Viem

Overview

Viem is a TypeScript interface for Ethereum that provides low-level stateless primitives for interacting with the blockchain. It focuses on developer experience, stability, bundle size, and performance.

Key features:

• Automatic type safety and inference
• Tree-shakable lightweight modules
• Optimized encoding/parsing
• Composable APIs

Core Concepts

Constants

Viem exports useful constants for common values:

import { maxUint256, maxUint128, maxInt256, zeroAddress, zeroHash } from "viem";

// Max values for uint types
maxUint256; // 2n ** 256n - 1n
maxUint128; // 2n ** 128n - 1n

// Zero values
zeroAddress; // 0x0000000000000000000000000000000000000000
zeroHash; // 0x0000000000000000000000000000000000000000000000000000000000000000

Utilities

Unit Conversion

Convert between wei and human-readable values:

import { parseEther, formatEther, parseUnits, formatUnits } from "viem";

// ETH <-> Wei (18 decimals)
parseEther("1"); // 1000000000000000000n
formatEther(1000000000000000000n); // "1"

// Custom decimals (e.g., USDC with 6 decimals)
parseUnits("100", 6); // 100000000n
formatUnits(100000000n, 6); // "100"

ABI Utilities

Parse human-readable ABIs:

import { parseAbi, parseAbiItem } from "viem";

const abi = parseAbi([
  "function balanceOf(address owner) view returns (uint256)",
  "function transfer(address to, uint256 amount) returns (bool)",
  "event Transfer(address indexed from, address indexed to, uint256 amount)",
]);

const item = parseAbiItem("function name() view returns (string)");

Address Utilities

import { getAddress, isAddress, isAddressEqual } from "viem";

// Checksum address
getAddress("0xabc..."); // "0xAbc..." (checksummed)

// Validation
isAddress("0x..."); // true/false
isAddressEqual("0xAbc...", "0xabc..."); // true (case-insensitive)

Clients

Public Client (Read Operations)

For querying blockchain state:

import { createPublicClient, http } from "viem";
import { mainnet } from "viem/chains";

const publicClient = createPublicClient({
  chain: mainnet,
  transport: http(), // or http("https://eth-mainnet.g.alchemy.com/v2/...")
});

// Read operations
const blockNumber = await publicClient.getBlockNumber();
const balance = await publicClient.getBalance({ address: "0x..." });
const block = await publicClient.getBlock();

Wallet Client (Write Operations)

For signing and sending transactions:

import { createWalletClient, http } from "viem";
import { privateKeyToAccount } from "viem/accounts";
import { mainnet } from "viem/chains";

const account = privateKeyToAccount("0x...");

const walletClient = createWalletClient({
  account,
  chain: mainnet,
  transport: http(),
});

// Send transaction
const hash = await walletClient.sendTransaction({
  to: "0x...",
  value: parseEther("0.1"),
});

Browser Wallet Client

For browser environments with injected wallets:

import "viem/window";
import { createWalletClient, custom } from "viem";
import { mainnet } from "viem/chains";

const walletClient = createWalletClient({
  chain: mainnet,
  transport: custom(window.ethereum!),
});

// Request accounts
const [address] = await walletClient.requestAddresses();

Extending Clients

Combine capabilities using .extend():

import { createWalletClient, http, publicActions } from "viem";
import { mainnet } from "viem/chains";

const client = createWalletClient({
  chain: mainnet,
  transport: http(),
}).extend(publicActions);

// Now supports both wallet and public actions
const balance = await client.getBalance({ address: "0x..." });
const hash = await client.sendTransaction({ to: "0x...", value: 1n });

Contract Interactions

Reading from Contracts

import { createPublicClient, http } from "viem";
import { mainnet } from "viem/chains";

const publicClient = createPublicClient({
  chain: mainnet,
  transport: http(),
});

const abi = [
  {
    inputs: [{ name: "owner", type: "address" }],
    name: "balanceOf",
    outputs: [{ name: "", type: "uint256" }],
    stateMutability: "view",
    type: "function",
  },
] as const;

const balance = await publicClient.readContract({
  address: "0x...", // Contract address
  abi,
  functionName: "balanceOf",
  args: ["0x..."], // Owner address
});

Writing to Contracts

const hash = await walletClient.writeContract({
  address: "0x...",
  abi,
  functionName: "transfer",
  args: ["0x...", parseUnits("100", 18)],
});

Simulating Before Writing

Simulate a transaction before sending:

const { request } = await publicClient.simulateContract({
  address: "0x...",
  abi,
  functionName: "transfer",
  args: ["0x...", parseUnits("100", 18)],
  account: "0x...",
});

const hash = await walletClient.writeContract(request);

Account Types

Local Accounts

For server-side or controlled environments:

import { privateKeyToAccount, mnemonicToAccount } from "viem/accounts";

// From private key
const account = privateKeyToAccount("0x...");

// From mnemonic
const account = mnemonicToAccount("legal winner thank year wave sausage ...");

JSON-RPC Accounts

For browser wallets:

import "viem/window";

const [account] = await window.ethereum!.request({
  method: "eth_requestAccounts",
});

Chain Configuration

Built-in Chains

import { mainnet, sepolia, arbitrum, optimism, polygon, base } from "viem/chains";

const client = createPublicClient({
  chain: arbitrum,
  transport: http(),
});

Custom Chains

import { defineChain } from "viem";

const customChain = defineChain({
  id: 123456,
  name: "My Chain",
  nativeCurrency: { name: "ETH", symbol: "ETH", decimals: 18 },
  rpcUrls: {
    default: { http: ["https://rpc.mychain.com"] },
  },
  blockExplorers: {
    default: { name: "Explorer", url: "https://explorer.mychain.com" },
  },
});

Documentation

This skill provides a reference for common operations. For comprehensive documentation, fetch the full viem docs using context7 MCP:

Library ID: /wevm/viem

Use context7 to query specific topics:

• clients – Client types and configuration
• actions – Available actions (read, write, wallet)
• accounts – Local and JSON-RPC accounts
• contract – Contract interactions (readContract, writeContract)
• abi – ABI encoding/decoding utilities
• chains – Supported chains and custom chain configuration

Fetching Documentation

To fetch viem documentation for a specific topic:

1. Resolve the library ID with mcp__context7__resolve-library-id (or use /wevm/viem directly)
2. Call mcp__context7__get-library-docs with the topic parameter

Example topics: "getting started", "wallet client", "send transaction", "read contract".