endpoint, and the seller's toll-free number dispatches the SMS. Your private key never leaves your machine. The seller never sees it.

Roadmap

• Now (2026-05-22): Base Sepolia testnet, TFV pending. Payment flow verified end-to-end; deliveries gated on Twilio approval.
• Once TFV clears: Real US SMS delivery, testnet pricing held while we collect early traffic.
• A few weeks out: A2P 10DLC Brand approval → higher per-day throughput tier.
• Mainnet flip: Production launch, USDC settles on Base mainnet; Solana facilitator on roadmap after.

License

MIT