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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.

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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

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How It Works

  1. Server advertises erc20ApprovalGasSponsoring in the PaymentRequired response extensions
  2. Client checks if Permit2 allowance is insufficient; if so, signs a raw approve(Permit2, MaxUint256) transaction without broadcasting it
  3. Client includes the signed transaction in extensions.erc20ApprovalGasSponsoring.info.signedTransaction
  4. 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.

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Server Usage

Advertise support for this extension in your route configuration: 
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(),
    },
  },
};

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Client Usage

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

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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.

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SDK Support

SDKSupported
TypeScript✅
Go✅
Python✅