Agentic Payments: x402 + MPP

Two complementary protocols for AI-agent and machine-to-machine payments on Stellar. Pick based on who depends on whom and how often the agent pays.

Quick decision

• Selling an API, want zero-XLM clients → x402 Seller in x402.md
• Calling an x402 API from an agent → x402 Buyer in x402.md
• Selling an API, no facilitator dependency → Charge mode in mpp.md
• Agent making many requests per session → Session mode in mpp.md
• Sold an API, now want agents to find it → Discovery in mpp.md
• Unsure → x402 (lowest friction to get started)

All protocols use USDC (SEP-41 SAC) by default; stellar:testnet / stellar:pubnet CAIP-2 network IDs.

Read the file that matches the task

This file carries the decision table, the shared testnet account setup, and the USDC address reference. The protocol playbooks live alongside it:

Related skills

• The SACs the protocols call → ../smart-contracts/SKILL.md
• USDC and other classic assets → ../assets/SKILL.md
• Wallets and signing in the buyer client → ../dapp/SKILL.md
• RPC simulation / submission patterns → ../data/SKILL.md
• SEP-41 (token interface) and related standards → ../standards/SKILL.md

Testnet setup (shared)

Both protocols need the same base setup: a client/payer account (signs and pays from a USDC balance) and a server/recipient account. Both need a USDC trustline.

One step is web-only (Captcha) and cannot be scripted: the Circle USDC faucet. Everything else can be automated — x402.md ships a setup.js that does steps 1–3 and writes a starter .env. (x402 additionally needs the web-only OZ Channels key generator; MPP needs no third-party key.)

1. Generate two keypairs

   node -e "const { Keypair } = require('@stellar/stellar-sdk'); for (const n of ['RECIPIENT','PAYER']) { const k = Keypair.random(); console.log(n, k.publicKey(), k.secret()); }"
   

2. Fund both with testnet XLM (friendbot)

   curl "https://friendbot.stellar.org?addr=RECIPIENT_G..."
   curl "https://friendbot.stellar.org?addr=PAYER_G..."
   

3. Add a USDC trustline to BOTH accounts — open Stellar Lab and add a USDC trustline to each G..., or run via SDK for each keypair:

   import * as StellarSdk from "@stellar/stellar-sdk";
   
   const horizon = new StellarSdk.Horizon.Server("https://horizon-testnet.stellar.org");
   // Circle's classic USDC issuer on Stellar testnet
   const USDC_ISSUER = "GBBD47IF6LWK7P7MDEVSCWR7DPUWV3NY3DTQEVFL4NAT4AQH3ZLLFLA5";
   
   async function addTrustline(secret) {
     const kp = StellarSdk.Keypair.fromSecret(secret);
     const acc = await horizon.loadAccount(kp.publicKey());
     const tx = new StellarSdk.TransactionBuilder(acc, {
       fee: StellarSdk.BASE_FEE,
       networkPassphrase: StellarSdk.Networks.TESTNET,
     })
       .addOperation(StellarSdk.Operation.changeTrust({
         asset: new StellarSdk.Asset("USDC", USDC_ISSUER),
       }))
       .setTimeout(60)
       .build();
     tx.sign(kp);
     return horizon.submitTransaction(tx);
   }
   
   // Repeat for both the recipient secret and the payer secret.
   await addTrustline(process.env.RECIPIENT_SECRET);
   await addTrustline(process.env.PAYER_SECRET);
   

   Without a trustline on the recipient, the SAC transfer settles into nothing and the request fails with op_no_trust.

4. Fund the PAYER with testnet USDC — open the Circle testnet faucet, select Stellar testnet, paste the payer's G.... Web Captcha; no API.

Two USDC addresses (don't confuse them)

USDC on Stellar has two addresses, used in different places. Mixing them up is a common stumble.

Use the exported constants instead of hard-coding when possible:

import { USDC_TESTNET_ADDRESS, USDC_PUBNET_ADDRESS } from "@x402/stellar";
// USDC_TESTNET_ADDRESS = "CBIELTK6YBZJU5UP2WWQEUCYKLPU6AUNZ2BQ4WWFEIE3USCIHMXQDAMA"
// USDC_PUBNET_ADDRESS  = "CCW67TSZV3SSS2HXMBQ5JFGCKJNXKZM7UQUWUZPUTHXSTZLEO7SJMI75"

x402's payTo route config and MPP's recipient are always a classic account (G...). The SAC address only appears where the config names the settlement asset (x402's custom asset price config, MPP's currency).