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

# Using System Contracts

> Understand how to best use system contracts on Abstract.

When building smart contracts on Abstract, you often need to interact directly with **system contracts**
to perform operations, such as:

* Deploying smart contracts with the [ContractDeployer](/how-abstract-works/system-contracts/list-of-system-contracts#contractdeployer).
* Paying gas fees to the [Bootloader](/how-abstract-works/system-contracts/bootloader).
* Using nonces via the [NonceHolder](/how-abstract-works/system-contracts/list-of-system-contracts#nonceholder).

## Installing system contracts

To use system contracts into your smart contracts, install the
[@matterlabs/zksync-contracts](https://www.npmjs.com/package/@matterlabs/zksync-contracts) package.

forge install matter-labs/era-contracts

<CodeGroup>
  ```bash Hardhat theme={null}
  npm install @matterlabs/zksync-contracts
  ```

  ```bash Foundry theme={null}
  forge install matter-labs/era-contracts
  ```
</CodeGroup>

Then, import the system contracts into your smart contract:

```solidity theme={null}
// Example imports:
import "@matterlabs/zksync-contracts/l2/system-contracts/interfaces/IAccount.sol";
import { TransactionHelper } from "@matterlabs/zksync-contracts/l2/system-contracts/libraries/TransactionHelper.sol";
import { BOOTLOADER_FORMAL_ADDRESS } from "@matterlabs/zksync-contracts/l2/system-contracts/Constants.sol";
```

## Available System Contract Helper Libraries

A set of libraries also exist alongside the system contracts to help you interact with them more easily.

| Name                                                                                                                                               | Description                                                                                               |
| -------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------- |
| [EfficientCall.sol](https://github.com/matter-labs/era-contracts/blob/main/system-contracts/contracts/libraries/EfficientCall.sol)                 | Perform ultra-efficient calls using zkEVM-specific features.                                              |
| [RLPEncoder.sol](https://github.com/matter-labs/era-contracts/blob/main/system-contracts/contracts/libraries/RLPEncoder.sol)                       | Recursive-length prefix (RLP) encoding functionality.                                                     |
| [SystemContractHelper.sol](https://github.com/matter-labs/era-contracts/blob/main/system-contracts/contracts/libraries/SystemContractHelper.sol)   | Library used for accessing zkEVM-specific opcodes, needed for the development of system contracts.        |
| [SystemContractsCaller.sol](https://github.com/matter-labs/era-contracts/blob/main/system-contracts/contracts/libraries/SystemContractsCaller.sol) | Allows calling contracts with the `isSystem` flag. It is needed to call ContractDeployer and NonceHolder. |
| [TransactionHelper.sol](https://github.com/matter-labs/era-contracts/blob/main/system-contracts/contracts/libraries/TransactionHelper.sol)         | Used to help custom smart contract accounts to work with common methods for the Transaction type.         |
| [UnsafeBytesCalldata.sol](https://github.com/matter-labs/era-contracts/blob/main/system-contracts/contracts/libraries/UnsafeBytesCalldata.sol)     | Provides a set of functions that help read data from calldata bytes.                                      |
| [Utils.sol](https://github.com/matter-labs/era-contracts/blob/main/system-contracts/contracts/libraries/Utils.sol)                                 | Common utilities used in Abstract system contracts.                                                       |

<Card title="System contract libraries source code" href="https://github.com/matter-labs/era-contracts/tree/main/system-contracts/contracts/libraries" icon="github">
  View all the source code for the system contract libraries.
</Card>

## The isSystem Flag

Each transaction can contain an `isSystem` flag that indicates whether the transaction
intends to use a system contract’s functionality. Specifically, this flag needs to be true
when interacting with the [ContractDeployer](/how-abstract-works/system-contracts/list-of-system-contracts#contractdeployer)
or the [NonceHolder](/how-abstract-works/system-contracts/list-of-system-contracts#nonceholder) system contracts.

To make a call with this flag, use the [SystemContractsCaller](https://github.com/matter-labs/era-contracts/blob/main/system-contracts/contracts/libraries/SystemContractsCaller.sol)
library, which exposes functions like `systemCall`, `systemCallWithPropagatedRevert`, and `systemCallWithReturndata`.

<Accordion title="Example transaction using the isSystem flag">
  ```solidity theme={null}
  import {SystemContractsCaller} from "@matterlabs/zksync-contracts/l2/system-contracts/libraries/SystemContractsCaller.sol";
  import {NONCE_HOLDER_SYSTEM_CONTRACT, INonceHolder} from "@matterlabs/zksync-contracts/l2/system-contracts/Constants.sol";
  import {TransactionHelper} from "@matterlabs/zksync-contracts/l2/system-contracts/libraries/TransactionHelper.sol";

  function validateTransaction(
      bytes32,
      bytes32,
      Transaction calldata _transaction
  ) external payable onlyBootloader returns (bytes4 magic) {

      // Increment nonce during validation
      SystemContractsCaller.systemCallWithPropagatedRevert(
          uint32(gasleft()),
          address(NONCE_HOLDER_SYSTEM_CONTRACT),
          0,
          abi.encodeCall(
              INonceHolder.incrementMinNonceIfEquals,
              (_transaction.nonce)
          )
      );

      // ... rest of validation logic here

  }
  ```
</Accordion>

### Configuring Hardhat & Foundry to use isSystem

You can also enable the `isSystem` flag for your smart contract development environment.

#### Hardhat

Add `enableEraVMExtensions: true` within the `settings` object of the `zksolc` object in the `hardhat.config.js` file.

<Accordion title="View Hardhat configuration">
  ```typescript theme={null}
  import { HardhatUserConfig } from "hardhat/config";
  import "@matterlabs/hardhat-zksync";

  const config: HardhatUserConfig = {
    zksolc: {
      version: "latest",
      settings: {
        // This is the current name of the "isSystem" flag
        enableEraVMExtensions: true, // Note: NonceHolder and the ContractDeployer system contracts can only be called with a special isSystem flag as true
      },
    },
    defaultNetwork: "abstractTestnet",
    networks: {
      abstractTestnet: {
        url: "https://api.testnet.abs.xyz",
        ethNetwork: "sepolia",
        zksync: true,
        verifyURL:
          "https://api-explorer-verify.testnet.abs.xyz/contract_verification",
      },
    },
    solidity: {
      version: "0.8.24",
    },
  };

  export default config;
  ```
</Accordion>

#### Foundry

Add the `enable_eravm_extensions = true` flag to the `foundry.toml` configuration file.

<Accordion title="View Foundry configuration">
  ```toml {7} theme={null}
  [profile.default]
  src = 'src'
  libs = ['lib']
  fallback_oz = true

  [profile.default.zksync]
  enable_eravm_extensions = true # Note: System contract calls (NonceHolder and ContractDeployer) can only be called with this set to true

  [etherscan]
  abstractTestnet = { chain = "11124", url = "https://api-sepolia.abscan.org/api", key = "TACK2D1RGYX9U7MC31SZWWQ7FCWRYQ96AD"}
  abstractMainnet = { chain = "2741", url = "", key = ""} # You can replace these values or leave them blank to override via CLI
  ```
</Accordion>