ERC-20 Approval Gas Sponsoring

The ERC-20 approval gas sponsoring extension enables gasless Permit2 approval for any ERC-20 token, including those that do not implement EIP-2612. The client signs (but does not broadcast) a standard approve(Permit2, MaxUint256) transaction, and the facilitator broadcasts it atomically before settling the Permit2 payment.

Overview

This is the universal fallback for gasless Permit2 onboarding. While EIP-2612 gas sponsoring is preferred for tokens that support it, this extension works with every ERC-20 token:

For Buyers: No gas needed — sign an approval transaction off-chain and the facilitator broadcasts it
For Sellers: Advertise this extension to support the widest range of ERC-20 tokens
For Facilitators: Broadcast the pre-signed approval and settle in an atomic batch, optionally funding the client’s gas if needed

How It Works

Server advertises erc20ApprovalGasSponsoring in the PaymentRequired response extensions
Client checks if Permit2 allowance is insufficient; if so, signs a raw approve(Permit2, MaxUint256) transaction without broadcasting it
Client includes the signed transaction in extensions.erc20ApprovalGasSponsoring.info.signedTransaction
Facilitator executes an atomic batch:
- Funds the client’s wallet with gas (if needed)
- Broadcasts the client’s signed approval transaction
- Calls x402ExactPermit2Proxy.settle() to complete the payment

The atomic batch ensures the approval and settlement happen together — there is no window for front-running between the approval and the payment.

Server Usage

Advertise support for this extension in your route configuration:

TypeScript
Go
Python

import { declareErc20ApprovalGasSponsoringExtension } from "@x402/extensions/erc20-approval-gas-sponsoring";

const routes = {
  "GET /api/data": {
    accepts: [{
      scheme: "exact",
      network: "eip155:84532",
      price: "$0.01",
      payTo: "0xYourAddress",
    }],
    extensions: {
      ...declareErc20ApprovalGasSponsoringExtension(),
    },
  },
};

import (
    "github.com/x402-foundation/x402/go/v2/extensions/erc20approvalgassponsor"
)

extensions := erc20approvalgassponsor.DeclareExtension()
// Include in your route's Extensions field

from x402.extensions.erc20_approval_gas_sponsoring import (
    declare_erc20_approval_gas_sponsoring_extension,
)

routes = {
    "GET /api/data": {
        "accepts": {
            "scheme": "exact",
            "network": "eip155:84532",
            "price": "$0.01",
            "payTo": "0xYourAddress",
        },
        "extensions": {
            **declare_erc20_approval_gas_sponsoring_extension(),
        },
    },
}

Client Usage

The ExactEvmScheme handles ERC-20 approval gas sponsoring automatically as a fallback when EIP-2612 is not available.

TypeScript
Go
Python

import { ExactEvmScheme } from "@x402/evm/exact/client";

// signTransaction and getTransactionCount capabilities are required
const scheme = new ExactEvmScheme(signer);
client.register("eip155:*", scheme);

// The scheme automatically signs an ERC-20 approval when:
// 1. The server advertises erc20ApprovalGasSponsoring
// 2. The asset transfer method is "permit2"
// 3. The client's Permit2 allowance is insufficient
// 4. EIP-2612 gas sponsoring is not available or not supported by the token

import (
    evm "github.com/x402-foundation/x402/go/v2/mechanisms/evm/exact/client"
)

scheme := evm.NewExactEvmScheme(signer)
client.Register("eip155:*", scheme)

// ERC-20 approval gas sponsoring is handled automatically
// as a fallback when EIP-2612 is not available

from x402 import x402Client
from x402.mechanisms.evm import EthAccountSignerWithRPC
from x402.mechanisms.evm.exact import register_exact_evm_client

account = Account.from_key("0xYourPrivateKey")
signer = EthAccountSignerWithRPC(account, rpc_url="https://sepolia.base.org")

client = x402Client()
register_exact_evm_client(client, signer)
# Automatically signs ERC-20 approval transaction when needed

When to Use

Use this extension when:

You want to support any ERC-20 token, not just those with EIP-2612
You’re using the permit2 asset transfer method
You want fully gasless onboarding for your users regardless of the token

This extension is typically advertised alongside EIP-2612 gas sponsoring. The client automatically selects the best option: EIP-2612 if the token supports it, ERC-20 approval otherwise.

SDK Support

SDK	Supported
TypeScript	✅
Go	✅
Python	✅

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ERC-20 Approval Gas Sponsoring

Overview

How It Works

Server Usage

Client Usage

When to Use

SDK Support

​Overview

​How It Works

​Server Usage

​Client Usage

​When to Use

​SDK Support

Overview

How It Works

Server Usage

Client Usage

When to Use

SDK Support