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Path: /apps/builder-codes/builder-codes> ## Documentation Index

Fetch the complete documentation index at: https://docs.base.org/llms.txt
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Accept B20 payments

Accept B20 token payments in your app and match each transaction to an order with onchain memos.

B20 is an ERC-20 superset. Standard transfer, transferFrom, approve, balanceOf, and ERC-2612 permit all work, so an app that accepts ERC-20 tokens accepts B20 with no code changes.

B20's new features include transfer policies, pausing, supply caps, and memos. This guide uses the memo: transferWithMemo works like transfer, but also attaches a bytes32 reference such as an order ID and emits a Memo event immediately after the standard Transfer event. Your app can read that Memo event to tie each payment to an order.

Tag a payment with a memo

This example reads the token's decimals, sends a payment tagged with an order ID, then reads the memo back from the receipt. It uses your configured viem walletClient and publicClient:

import { parseUnits, stringToHex, hexToString, parseEventLogs } from 'viem';

const TOKEN    = '0xB200...'; // the B20 token you accept
const MERCHANT = '0x...';     // where payments land

const ABI = [
  { type: 'function', name: 'decimals', stateMutability: 'view', inputs: [], outputs: [{ type: 'uint8' }] },
  { type: 'function', name: 'transferWithMemo', stateMutability: 'nonpayable',
    inputs: [{ name: 'to', type: 'address' }, { name: 'amount', type: 'uint256' }, { name: 'memo', type: 'bytes32' }],
    outputs: [{ type: 'bool' }] },
  { type: 'event', name: 'Memo', inputs: [
    { name: 'caller', type: 'address', indexed: true },
    { name: 'memo',   type: 'bytes32', indexed: true },
  ] },
];

// Read decimals because B20 tokens range from 6 to 18.
const decimals = await publicClient.readContract({ address: TOKEN, abi: ABI, functionName: 'decimals' });

// Pay 10 tokens, tagging the transfer with an order ID.
const hash = await walletClient.writeContract({
  address: TOKEN, abi: ABI, functionName: 'transferWithMemo',
  args: [MERCHANT, parseUnits('10', decimals), stringToHex('order-42', { size: 32 })],
});

// The Memo event carries the order ID back. Read it from the receipt.
const receipt = await publicClient.waitForTransactionReceipt({ hash });
const [memo] = parseEventLogs({ abi: ABI, logs: receipt.logs, eventName: 'Memo' });
console.log(hexToString(memo.args.memo, { size: 32 }).replace(/\0+$/, '')); // "order-42"

To collect with an allowance instead of a direct transfer, use transferFromWithMemo. It emits the same Memo event.

Handle B20-specific reverts

A B20 transfer can revert where a standard ERC-20 would not. Surface these so a failed payment is visible, not silent:

• PolicyForbids: the sender or recipient is not authorized by the token's transfer policy. Most tokens are open by default, but a regulated issuer can gate transfers with an allowlist or blocklist.
• A paused transfer: the issuer paused the token's TRANSFER feature.

Call publicClient.simulateContract with the same arguments before sending. It raises these as typed errors before the user signs, so you can show the reason instead of a failed transaction.

Related pages

• B20 token standard: the full interface, including memos, policies, pausing, and roles.
• Query B20 events: index Transfer and Memo events with the CDP SQL API to reconcile payments against orders at scale.
• Launch a B20 Token: create your own B20 token.