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# Set Up Anvil and Fork

## Overview

[`testActions`](/docs/actions/test) adds test-node controls to a Client. The default mode targets
Anvil, while `mode: 'hardhat'` and `mode: 'ganache'` adapt supported Actions to those nodes.

## Recipes

These recipes assume [Anvil](https://book.getfoundry.sh/anvil/) is installed and listening on its
default port.

### Connect to Anvil

Extend the same Client with public, wallet, and test Actions so a test can arrange state, perform an
operation, and assert its result.

:::code-group
```ts twoslash [example.ts]
import { client } from './viem.config'

const blockNumber = await client.block.getNumber() // [!code focus]
// @log: 0n
```

```ts twoslash [viem.config.ts] filename="viem.config.ts"
// [!include ~/snippets/docs/test.config.ts:setup]
```
:::

### Start a Mainnet Fork

Pass an authenticated production RPC URL to Anvil when the test needs deployed contracts or live
chain state.

```sh
anvil --fork-url "$MAINNET_RPC_URL"
```

Pin the fork for reproducible tests.

```sh
anvil --fork-url "$MAINNET_RPC_URL" --fork-block-number 20000000
```

### Reset a Fork

Use [`state.reset`](/docs/actions/test/state/reset) to restore the configured fork, or select a new
RPC URL and block without restarting Anvil.

:::code-group
```ts twoslash [example.ts]
import { client } from './viem.config'

await client.state.reset({ // [!code focus]
  blockNumber: 20_000_000n, // [!code focus]
  jsonRpcUrl: 'https://eth.merkle.io', // [!code focus]
}) // [!code focus]
```

```ts twoslash [viem.config.ts] filename="viem.config.ts"
// [!include ~/snippets/docs/test.config.ts:setup]
```
:::

## Best Practices

### Pin Fork Inputs

Pin the block number and control the RPC URL in test configuration. Assertions against an unpinned
fork change as mainnet advances.

### Keep Test Nodes Private

Test Actions can overwrite balances, code, storage, and chain state. Never expose an Anvil RPC
endpoint to an untrusted network.

## See More

<Cards>
  <Card icon="lucide:user-cog" title="Manipulate Account State" description="Impersonate Accounts and seed the state a test needs." to="/docs/guides/testing/accounts" />

  <Card icon="lucide:undo-2" title="Snapshot and Restore" description="Return to a known state without restarting the node." to="/docs/guides/testing/snapshots" />
</Cards>