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Add Transaction Preview to a MetaMask Snap

How to Add Transaction Preview to a MetaMask Snap Using Tenderly Simulation API

This tutorial demonstrates how to add a “transaction preview” option to a wallet using Tenderly Simulation API. We’ll show you how to add this option to the MetaMask wallet.

The “transaction preview” feature allows users to see the exact outcome of their transactions before submitting them to the production network. Wallets equipped with this feature make it easier for users to understand the financial implications of their transactions before making a commitment.

Simulations are executed on a fork of the latest state of the blockchain in a safe, risk-free environment. This helps wallet users prevent sending malicious or error-prone transactions.

You can find the complete demo repository on GitHub:

Tenderly MetaMask Snap Demo

This tutorial is for demonstration purposes only, specifically to illustrate how to add a transaction preview option to a MetaMask Snap. It is not intended nor recommended for any other usage.

Project overview

This project shows you how to integrate transaction simulations into the MetaMask wallet. We’ll be working with MetaMask Snaps.

You’ll learn how to connect to the Tenderly Simulation API, send the transaction data, retrieve the simulation results, and display them to the user via the MetaMask UI. This includes asset changes in dollars, native-asset balance changes, output value, storage changes, event logs, and call traces.

Simulated transactions can then also be opened in Tenderly for further debugging and testing.

Below is a list of features that we’ll be adding to the MetaMask Snap. Similar features can also be added to any wallet to help users build confidence when sending transactions.

Simulation debuggingAuto-generated link to inspect and debug the simulation in Tenderly.
Publicly sharable simulationsAbility to enable/disable public sharing of simulations.
Asset changes displayed in dollar valueHuman-readable data, including asset changes automatically converted into dollars.
Native-asset balance changesAbility to track changes in their native-asset balance during the execution of the contract.
Output valueSee the result of the contract call, displaying what the contract is set to return.
Storage changesSee any changes made to the contract’s storage during the execution.
Event logsSee events that were emitted during the contract’s execution.
Call tracesBreakdown of all the function calls that happened during the execution.


Before you start building the MetaMask Snap, make sure to disable the “real” version of MetaMask and install the MetaMask Flask Development Plugin.

Let’s start building! 👷‍♂️

Step 1: Setting up a MetaMask Snap project

Install MetaMask’s official Create Snap CLI and create a new project.

yarn create @metamask/snap project-name
npm create @metamask/snap project-name

Next, we need to start the Snap project and serve the front end on https://localhost:8000.

From the root of the newly created project, install the project dependencies using Yarn.


Start the development server.

yarn start

Setting the correct permissions

Once you’ve installed the Snap, find the /packages/snap/snap.manifest.json file and set the permissions as shown below. Learn about MetaMask permissions here.

  // ...
  "initialPermissions": {
    "snap_dialog": {},
    "endowment:rpc": {
      "dapps": true,
      "snaps": false
    "endowment:transaction-insight": {
      "allowTransactionOrigin": true
    "endowment:network-access": {},
    "snap_manageState": {},
    "endowment:ethereum-provider": {
      "dapps": true,
      "snaps": true

When you run the app and try to connect the wallet, a permission request will pop up.

Click Connect and Approve & install buttons to continue.

Tenderly Docs

Install MetaMask Snap Permissions

Step 2: Generating Tenderly API access tokens

The transaction preview option in the MetaMask Snap is powered by the Tenderly Simulation API.

Before you can start using the API, you need to generate access tokens. Follow these steps:

  1. Log into Tenderly or create a free account here.
  2. Go to the Authorization page and click on the Generate Access Token button.

If you need help with API authentication, please follow this guide:

Tenderly Docs

Generate Access Token from Authorization page

Step 3: Writing the logic for the Snap

The core logic that makes the Snap work is stored in three files:

  • Credentials Access (credentials-access.ts): Manages the Tenderly API authentication and access.
  • Simulation (simulation.ts): The core logic for interacting with the Tenderly API to simulate transactions and fetch simulation results.
  • Formatter (formatter.ts): Logic for parsing simulation results and formatting them in a user-friendly way in the MetaMask UI.

Credentials Access

We can store all methods responsible for requesting, updating, and fetching Tenderly API credentials in a file called credentials-access.ts. View the source code here.

The most important methods include:

  • fetchCredentials(): This function retrieves the credentials of a Tenderly project. If no credentials are stored, it triggers handleUpdateTenderlyCredentials to request new ones.
  • handleUpdateTenderlyCredentials(): This function handles the process of updating Tenderly project credentials. It gets new credentials by calling requestNewTenderlyCredentials and then saves them using the snap_manageState method with an 'update' operation.
  • requestNewTenderlyCredentials(): This function requests new Tenderly credentials. It calls requestCredentials to receive the raw credentials data and verifies its correctness before returning an object with accountId, projectId, and accessToken.

snap_manageState is a built-in method that allows the Snap to persist up to 100 MB of data to the disk. Learn more here.


Next, we need to create the logic that sends transaction data to the Tenderly Simulation API, simulates it, and retrieves the results.

We’ll use the simulate API endpoint for that.{accountId}/project/{projectId}/simulate

To keep things organized, let’s store the simulation logic in a file called simulation.ts. View the source code here.

To send transaction data to the Tenderly Simulation API, we need to create two key methods:

  • simulate(): This is the main function that handles the simulation of a transaction. It fetches the API access credentials, submits the transaction data to the Tenderly Simulation API, and handles any errors returned by the API.
export async function simulate(
  transaction: { [key: string]: Json },
  transactionOrigin: string,
): Promise<Panel> {
  const credentials = await fetchCredentials(transactionOrigin);
  if (!credentials) {
    return panel([text('🚨 Tenderly access token updated. Please try again.')]);
  const simulationResponse = await submitSimulation(transaction, credentials);
  const err = catchError(simulationResponse, credentials);
  return err || formatResponse(simulationResponse, credentials);
  • submitSimulation(): This function sends a request to the API along with the transaction data to be simulated.

When the transaction is simulated, we can make it publicly accessible by calling the share API endpoint. Making the simulation shareable allows you to copy the link to the transaction and send it to anyone. Shared transactions can be viewed without a Tenderly account.

async function submitSimulation(
  transaction: { [key: string]: Json },
  credentials: TenderlyCredentials,
) {
  const chainId = await ethereum.request({ method: 'eth_chainId' });
  const response = await fetch(
      method: 'POST',
      body: JSON.stringify({
        from: transaction.from,
        gas: hex2int(transaction.gas),
        value: hex2int(transaction.value),
        network_id: hex2int(chainId as string),
        save: true,
        save_if_fails: true,
        simulation_type: 'full',
        generate_access_list: false,
        source: 'tenderly-metamask-snap',
      headers: {
        'Content-Type': 'application/json',
        'X-Access-Key': credentials.accessToken,
  const parsedResponse = await response.json();
  // Make the simulation publicly accessible
  if (parsedResponse?.simulation?.id) {
    await fetch(
        method: 'POST',
        headers: {
          'Content-Type': 'application/json',
          'X-Access-Key': credentials.accessToken,
  return parsedResponse;


Once the API returns the simulation results, we need to parse this data and format it in a user-friendly way to be displayed in the MetaMask UI.

We can store this logic in a file called formatter.ts. View the source code here.

The core formatting methods include:

  • formatResponse(): This function receives the raw data and credentials of a Tenderly project simulation, calls the individual formatter functions for each relevant section (like balance changes, output value, asset changes, etc.), and returns a panel with all the formatted outputs.
  • formatBalanceDiff(): This function generates a panel showing balance changes for each account involved in the transaction.
  • formatOutputValue(): This function creates a panel that presents the output values of the transaction if any exist. It also decodes the output, if possible.
  • formatAssetChanges(): This function formats a panel to show any asset changes, differentiating between ERC20, ERC721, and other changes.
  • formatStorageChanges(): This function creates a panel that lists any changes to storage that occurred during the transaction. It uniquely formats addresses and nested data structures for clarity.
  • formatEventLogs(): This function presents the event logs, if they exist, for each transaction. The logs are formatted for readability and include input values.
  • formatCallTrace(): This function produces a formatted visual hierarchy of call traces, showing nested calls recursively.
  • formatSimulationUrl(): This function returns a link to the full details of the simulation on the Tenderly Dashboard and a separate shareable link.

Step 4: Displaying simulation data in the MetaMask UI

The MetaMask Snap mono repo provides a set of predefined UI elements and layout configurations. We’ll use these elements to extend the existing MetaMask UI to display the simulation results.

When you install our MetaMask example, we show you how to run different types of simulations:

  • Successful transaction simulation with a predefined payload (successful ERC-20 transfer and NFT transfer)
  • Failed transaction simulation with a predefined payload
  • Custom transaction simulation with any payload
Tenderly Docs

Tenderly Snap UI

For the purpose of this tutorial, we’ll show you how to run simulations with a predefined payload and how to add your custom payload and execute the simulation.

Successful simulation with a predefined payload

To initiate a simulation, we’ll write a function that will send the predefined transaction data from our wallet address and call the eth_sendTransaction RPC method.

// packages/site/src/utils/snap.ts
export const sendTransaction = async (data: any): Promise<any> => {
  try {
    const [from] = (await window.ethereum.request({
      method: 'eth_requestAccounts',
    })) as string[];
    if (!from) {
      return Promise.reject(Error('Failed to get an account'));
    return window.ethereum.request({
      method: 'eth_sendTransaction',
      params: [
  } catch (e) {
    return e;

The eth_sendTransaction method call will trigger the onTransaction handler, which will call our custom simulate function, which was defined in simulation.ts.

// packages/snap/src/index.ts
export const onTransaction: OnTransactionHandler = async ({ transaction, transactionOrigin }) => {
  if (!isObject(transaction) || !hasProperty(transaction, 'to')) {
    return {
      content: {
        value: 'Unknown transaction type',
        type: NodeType.Text,
  const simulationResponse = await simulate(transaction, transactionOrigin || '');
  return {
    content: {
      children: simulationResponse.children,
      type: NodeType.Panel,

In the onTransaction handler, the simulate function is called with the transaction and its origin as arguments.

The simulate function is responsible for fetching access credentials, submitting the transaction data to the Tenderly API, handling any errors, and formatting the API response for output.

The results of the simulation are displayed within the MetaMask UI. The images below show a successful ERC20 token transfer of 1 USDC to demo.eth. The output is generated by the functions defined in the formatter.ts file.

Tenderly Docs

ERC20 Transfer - send 1 USDC to demo.eth

For example, the Asset Changes block, which shows us the dollar value changes resulting from the simulation, is generated by the formatAssetChanges() function.

You also get a link to the simulated transaction which you can open in Tenderly and inspect it further with Debugger or resimulate it with different values.

This link is generated by the formatSimulationUrl() function defined in the formatter.ts file.

export function formatSimulationUrl(data: any, credentials: TenderlyCredentials): Component[] {
  const simulationUrl = `${credentials.accountId}/${credentials.projectId}/simulator/${data.simulation?.id}`;
  const sharedSimulationUrl = `${data.simulation?.id}`;
  return [
    heading('Tenderly Dashboard:'),
    text('See full simulation details in Tenderly.'),
    text(`**Status:** ${data.transaction?.status ? 'Success ✅' : 'Failed ❌'}`),
    text('Share simulation details with others! 🤗'),

Failed transaction with a predefined payload

For the second example, we’ll simulate a failed transaction with a predefined payload.

The image below shows a failed ERC20 token transfer of 1,000,000 USDC to demo.eth.

Tenderly Docs

Preview of the failed transaction

We also get a link to the Tenderly Dashboard, where the user can inspect the stack trace, events, state changes, gas consumption, etc. This is particularly useful for understanding why the transaction failed.

The user can smoothly continue debugging the failed transaction and resimulating it to test different solutions.

Simulation with a custom payload

To run a simulation with a custom payload, you need to add an object that aligns with the Ethereum transaction specification.

Tenderly Docs

Custom payload UI

It can contain the following fields:

  • from: DATA, 20 Bytes - The address the transaction is sent from.
  • to: DATA, 20 Bytes - (optional when creating a new contract) The address the transaction is directed to.
  • gas: QUANTITY - (optional, default: 90000) Integer of the gas provided for the transaction execution. It will return unused gas.
  • gasPrice: QUANTITY - (optional, default: To-Be-Determined) Integer of the gasPrice used for each paid gas.
  • value: QUANTITY - (optional) Integer of the value sent with this transaction.
  • data: DATA - The compiled code of a contract OR the hash of the invoked method signature and encoded parameters.
  • nonce: QUANTITY - (optional) Integer of a nonce. This allows you to overwrite your own pending transactions that use the same nonce.

All numerical values should be hexadecimal strings, and addresses should be Ethereum addresses (20 bytes).

Tenderly Docs

Send a custom payload

After entering the custom payload, users can initiate the simulation using the same sendTransaction function as before. This passes the custom payload to the onTransaction handler, which then triggers the simulate function to run the simulation.

Next steps

In this tutorial, you learned how to add a transaction preview option to a MetaMask Snap powered by the Tenderly Simulation API. You can find the source code for this project on GitHub.

Transaction simulation on Tenderly can be initiated in three ways, depending on your project and needs. Learn how to integrate simulations into your dapps with these help resources: