> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ostium.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Testnet

> Use the Ostium SDK on Arbitrum Sepolia by enabling the testnet option on any client mode.

All client modes support `testnet: true`.

When enabled, the SDK switches to Arbitrum Sepolia defaults for:

* contracts
* subgraph endpoint
* builder API endpoint
* default Pimlico sponsor URL

## Example

```ts theme={null}
const client = await OstiumClient.createDelegatedAndGasless({
  delegatePrivateKey: process.env.DELEGATE_PRIVATE_KEY as `0x${string}`,
  traderAddress: '0xTraderAddress',
  testnet: true,
});
```

## Notes

* if you omit `pimlicoUrl` in gasless modes, the SDK uses the default Sepolia sponsor URL when `testnet: true`
* you can still override `rpcUrl`, `subgraphUrl`, `builderApiUrl`, or `pimlicoUrl` manually
* make sure the trader account, approvals, and delegation setup all happen on the same network

See [Client Configuration](/developer/client-modes/configuration) for the full list of parameters every mode accepts.