Architecture

Understanding how the X402 Facilitator enables gasless payments on ApeChain.

High-Level Overview

The X402 Facilitator acts as a trusted relayer between paying agents and the blockchain. It abstracts away gas management, transaction submission, and nonce tracking - letting developers focus on building great products.

System Components

🤖

Agent / Client

Your app or AI agent that needs to make payments. Uses the SDK to sign permits.

⚡

Facilitator API

Verifies permits and submits transactions. Pays gas fees on behalf of users.

📜

Settlement Contract

On-chain contract that executes permit + transfer atomically.

Payment Flow

1
Agent creates invoice
Specifies recipient, token, amount, and chain
2
Agent signs EIP-2612 permit
Off-chain signature - costs 0 gas
3
Facilitator verifies permit
Checks signature, balance, nonce, recipient
4
Facilitator settles on-chain
Submits tx to Settlement Contract, pays gas in $APE
5
Payment complete
Agent receives txHash confirmation

EIP-2612 Permits

The magic behind gasless payments is EIP-2612, an extension to ERC-20 tokens that allows approvals via signatures instead of transactions.

Traditional ERC-20 Flow (requires 2 transactions)

  1. User calls approve(spender, amount) - pays gas
  2. Spender calls transferFrom() - pays gas

EIP-2612 Permit Flow (1 transaction, user pays 0 gas)

  1. User signs permit message off-chain - FREE
  2. Relayer calls permit() + transferFrom() - relayer pays gas

Settlement Contract

The Settlement Contract is a simple, stateless contract that combines permit() andtransferFrom() into a single atomic transaction:

function settle(
    address token,
    address owner,
    address recipient,
    uint256 amount,
    uint256 deadline,
    uint8 v, bytes32 r, bytes32 s
) external {
    // 1. Execute the permit (approve)
    IERC20Permit(token).permit(
        owner, address(this), amount, deadline, v, r, s
    );
    
    // 2. Transfer tokens to recipient
    IERC20(token).transferFrom(owner, recipient, amount);
}

Key properties:

  • Atomic - If transfer fails, permit is not consumed
  • Stateless - No storage, minimal gas overhead
  • Non-custodial - Contract never holds funds

Security Model

✅ What the Facilitator CAN do

  • • Submit your signed permit to the blockchain
  • • Transfer the exact amount you authorized
  • • Send funds only to the recipient you specified

❌ What the Facilitator CANNOT do

  • • Change the recipient address
  • • Transfer more than the signed amount
  • • Reuse your permit (nonce protection)
  • • Access funds without your signature

Network Details

NetworkApeChain Mainnet
Chain ID33139
RPC URLhttps://apechain.calderachain.xyz/http
Block Explorerapescan.io
Settlement Contract0xe24a8dbf205ee116c991f60686f778a2337a844f
Native Token$APE (used for gas)