Preventing Duplicate Transactions

Your backend server may implement retries to handle when an HTTP response from Engine was not received successfully. In these cases, set an idempotency key to drop duplicate submissions of the same transaction.

This approach prevents costly and irreversible errors like transferring funds or minting NFTs more than once.


Set the x-idempotency-key header to a unique string.

Engine only submits a transaction if there was not another transaction sent with the same idempotency key in the past 24 hours. Subsequent calls with the same idempotency key return the original transaction queueId.


This example safely retries a request up to three times if a 2xx is not received.

const idempotencyKey = "27R82mBV"; // Some unique identifier from your app
const numRetries = 3;
let queueId: string | undefined;
for (let i = 0; i < numRetries; i++) {
const resp = await fetch(url, {
method: "POST",
headers: {
Authorization: `Bearer ${process.env.THIRDWEB_ACCESS_TOKEN}`,
"Content-Type": "application/json",
"x-idempotency-key": idempotencyKey,
body: JSON.stringify({
to: "0x4Ff9aa707AE1eAeb40E581DF2cf4e14AffcC553d",
amount: "42000000000",
// If 2xx, break. Else try again.
if (resp.ok) {
queueId = (await resp.json()).result;
if (queueId) {
console.log("Transaction submitted:", queueId);
} else {
console.error("Unable to submit transaction.");