Simulation API


The simulator enables you to seamlessly fork the chain and simulate the execution of any transactions as if time has frozen. You can fork (aka stop) any network at any block. This can help you validate any hypothesis you have, ranging from understanding how smart contract logic is behaving in some specific environment setup to creating the whole “sandbox” environment of the network.
Here are some examples of what you can achieve:
  • Dry-run user transactions, saving them money on failed transactions and improving UX.
  • DApp playground mode - Let users try out your dApp without spending any gas with all production data readily available.
  • Staging and QA environments for a blockchain project.
  • CI/CD for your blockchain project.
  • Validating complex DAO governance proposals.

How to start using Tenderly Simulation API

Follow the next steps in order to start using simulations:
  1. 1.
    Create a one-off simulation
  2. 2.
    Create Fork environment
  3. 3.
    Add Ether to your test address
  4. 4.
    Execute transaction in Tenderly Fork
  5. 5.
    Delete the fork

0 Set up the API

To get your Access Token, go to the Settings > Authorization and click on Create Access Token. If you want to create an organization token you can do so from your organization’s settings page.
The token you get will be used in API authentication.
Do not share the API Access token freely.
You should have these variables exposed to your code, as shown in this .env file:
TENDERLY_ACCESS_KEY=... # obtain it from the dashboard
TENDERLY_PROJECT=... # project slug
TENDERLY_USER=... # your username
You can find more details here:

1 Create a one-off simulation

We can start with a single simulation that is not recording any state. You can see the output of the transaction, if it failed or not, without actually sending it to the chain and spending gas. You can simulate any transaction using any given contract deployed on networks available in Tenderly.
Let’s execute a simple ERC20 transfer simulation on the DAI contract.
API Typescript
dotenv.config(); // load environment variables using dotenv
const SIMULATE_URL = `${TENDERLY_USER}/project/${TENDERLY_PROJECT}/simulate`
// set up your access-key, if you don't have one or you want to generate new one follow next link
const opts = {
headers: {
'X-Access-Key': TENDERLY_ACCESS_KEY as string,
const body = {
// standard TX fields
"network_id": "1",
"from": senderAddr,
"to": contract.address,
"gas": 21204,
"gas_price": "0",
"value": 0,
// simulation config (tenderly specific)
"save_if_fails": true,
"save": false,
"simulation_type": "quick"
const resp = await, body, opts);
When transactions are simulated via API, by default they don’t get persisted. To help the debugging using the Dashboard, you can configure save_if_fails flag. If you need to persist the transaction in case of success too, you can set save flag to true. Both of these are false by default.
Transactions are executed by default in full mode. To speed up simulations even further, you can set simulation_type to quick. Full simulations use the source code of the smart contract to generate call traces, whereas quick simulations use only the bytecode and thus take less time.

2 Create a Fork environment

Next, after we have executed a one-off simulation let’s execute them together. We are going to execute them under the “forked” environment. You can imagine it as if time has stopped on a specific block and only we can execute simulated transactions that would apply to the desired chain.
Here we are going to fork the Ethereum Mainnet on block 14386016:
API Typescript
import * as dotenv from "dotenv"
import axios from 'axios';
dotenv.config(); // load environment variables using dotenv
// set up your access-key, if you don't have one or you want to generate new one follow next link
const opts = {
headers: {
'X-Access-Key': TENDERLY_ACCESS_KEY as string,
const body = {
"network_id": "1",
"block_number": 14386016,
}, body, opts)
.then(res => {
console.log(`Forked with fork ID ${}. Check the Dashboard!`);
}).catch(err => console.error(err))

2.1 Using Fork with ethers.js

After creating a Tenderly Fork, you can interact with its JSON-RPC API, returned through simulation_fork.rpc_url. After, you can spawn ethers.js with any other network you are using or plan to use.
Tenderly will create 10 test addresses with 100 ETH each. By default, ethers.js will do transaction signing with the 0th address as the default signer.
API Typescript
import {ethers} from "ethers";
const forkId =
const forkRPC = `${forkId}`
const provider = new ethers.providers.JsonRpcProvider(forkRPC);
const signer = provider.getSigner()

3 Add Ether to your test address

As one of the first actions on top of the Forked environment let’s add Ether to your wallet address that would be used to send transactions. There are a couple of options for how we can do this, we can just simulate transactions and transfer Ethers from one address to our own (every address is automatically unlocked) or we can just mint new Ether.
Let’s see how we can mint new Ether to our test address. Here we’re using the custom Tenderly JSON-RPC endpoint tenderly_addBalance.
API Typescript
const params = [
ethers.utils.hexValue(100) // hex encoded wei amount
await provider.send('tenderly_addBalance', params)

4 Execute transaction in Tenderly Fork

Now that we have enough Ethers to cover all gas costs for our transactions. Let’s start to chain transaction simulations, we are going to execute approve() and transferFrom() functions on top of the DAI contract.
When specifying BigNumberish fields for transaction payload in a form of a hexadecimal string, these must not contain leading zeros. Use ethers.utils.hexValue(aBigNumberish) to convert the given bigNumberish value into acceptable form with no leading zeros.
We can use the provider we have created in 2.1 with ethers.js.
API Typescript
const DAI_ADDRESS = "0x6B175474E89094C44Da98b954EedeAC495271d0F";
const DAI_ABI = "{...}";
const daiContract = new ethers.Contract(DAI_ADDRESS , DAI_ABI , signer);
// converts 1 ether into wei, stripping the leading zeros
const tokenAmount = ethers.utils.hexValue(utils.parseEther('1').toHexString(10));
// Keep in mind that every account is automatically unlocked when performing simulations.
// This enables you to impersonate any address and send transactions.
const unsignedTx = await contract.populateTransaction.approve(await signer.GetAddress(), tokenAmount)
const transactionParameters = [{
to: contract.address,
gas: ethers.utils.hexValue(3000000),
gasPrice: ethers.utils.hexValue(1),
value: ethers.utils.hexValue(0)
const txHash = await provider.send('eth_sendTransaction', transactionParameters)
const respTxTransfer = await daiContract.transferFrom(ZERO_ADDRESS, YOUR_ADDRESS, tokenAmount);

5 Delete the fork

At the end of your execution, we should delete the Fork. This would remove any unnecessary clutter from the UI and also our infrastructure would be grateful 🙏
API Typescript
await axios.delete(TENDERLY_FORK_ACCESS_URL, opts)
In the following articles you will find specific code examples in some places. If you want to browse our code repo immediately you can find it in this GitHub repo.
You can check out the plans and pricing for the Simulation API here. You can also check out all Public API endpoints here - this documentation space is under continuous development, so if you need some API endpoints that aren't available or other API needs that aren't listed publicly, please ping us via Intercom on the Tenderly Dashboard and we'll do our best to meet your needs 🖖