## `chainId`

### Signature

```solidity
function chainId(uint256 chainId) external;
```

### Description

Sets `block.chainid` to the specified value.

### Parameters

| Parameter | Type      | Description                 |
|-----------|-----------|-----------------------------|
| `chainId` | `uint256` | The new chain ID to set     |

### Examples

```solidity [test/ChainId.t.sol]
function testChainId() public {
    vm.chainId(31337);
    assertEq(block.chainid, 31337);
}
```

### Gotchas

:::note
Changing the chain ID affects how signatures are validated since chain ID is typically included in EIP-712 typed data signatures. Make sure to account for this when testing signature-based functionality.
:::

### Related Cheatcodes

* [`createSelectFork`](/reference/cheatcodes/create-select-fork) - Creates and selects a fork at a specific block
* [`warp`](/reference/cheatcodes/warp) - Sets `block.timestamp`
* [`roll`](/reference/cheatcodes/roll) - Sets `block.number`