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Gasless Payments for AI Agents with EIP-712 Intents

#ai #ethereum #solidity #tutorial

AI agents can't hold ETH for gas. They shouldn't have to. Here's how to build a payment system where agents sign typed data and never touch gas or funds.

The problem

Your AI agent needs to pay for an API, buy data, or settle a trade. Traditional approach: give it a wallet with ETH + USDC. But then you're managing gas balances, handling failed transactions, and praying the agent doesn't get drained.

The solution: EIP-712 payment intents

Instead of executing transactions, agents sign intents - structured messages that say "I want to pay X to Y." A relayer picks up the intent, validates it against spending policies, and submits the on-chain transaction.

The agent never needs ETH. Never submits a transaction. Never holds funds.

Step 1: Define the intent structure 

struct PaymentIntent {
    address bot;      // The agent's address
    address to;       // Payment recipient
    address token;    // ERC-20 token (e.g. USDC)
    uint256 amount;   // Amount in token decimals
    uint256 deadline; // Expiry timestamp
    bytes32 ref;      // Reference hash (memo, invoice ID, etc.)
}
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Step 2: Agent signs the intent 

import { AxonClient } from '@axonfi/sdk';

const axon = new AxonClient({
  relayerUrl: 'https://relay.axonfi.xyz',
  botPrivateKey: process.env.BOT_PRIVATE_KEY,
  vaultAddress: '0xYourVault',
  chainId: 8453,
});

// Signs EIP-712 typed data + sends to relayer
const { txHash } = await axon.pay({
  to: recipientAddress,
  amount: '10.00', // 10 USDC
  memo: 'Monthly API subscription',
});
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The SDK handles:

  • EIP-712 domain separator (chain-aware, vault-specific)
  • Deadline calculation (15 min default)
  • Reference hashing (SHA-256 of memo)
  • Signature generation
  • HTTP submission to the relayer

Step 3: Relayer validates and executes

The relayer checks:

  1. Is the bot registered on this vault?
  2. Is the signature valid for this intent?
  3. Is the amount within the bot's per-tx cap?
  4. Has the bot exceeded daily/weekly limits?
  5. Is the destination whitelisted (if whitelist is set)?
  6. Is the destination blacklisted?
  7. If amount > AI threshold, run 3-agent verification

Only after all checks pass does it call the vault contract.

Step 4: Vault enforces on-chain 

function executePayment(
    PaymentIntent calldata intent,
    bytes calldata signature
) external onlyRelayer {
    require(bots[intent.bot].isActive, "Bot not active");
    require(block.timestamp <= intent.deadline, "Expired");
    require(intent.amount <= bots[intent.bot].maxPerTxAmount, "Over limit");

    // Verify EIP-712 signature
    bytes32 digest = _hashTypedDataV4(keccak256(abi.encode(...)));
    require(ECDSA.recover(digest, signature) == intent.bot, "Bad sig");

    // Transfer
    IERC20(intent.token).transfer(intent.to, intent.amount);
}
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The on-chain maxPerTxAmount is the hard ceiling. Even if the relayer is compromised, the contract enforces per-bot limits.

Python version 

from axonfi import AxonClient

axon = AxonClient(
    relayer_url="https://relay.axonfi.xyz",
    bot_private_key="0x...",
    vault_address="0x...",
    chain_id=8453,
)

result = axon.pay(
    to="0xrecipient",
    amount="5.00",
    memo="Data purchase",
)
print(result["txHash"])
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LangChain integration 

from langchain_axon import AxonToolkit

toolkit = AxonToolkit(
    relayer_url="https://relay.axonfi.xyz",
    bot_private_key="0x...",
    vault_address="0x...",
    chain_id=8453,
)

tools = toolkit.get_tools()
# Gives your agent: pay, swap, execute_protocol, get_balance, get_vault_value
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Why this matters for agent builders

  • No gas management - your agent loop doesn't need to check ETH balances or handle gas estimation
  • No fund management - one vault per fleet, not one wallet per agent
  • Instant revocation - pause a bot in one click, funds stay safe
  • Audit trail - every payment is a signed intent with a memo, queryable on-chain
  • Works on any EVM chain - same code on Base, Arbitrum, or any L2

Get started 

npm install @axonfi/sdk
# or
pip install axonfi
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Source: github.com/axonfi

The contracts are deployed on Base and Arbitrum. Testnet faucets and addresses are in TESTNET.md.

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