SDK & reference
HPP works with the standard @x402 SDK maintained by the
x402 Foundation. There is no HPP-specific fork — HPP
Mainnet and Sepolia are recognized via their CAIP-2 network identifiers, the same way the SDK
recognizes Base, Polygon, and other EVM chains.
Packages
| Package | Used by | Purpose |
|---|---|---|
@x402/core | both | x402Client, HTTPFacilitatorClient, RoutesConfig types. |
@x402/evm | both | EVM scheme clients (exact, upto) and the DEFAULT_STABLECOINS asset map. |
@x402/express | sellers | paymentMiddleware and x402ResourceServer for Express. |
@x402/extensions | sellers | Route extensions, e.g. declareEip2612GasSponsoringExtension() for gasless upto. |
@x402/fetch | buyers | wrapFetchWithPayment to make a paying fetch. |
viem | buyers | Wallet account and chain utilities (privateKeyToAccount, defineChain). |
Scheme clients are imported per role and scheme:
// Seller (resource server)
import { ExactEvmScheme } from "@x402/evm/exact/server";
import { UptoEvmScheme } from "@x402/evm/upto/server";
// Buyer (client)
import { ExactEvmScheme } from "@x402/evm/exact/client";
import { UptoEvmScheme } from "@x402/evm/upto/client";
HPP network identifiers
x402 uses CAIP-2 network identifiers. Use these values for
the network field everywhere in the SDK:
| Network | x402 network |
|---|---|
| HPP Mainnet | eip155:190415 |
| HPP Sepolia | eip155:181228 |
Asset
| Field | Value |
|---|---|
| USDC.e address (both chains) | 0x401eCb1D350407f13ba348573E5630B83638E30D |
| Decimals | 6 |
| EIP-712 domain | name Bridged USDC, version 2 |
Register it in the SDK's default-asset map so EIP-3009 domains are filled in automatically — see Quickstart: Sellers → Register USDC.e.
Discovery
x402 includes an optional discovery mechanism (the "bazaar" extension) that lets sellers publish their paid endpoints so buyers and agents can find them. A seller opts in by declaring the discovery extension on its routes; HPP operates a discovery service that indexes published HPP x402 services. Discovery is not required to accept or make payments — it is an add-on for marketplace-style use.
Links
- x402 specification & SDK — https://github.com/x402-foundation/x402
- HPP Facilitator — endpoints and schemes
- HPP Networks & Token — chain IDs and RPC
- HPP Network Information — WSS, Chainlist, explorer