Mainnet execution simulation

Mainnet Execution Simulation (MXS) allows you to test your Clarity smart contracts using actual data and state from the Stacks mainnet. This powerful feature helps ensure your contracts behave as expected in a live environment by enabling you to:

• Test contract interactions against real-world data and dependencies.
• Simulate any historical or recent mainnet transaction to observe its outcome or evaluate its execution cost.

This guide focuses on using MXS within the context of unit tests using Vitest, which is the primary use case.

In this guide, you will learn:

1. 1What Mainnet Execution Simulation (MXS) is and why it's valuable.
2. 2How to enable and configure MXS in your Clarinet project.
3. 3How to write and run unit tests using MXS.
4. 4Current limitations of the MXS feature.

What is Mainnet execution simulation?

When developing smart contracts, testing against realistic conditions is crucial. Simnet provides an isolated testing environment, but it lacks the complexity and historical state of the live Stacks mainnet.

MXS bridges this gap by allowing your unit tests, executed via the Clarinet JS SDK and Vitest, to interact with a simulated environment that mirrors the Stacks mainnet state at a specific block height. This means you can:

• Validate contract logic against real data: Call mainnet contracts or check data derived from mainnet state directly within your tests.
• (Re)simulate transactions: Execute specific mainnet transactions to analyze their exact results, trace their execution, or estimate their costs without deploying or spending actual STX.

Enable MXS in your project

To use MXS in your unit tests, you need to enable it in your Clarinet.toml file. Add the following section:

[repl.remote_data]
# Enable mainnet execution simulation
enabled = true
# Specify the Stacks block height to fork from (optional, default to latest)
initial_height = 522000
# Specify the API URL to use (optional, default to https://api.hiro.so)
api_url = 'https://api.hiro.so'

• enabled = true: This flag activates the MXS feature for your tests.
• initial_height = <block_height>: This optional setting specifies the Stacks block height at which the simulation should start.
  • If omitted, Clarinet defaults to the latest finalized Stacks block height at the time of execution.
  • Recommendation: Setting a specific initial_height is highly recommended for consistent and reproducible test results, as the mainnet state constantly changes.
• api_url = 'https://api.hiro.so': This optional setting specifies the API URL to use.

Write tests with MXS

Once MXS is enabled, your Vitest tests running via the Clarinet JS SDK (npm run test) will automatically operate against the specified mainnet state. You don't need to change your test writing approach significantly.

The simnet object, typically used for interacting with the simulated environment in tests, will now interact with the mainnet state snapshot defined by initial_height.

For example, you could write a test that calls a function on a mainnet contract like pox-4:

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import { describe, it, expect } from "vitest";
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import { Cl } from "@stacks/transactions";
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const accounts = simnet.getAccounts();
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const deployer = accounts.get("deployer")!; // Or any other account
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describe("pox-4 reward cycle", () => {
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  it("returns pox reward cycle id", () => {
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    // Call the current-pox-reward-cycle function on the mainnet pox-4 contract
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    const call = simnet.callReadOnlyFn(
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      "SP000000000000000000002Q6VF78.pox-4", // Mainnet pox-4 contract principal
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      "current-pox-reward-cycle",
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      [], // No arguments needed for this function
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      deployer // Caller address doesn't impact read-only calls usually
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    );
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    // Assert that the call was successful and returns cycle number u109
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    expect(call.result).toBeUint(109);
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    // You can further assert other values by tweaking the initial_height in `Clarinet.toml`
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    // For example, if initial_height = 299000, the value might be:
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    // expect(call.result).toBeUint(98);
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    console.log("Current POX reward cycle:", Cl.prettyPrint(call.result));
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  });
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});
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This test uses simnet.callReadOnlyFn just like in standard unit tests, but because MXS is enabled, the call targets the actual pox-4 contract state at the initial_height specified in Clarinet.toml.

Test MXS in the playground

You can quickly experiment with MXS without setting up a full project using the Clarinet Playground.

1. 1Go to: https://play.hiro.so/?remote_data=true
   • The ?remote_data=true query parameter enables MXS.
2. 2In the command input field, call a mainnet contract function. For example, to call current-pox-reward-cycle on pox-4:
   Terminal

   $ 
   contract-call? 'SP000000000000000000002Q6VF78.pox-4 current-pox-reward-cycleCopy

The playground will execute the call against the latest mainnet state and display the result.

Setting up an API Key for MXS

While you can use Mainnet Execution Simulation (MXS) without an API key, you may run into rate limits that could disrupt your development workflow. We recommend using an API key to ensure reliable and uninterrupted access to MXS functionality.

To use an API key with MXS, set the HIRO_API_KEY environment variable in your terminal.

$ 
export HIRO_API_KEY="<your-api-key>"Copy

Replace <your-api-key> with the actual API key you obtain from the Hiro Platform.

Currently, the HIRO_API_KEY is specifically utilized by the Mainnet Execution Simulation (MXS) feature within Clarinet. Other Clarinet features do not use this API key at this time.

Limitations

Currently, MXS does not support Clarity functions that rely on querying specific information related to burn blocks or miner tenures. The following functions are not implemented in the simulation environment:

• (get-burn-block-info? pox-addrs <burn-block-height>)
• (get-tenure-info? block-reward <stacks-block-height>)
• (get-tenure-info? miner-spend-total <stacks-block-height>)
• (get-tenure-info? miner-spend-winner <stacks-block-height>)

Calls to these functions within an MXS context will result in runtime errors.

Next steps