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# Manipulate Account State

## Overview

Test Actions can arrange Account and contract state without replaying every transaction that
produced it. Use these controls only against a local test node.

## Recipes

These recipes assume you have [set up an Anvil Client](/docs/guides/testing/anvil).

### Impersonate an Account

[`address.impersonate`](/docs/actions/test/address/impersonate) lets the node send from an address
whose private key is unavailable. Stop impersonating it after the operation.

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

const address = '0xA0Cf798816D4b9b9866b5330EEa46a18382f251e'

await client.address.impersonate({ address }) // [!code focus]
try {
  await client.transaction.send({ // [!code focus]
    account: Account.from(address), // [!code focus]
    to: '0x70997970c51812dc3a010c7d01b50e0d17dc79c8', // [!code focus]
    value: Value.fromEther('1'), // [!code focus]
  }) // [!code focus]
} finally {
  await client.address.stopImpersonating({ address }) // [!code focus]
}
```

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

### Seed a Balance and Nonce

Use [`address.setBalance`](/docs/actions/test/address/setBalance) and
[`address.setNonce`](/docs/actions/test/address/setNonce) to place an Account at the exact starting
state a test expects.

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

const address = '0xA0Cf798816D4b9b9866b5330EEa46a18382f251e'

await client.address.setBalance({ // [!code focus]
  address, // [!code focus]
  value: Value.fromEther('10'), // [!code focus]
}) // [!code focus]
await client.address.setNonce({ address, nonce: 42 }) // [!code focus]
```

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

### Modify Contract State

Use [`address.setCode`](/docs/actions/test/address/setCode) to replace runtime bytecode, or
[`address.setStorageAt`](/docs/actions/test/address/setStorageAt) to write one known storage slot.

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

const address = '0xe846c6fcf817734ca4527b28ccb4aea2b6663c79'

await client.address.setCode({ address, bytecode: '0x60006000f3' }) // [!code focus]
await client.address.setStorageAt({ // [!code focus]
  address, // [!code focus]
  index: 0, // [!code focus]
  value: '0x000000000000000000000000000000000000000000000000000000000000002a', // [!code focus]
}) // [!code focus]
```

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

## Best Practices

### Restore Every Mutation

Wrap stateful tests in a snapshot and revert it after the assertion. This prevents one test from
changing the starting state of another.

### Derive Storage Slots

Use the contract layout and encoding rules to calculate storage positions. Guessing a slot can
silently alter unrelated state.

## See More

<Cards>
  <Card icon="lucide:undo-2" title="Snapshot and Restore" description="Isolate every state mutation behind a reversible checkpoint." to="/docs/guides/testing/snapshots" />

  <Card icon="lucide:file-check-2" title="Test Contract Interactions" description="Exercise typed reads and writes against a fork." to="/docs/guides/testing/contracts" />
</Cards>