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Path: /apps/builder-codes/builder-codes> ## Documentation Index
> Fetch the complete documentation index at: https://docs.base.org/llms.txt
> Use this file to discover all available pages before exploring further.

# 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`:

```js pay-with-memo.js lines highlight={22-23,28-29} wrap theme={null}
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](/base-chain/specs/upgrades/beryl/b20): the full interface, including memos, policies, pausing, and roles.
* [Query B20 events](https://docs.cdp.coinbase.com/data/sql-api/b20-events): index `Transfer` and `Memo` events with the CDP SQL API to reconcile payments against orders at scale.
* [Launch a B20 Token](/get-started/launch-b20-token): create your own B20 token.

Start at /apps/builder-codes/builder-codes and fetch https://docs.base.org/llms.txt to locate the relevant documentation structure and nearby pages. Add the supplied “Accept B20 payments” guide, including its JavaScript example and related links, then verify that the page renders correctly and its links and code sample are intact.

Written by the indexing model from the issue text.