Viem Celo

Project Setup

Public Client (apps/api/src/lib/celo-client.ts)

import { createPublicClient, http } from 'viem';
import { celo } from 'viem/chains';

export const celoClient = createPublicClient({
  chain: celo,
  transport: http(process.env.CELO_RPC_URL || 'https://forno.celo.org'),
});

Chain Config

• Chain: Celo Mainnet
• Chain ID: 42220
• Import: import { celo } from 'viem/chains'
• Default RPC: https://forno.celo.org
• Block time: ~5 seconds

Common Patterns

Reading Contracts

const result = await celoClient.readContract({
  address: CONTRACT_ADDRESS,
  abi: contractAbi,
  functionName: 'someFunction',
  args: [arg1, arg2],
});

Encoding Transaction Data

import { encodeFunctionData } from 'viem';

const data = encodeFunctionData({
  abi: contractAbi,
  functionName: 'swapIn',
  args: [provider, exchangeId, tokenIn, tokenOut, amountIn, minAmountOut],
});

Format / Parse Units

import { formatUnits, parseUnits } from 'viem';

// Mento tokens: 18 decimals
formatUnits(1000000000000000000n, 18); // "1.0"
parseUnits('100', 18); // 100000000000000000000n

// USDC/USDT: 6 decimals
formatUnits(1000000n, 6); // "1.0"
parseUnits('100', 6); // 100000000n

Address Types

Always use Address type from viem for addresses:

import type { Address } from 'viem';

const addr: Address = '0x765DE816845861e75A25fCA122bb6898B8B1282a';

Celo-Specific: feeCurrency (CIP-64)

Celo supports paying gas fees in tokens other than CELO. Use the feeCurrency field:

const hash = await walletClient.sendTransaction({
  to: '0x...',
  data: '0x...',
  feeCurrency: '0x765DE816845861e75A25fCA122bb6898B8B1282a', // Pay gas in USDm
});

Common fee currencies on Celo:

• USDm (cUSD): 0x765DE816845861e75A25fCA122bb6898B8B1282a
• USDC: 0xcebA9300f2b948710d2653dD7B07f33A8B32118C

Environment Variables

Important Notes

• All Mento stablecoin tokens use 18 decimals
• USDC and USDT on Celo use 6 decimals
• Use getTokenDecimals() from packages/shared to get correct decimals
• Always lowercase addresses when using them as map keys
• The project uses viem v2 — check package.json for exact version