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

# Storage state diffs

> Track EVM storage state changes with addStateDiff() — subscribe to storage slot writes for any contract address.

<Tip>
  State diffs for historical blocks are [currently available](/en/data/evm) from [SQD Network](/en/network) on the same basis as all other data stored there: for free. If you deploy a squid that indexes traces [in real-time](/en/sdk/squid-sdk/resources/unfinalized-blocks) to SQD Cloud and use our [RPC addon](/en/cloud/resources/rpc-proxy), the necessary `trace_` or `debug_` RPC calls made will be counted alongside all other calls and [the price](/en/cloud/pricing#rpc-requests) will be computed for the total count. There are no surcharges for traces or state diffs.
</Tip>

#### `addStateDiff(options)`

Subscribe to changes in the [contract storage](https://coinsbench.com/solidity-layout-and-access-of-storage-variables-simply-explained-1ce964d7c738). This allows for tracking the contract state changes that are difficult to infer from events or transactions, such as the changes that take into account the output of internal calls. `options` has the following structure:

```typescript theme={"system"}
{
  // data requests
  address?: string[]
  key?: string[]
  kind?: ('=' | '+' | '*' | '-')[]
  range?: {from: number, to?: number}

  // related data retrieval
  transaction?: boolean
}
```

The data requests here are:

* `address`: the set of addresses of contracts to track. Leave undefined to subscribe to state changes of all contracts from the whole network.
* `key`: the set of storage keys that should be tracked. Regular hexadecimal contract storage keys and [special keys](/en/sdk/squid-sdk/reference/processors/evm-batch/field-selection#state-diffs) (`'balance'`, `'code'`, `'nonce'`) are allowed. Leave undefined to subscribe to all state changes.
* `kind`: the set of diff kinds that should be tracked. Refer to the [`StateDiff` section](/en/sdk/squid-sdk/reference/processors/evm-batch/field-selection#state-diffs) of data items documentation for an explanation of the meaning of the permitted values.
* `range`: the range of blocks within which the storage changes should be tracked.

Enabling the `transaction` flag will cause the processor to retrieve the transaction that gave rise to each state change and add it to the [`transactions` iterable of block data](/en/sdk/squid-sdk/reference/processors/evm-batch/context-interfaces).

Note that state diffs can also be requested by the [`addTransaction()`](/en/sdk/squid-sdk/reference/processors/evm-batch/transactions) method as related data.

Selection of the exact data to be retrieved for each state diff item and its optional parent transaction is done with the `setFields()` method documented on the [Field selection](/en/sdk/squid-sdk/reference/processors/evm-batch/field-selection) page. Unlike other data items, state diffs do not have any fields that can be enabled, but some can be disabled for improved sync performance.