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Use webhooks for Engine to notify your backend server of crucial transaction or backend wallet events.

http://localhost:* is allowed. Otherwise endpoint URLs must use https.

Supported events

all_transactionSubscribe to all transaction events (queued, sent, mined, errored, retried).
queued_transactionA transaction was queued by Engine.
sent_transactionA transaction was sent to an RPC provider.
mined_transactionA transaction was mined on the blockchain
errored_transactionA transaction was errored out.
retried_transactionA transaction was retried.
cancelled_transactionA transaction was cancelled.
backend_wallet_balanceThe backend wallet's balance dropped below minWalletBalance.


Backend wallet balance

Define the minWalletBalance (in the blockchain native coin) to maintain in each backend wallet. The default value is value is 2000000000000000 wei (e.g. 0.002 ETH). A webhook event will be sent if a backend wallet balance falls below this value.

To read or update this value, call GET/POST /configuration/backend-wallet-balance.

Transaction events

Create a webhook by calling POST /webhooks/create.

Webhook verification

To secure your webhook endpoint, verify the request originated from Engine.

Check the signature

The payload body is signed with the webhook secret and provided in the X-Engine-Signature request header.

This code example checks if the signature is valid:

const generateSignature = (
body: string,
timestamp: string,
secret: string,
): string => {
const payload = `${timestamp}.${body}`;
return crypto.createHmac("sha256", secret).update(payload).digest("hex");

const isValidSignature = (
body: string,
timestamp: string,
signature: string,
secret: string,
): boolean => {
const expectedSignature = generateSignature(body, timestamp, secret);
return crypto.timingSafeEqual(

Check the timestamp

The event timestamp is provided in the X-Engine-Timestamp request header.

This code example checks if the event exceeds a given expiration duration:

export const isExpired = (
timestamp: string,
expirationInSeconds: number,
): boolean => {
const currentTime = Math.floor( / 1000);
return currentTime - parseInt(timestamp) > expirationInSeconds;

Example webhook endpoint

This NodeJS code example listens for event notifications on the /webhook endpoint:

import express from "express";
import bodyParser from "body-parser";
import { isValidSignature, isExpired } from "./webhookHelper";

const app = express();
const port = 3000;

const WEBHOOK_SECRET = "<your_webhook_auth_secret>";

app.use(bodyParser.text());"/webhook", (req, res) => {
const signatureFromHeader = req.header("X-Engine-Signature");
const timestampFromHeader = req.header("X-Engine-Timestamp");

if (!signatureFromHeader || !timestampFromHeader) {
return res.status(401).send("Missing signature or timestamp header");

if (
) {
return res.status(401).send("Invalid signature");

if (isExpired(timestampFromHeader, 300)) {
// Assuming expiration time is 5 minutes (300 seconds)
return res.status(401).send("Request has expired");

// Process the request
res.status(200).send("Webhook received!");

app.listen(port, () => {
console.log(`Server started on http://localhost:${port}`);

Payload format

The webhook request to your backend follows this format.

Method: POST


  • Content-Type: application/json
  • X-Engine-Signature: <payload signature>
  • X-Engine-Timestamp: <Unix timestamp in seconds>
"chainId": 80001,
"data": "0xa9059cbb0000000000000000000000003ecdbf3b911d0e9052b64850693888b008e183730000000000000000000000000000000000000000000000000000000000000064",
"value": "0x00",
"gasLimit": "39580",
"nonce": 1786,
"maxFeePerGas": "2063100466",
"maxPriorityFeePerGas": "1875545856",
"fromAddress": "0x3ecdbf3b911d0e9052b64850693888b008e18373",
"toAddress": "0x365b83d67d5539c6583b9c0266a548926bf216f4",
"gasPrice": "1875545871",
"transactionType": 2,
"transactionHash": "0xc3ffa42dd4734b017d483e1158a2e936c8a97dd1aa4e4ce11df80ac8e81d2c7e",
"signerAddress": null,
"accountAddress": null,
"target": null,
"sender": null,
"initCode": null,
"callData": null,
"callGasLimit": null,
"verificationGasLimit": null,
"preVerificationGas": null,
"paymasterAndData": null,
"userOpHash": null,
"functionName": "transfer",
"functionArgs": "0x3ecdbf3b911d0e9052b64850693888b008e18373,100",
"extension": "none",
"deployedContractAddress": null,
"deployedContractType": null,
"queuedAt": "2023-09-29T22:01:31.031Z",
"processedAt": "2023-09-29T22:01:38.754Z",
"sentAt": "2023-09-29T22:01:41.580Z",
"minedAt": "2023-09-29T22:01:44.000Z",
"cancelledAt": null,
"retryCount": 0,
"retryGasValues": false,
"retryMaxPriorityFeePerGas": null,
"retryMaxFeePerGas": null,
"errorMessage": null,
"sentAtBlockNumber": 40660021,
"blockNumber": 40660026,
"queueId": "1411246e-b1c8-4f5d-9a25-8c1f40b54e55",
"status": "mined"