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Develop a snap

A snap can extend the dapp-facing MetaMask JSON-RPC API in arbitrary ways, or integrate with and extend the functionality of MetaMask using the Snaps JSON-RPC API and permissions.

Before developing a snap, it's important to understand:

You can get started quickly using the Snaps template or follow a tutorial.

This page describes additional important steps when developing a snap.

Detect the user's MetaMask version​

When developing a website that depends on Snaps, you need to know whether the user has MetaMask Flask installed.

We recommend calling the web3_clientVersion MetaMask RPC method to obtain this information. The following example uses the @metamask/detect-provider package to get the provider object from MetaMask first:

import detectEthereumProvider from '@metamask/detect-provider';

// This resolves to the value of window.ethereum or null
const provider = await detectEthereumProvider();

// web3_clientVersion returns the installed MetaMask version as a string
const isFlask = (
  await provider?.request({ method: 'web3_clientVersion' })
)?.includes('flask');

if (provider && isFlask) {
  console.log('MetaMask Flask successfully detected!');

  // Now you can use Snaps!
} else {
  console.error('Please install MetaMask Flask!', error);
}

Test your snap​

Test your snap by hosting it locally using mm-snap serve, installing it in Flask, and calling its API methods from a web page.

Debug your snap​

To debug your snap, use console.log and inspect the MetaMask background process. You can add your log statements in your source code and build your snap, or add them directly to your snap bundle and use mm-snap manifest --fix to update the shasum in your snap manifest file. The manifest shasum must match the contents of your bundle at the time MetaMask fetches your snap.

note

Because adding logs modifies the snap source code, you must re-install the snap whenever you add a log statement.

The snap log output is only visible in the extension background process console. If you're using a Chromium browser, use the following steps to inspect the background process and view its console:

  1. Go to chrome://extensions.
  2. Toggle Developer mode on in the top right corner.
  3. Find MetaMask Flask, and select Details.
  4. Under Inspect views, select background.html.

The log output displays in the console that pops up.

Publish your snap​

Snaps are npm packages, so publishing a snap is as simple as publishing an npm package. Refer to the npm CLI documentation for details on publishing to the public registry. The following details are specific to Snaps:

  • The version in package.json and snap.manifest.json must match.
  • The image specified in iconPath in the manifest file is used as the icon displayed when installing and displaying confirmations from the snap.

After publishing the snap, any dapp can connect to the snap by using the snap ID npm:[packageName].

Distribute your snap​

Since snaps are currently intended for a developer audience, MetaMask doesn't currently facilitate distributing snaps to a wide audience. If you have a website that expects the user to install a snap, ask the user to install MetaMask Flask, and then ask the user to install the snap using the wallet_enable API method.

In the future, MetaMask will create some way for users to more easily discover snaps, but everyone can always build, publish, and use snaps without MetaMask's permission. (Although we may try to make it difficult to use known scams.)

Resources and tools​

You can review the growing number of example snaps maintained by MetaMask, as well as the following fully functional and open source snaps:

MetaMask also maintains tools to help developers build, debug, and maintain snaps:

  • Template snap - A template that includes TypeScript/React and vanilla JavaScript options and a CLI for building, packaging, and deploying your snap and a companion dapp.
  • Snaps Truffle Box - A template that combines the TypeScript template snap and Truffle so you can easily test snaps that use smart contracts with Ganache.
  • Test snaps - A collection of test snaps and a dapp for evaluating them.

If you have any questions, ask them on GitHub discussions, and if you encounter any issues, please open a GitHub issue.