npm install @metamask/connect-multichain

npm install @solana/web3.js

import { createMultichainClient, getInfuraRpcUrls } from '@metamask/connect-multichain';

const client = await createMultichainClient({
  dapp: {
    name: 'My Multichain DApp',
    url: window.location.href,
    iconUrl: 'https://mydapp.com/icon.png', // optional (or use base64Icon for embedded icons)
  },
  api: {
    supportedNetworks: {
      ...getInfuraRpcUrls({ infuraApiKey: 'YOUR_INFURA_API_KEY', caipChainIds: ['eip155:1', 'eip155:137', 'solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp'] }),
    },
  },
  ui: {
    headless: false, // set true for custom QR rendering
  },
});

// connect() resolves with no value (Promise<void>)
await client.connect(
  [
    'eip155:1',      // Ethereum mainnet
    'eip155:137',    // Polygon
    'solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp', // Solana mainnet
  ],
  [], // caipAccountIds (empty for initial connection)
);

// Session data arrives via the wallet_sessionChanged event (see Step 6),
// or read it on demand:
const session = await client.provider.getSession();
console.log(session?.sessionScopes); // approved scopes with their accounts

await client.connect(
  ['eip155:1'],
  ['eip155:1:0xYourAddress'],
);

// Read: eth_getBalance via RPC node
const balance = await client.invokeMethod({
  scope: 'eip155:1',
  request: {
    method: 'eth_getBalance',
    params: ['0xYourAddress', 'latest'],
  },
});

// Sign: personal_sign via wallet
const signature = await client.invokeMethod({
  scope: 'eip155:1',
  request: {
    method: 'personal_sign',
    params: ['0x48656c6c6f', '0xYourAddress'],
  },
});

// Send transaction via wallet
const txHash = await client.invokeMethod({
  scope: 'eip155:137',
  request: {
    method: 'eth_sendTransaction',
    params: [{
      from: '0xYourAddress',
      to: '0xRecipient',
      value: '0x2386F26FC10000', // 0.01 ETH in wei (hex)
      gas: '0x5208',
    }],
  },
});

const SOLANA_MAINNET = 'solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp';

// Sign a message
const signResult = await client.invokeMethod({
  scope: SOLANA_MAINNET,
  request: {
    method: 'signMessage',
    params: {
      account: { address: 'YourSolanaAddress' },
      message: btoa('Hello from Solana!'), // base64-encoded message bytes
    },
  },
});
// signResult: { signature: <base58 string>, signedMessage: <base64>, signatureType: 'ed25519' }

// Sign and send a transaction
const txResult = await client.invokeMethod({
  scope: SOLANA_MAINNET,
  request: {
    method: 'signAndSendTransaction',
    params: {
      account: { address: 'YourSolanaAddress' },
      transaction: base64EncodedTransaction, // base64-encoded serialized transaction
      scope: SOLANA_MAINNET,
    },
  },
});
// txResult: { signature: <base58 transaction signature> }

// Session state changes (accounts added/removed, scopes changed)
// Payload is SessionData | undefined — scopes live under sessionScopes
client.on('wallet_sessionChanged', (session) => {
  const scopes = session?.sessionScopes ?? {};
  console.log('Approved scopes:', Object.keys(scopes));
  // Accounts are CAIP-10 strings, e.g. 'eip155:1:0xabc...' — take the last segment for the address
});

// Connection status changes
client.on('stateChanged', (status) => {
  // status: 'loaded' | 'pending' | 'connecting' | 'connected' | 'disconnected'
  console.log('Connection status:', status);
});

// display_uri fires during 'connecting' state — headless QR code flow
client.on('display_uri', (uri: string) => {
  renderQrCode(uri);
});

const client = await createMultichainClient({
  dapp: { name: 'My DApp', url: window.location.href },
  api: {
    supportedNetworks: getInfuraRpcUrls({ infuraApiKey: 'YOUR_INFURA_API_KEY' }),
  },
  ui: { headless: true },
});

client.on('display_uri', (uri: string) => {
  // Render your own QR code or deeplink UI
  showCustomQrModal(uri);
});

await client.connect(
  ['eip155:1', 'solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp'],
  [],
);

// Hide QR modal on successful connection
hideCustomQrModal();

// Disconnect only Solana scope — EVM session stays active
await client.disconnect(['solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp']);

// Full disconnect — revoke all scopes, terminate session
await client.disconnect();

import { RPCInvokeMethodErr } from '@metamask/connect-multichain';

try {
  await client.connect(['eip155:1'], []);
} catch (err: any) {
  if (err?.message?.includes('Existing connection is pending')) {
    // MWP: a previous connect() is still pending — do NOT retry.
    // Show "Check your MetaMask Mobile app to continue" message.
    // (This error has no numeric code.)
  } else if (err?.code === 4001 || /reject|denied|cancel/i.test(err?.message ?? '')) {
    // User rejected the connection — show retry UI
  } else {
    console.error('Connection error:', err);
  }
}

// invokeMethod errors are wrapped in RPCInvokeMethodErr (err.code === 53).
// The wallet's original EIP-1193 / JSON-RPC code is on err.rpcCode
// (with err.rpcMessage and err.rpcData for revert data).
try {
  await client.invokeMethod({
    scope: 'eip155:1',
    request: { method: 'personal_sign', params: ['0x48656c6c6f', '0xYourAddress'] },
  });
} catch (err) {
  if (err instanceof RPCInvokeMethodErr && err.rpcCode === 4001) {
    // User rejected the signature — not an app error
  } else {
    throw err;
  }
}