B20 payment memos

Payment memos attached to B20 transfers: pairs each Memo(address,bytes32) log with its Transfer at the previous log index (same tx, same token). Send GET /api/b20-memos with the required field token and pay $0.005 per call over x402 or MPP (there is no free tier). It returns a JSON object with token, mode, fromBlock, toBlock, scannedFromBlock and 3 more.

Give a tx hash for one transaction, or scan a block window. Returns memoHex always and memoText when printable UTF-8. ?token=0xb200…&tx=0x…|&blocks=50000&address=0x…&limit=50

Category: Payments & x402 · Tags: b20 base memo payments logs transfer

TRY IN PLAYGROUND →

Parameters

Example request

curl -i "https://agent402.tools/api/b20-memos?token=0xb200000000000000000000000000000000000001&blocks=1000"

Without payment this returns HTTP 402 Payment Required with the exact price for b20-memos; any x402 v2 or MPP client pays it and retries.

Example response

{
  "token": "0xb200…0001",
  "mode": "window",
  "fromBlock": 1,
  "toBlock": 2,
  "scannedFromBlock": 1,
  "truncated": false,
  "count": 0,
  "memos": []
}

From an MCP client

catalog.call {
  "slug": "b20-memos",
  "params": {
    "token": "0xb200000000000000000000000000000000000001",
    "blocks": 1000
  }
}

The hosted connector at https://agent402.tools/mcp needs a payment for b20-memos; the stdio package pays it from a wallet or from AGENT402_CREDITS_KEY. Local install: npx -y agent402-mcp.

Errors and behavior

• token is required. An input the tool rejects returns an HTTP 4xx whose body carries error, tool, expected, required and example, so the caller can correct it.
• A paid call that ends in any status of 400 or above is not charged over x402, MPP or a prepaid credits key: settlement is cancelled when the tool fails. The exception is a Tempo push credential, a transfer the buyer sent before the call: it settles before the tool runs, so if the tool then fails the payment is recorded as a refund owed to the paying wallet.
• Wallet-only: this tool reaches the network or stored state, so it has no proof-of-work tier. A prepaid card-credits key issued earlier (Authorization: Bearer a402_...) also pays it.
• A POST with a JSON body to /api/b20-memos is served as this GET, with the body as the input.
• An Idempotency-Key header makes a retried paid call replay the first 200 instead of charging again (an answer larger than 1 MB is not replayed).

Paid call (JavaScript agent)

import { wrapFetchWithPayment } from "@x402/fetch";
import { x402Client } from "@x402/core/client";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

const client = new x402Client();
client.setSpendControls?.(false); // keep your own spending ceiling in code
registerExactEvmScheme(client, { signer: privateKeyToAccount(KEY) });
const payFetch = wrapFetchWithPayment(fetch, client);

const res = await payFetch("https://agent402.tools/api/b20-memos?token=0xb200000000000000000000000000000000000001&blocks=1000");

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