Using @x402-avm/core and @x402-avm/avm Packages

Build custom x402 payment integrations on Algorand using the core TypeScript SDK packages directly. These packages provide the client, resource server, and facilitator primitives for the HTTP 402 payment protocol.

Prerequisites

Before using this skill, ensure:

1. Node.js 18+ is installed
2. TypeScript project is set up
3. Algorand account is available (for signing transactions)
4. algosdk is installed as a peer dependency

Core Workflow: The x402 Payment Flow

The x402 protocol follows a standard HTTP 402 flow across three participants:

Client                    Resource Server              Facilitator
  |                             |                           |
  |-- GET /resource ----------->|                           |
  |<-- 402 PaymentRequired -----|                           |
  |                             |                           |
  |  (sign payment txns)        |                           |
  |                             |                           |
  |-- GET /resource ----------->|                           |
  |   X-PAYMENT: <payload>      |-- verify(payload) ------->|
  |                             |<-- { isValid: true } -----|
  |<-- 200 OK + content --------|                           |
  |                             |-- settle(payload) ------->|
  |                             |<-- { txId: "..." } -------|

How to Proceed

Step 1: Install Dependencies

npm install @x402-avm/core @x402-avm/avm algosdk

Step 2: Create a Client

The x402Client automatically handles 402 responses by creating and signing payments, then retrying the request with the X-PAYMENT header.

import { x402Client } from "@x402-avm/core/client";
import { registerExactAvmScheme } from "@x402-avm/avm/exact/client";
import type { ClientAvmSigner } from "@x402-avm/avm";
import algosdk from "algosdk";

const secretKey = Buffer.from(process.env.AVM_PRIVATE_KEY!, "base64");
const address = algosdk.encodeAddress(secretKey.slice(32));

const signer: ClientAvmSigner = {
  address,
  signTransactions: async (txns, indexesToSign) => {
    return txns.map((txn, i) => {
      if (indexesToSign && !indexesToSign.includes(i)) return null;
      const decoded = algosdk.decodeUnsignedTransaction(txn);
      return algosdk.signTransaction(decoded, secretKey).blob;
    });
  },
};

const client = new x402Client({ schemes: [] });
registerExactAvmScheme(client, { signer });

const response = await client.fetch("https://api.example.com/premium/data");

Step 3: Create a Resource Server

The x402ResourceServer is transport-agnostic. Use it with Express, Fastify, Hono, or any framework.

import { x402ResourceServer, HTTPFacilitatorClient } from "@x402-avm/core/server";
import { registerExactAvmScheme } from "@x402-avm/avm/exact/server";
import { ALGORAND_TESTNET_CAIP2, USDC_TESTNET_ASA_ID } from "@x402-avm/avm";

const facilitatorClient = new HTTPFacilitatorClient({
  url: "https://facilitator.example.com",
});

const server = new x402ResourceServer(facilitatorClient);
registerExactAvmScheme(server);

Step 4: Create a Facilitator

The x402Facilitator verifies payment signatures and settles transactions on-chain.

import { x402Facilitator } from "@x402-avm/core/facilitator";
import { registerExactAvmScheme } from "@x402-avm/avm/exact/facilitator";
import type { FacilitatorAvmSigner } from "@x402-avm/avm";

const facilitator = new x402Facilitator();
registerExactAvmScheme(facilitator, {
  signer: myFacilitatorSigner,
  networks: ALGORAND_TESTNET_CAIP2,
});

Step 5: Implement AVM Signers

Two signer interfaces exist:

• ClientAvmSigner: For clients paying for resources. Compatible with @txnlab/use-wallet.
• FacilitatorAvmSigner: For facilitators verifying and settling payments. Manages Algod clients, simulation, and submission.

Step 6: Apply Payment Policies (Optional)

Policies filter payment requirements before the client selects one.

import { PaymentPolicy } from "@x402-avm/core/client";
import { ALGORAND_TESTNET_CAIP2 } from "@x402-avm/avm";

const preferTestnet: PaymentPolicy = (version, requirements) => {
  return requirements.filter(r => r.network === ALGORAND_TESTNET_CAIP2);
};

registerExactAvmScheme(client, {
  signer,
  policies: [preferTestnet],
});

Important Rules / Guidelines

1. Always register the AVM scheme before using client, server, or facilitator -- call registerExactAvmScheme() after construction
2. Use CAIP-2 network identifiers in SDK code -- import ALGORAND_TESTNET_CAIP2 / ALGORAND_MAINNET_CAIP2 from @x402-avm/avm
3. Signer separation -- Protocol interfaces live in the SDK (@x402-avm/avm), implementations using algosdk live in your application code
4. TypeScript algosdk uses raw Uint8Array -- No base64 conversion needed (unlike the Python SDK)
5. Fee abstraction is automatic -- When PaymentRequirements include a feePayer, the scheme creates a 2-transaction atomic group automatically
6. Private key format -- AVM_PRIVATE_KEY is Base64-encoded 64-byte key (32-byte seed + 32-byte pubkey), address derived from secretKey.slice(32)
7. Transaction groups -- All x402-avm payments use Algorand atomic transaction groups; the paymentGroup array in the payload contains base64-encoded msgpack transaction bytes

Common Errors / Troubleshooting

Error	Cause	Solution
No scheme registered for network	AVM scheme not registered	Call registerExactAvmScheme() on the client/server/facilitator
Invalid key length: expected 64	Wrong private key format	Ensure AVM_PRIVATE_KEY is Base64 of 64-byte key
Simulation failed	Transaction would fail on-chain	Check sender balance, USDC opt-in, correct receiver
signer not found for address	Address mismatch	Verify signer address matches the account paying
Group ID mismatch	Inconsistent atomic group	Use algosdk.assignGroupID() before encoding
Fee too high	Fee exceeds MAX_REASONABLE_FEE	Check fee calculation; max is 10 ALGO
No payment requirements matched	Policies filtered all options	Review policy logic; ensure at least one requirement passes
Transaction rejected	User cancelled in wallet	Handle rejection gracefully in UI

References / Further Reading

• REFERENCE.md - Full API reference for @x402-avm/core and @x402-avm/avm
• EXAMPLES.md - Complete code examples
• x402-avm Examples Repository
• x402-avm Documentation