# OP Stack support

Description: Reference documentation for OP Stack chain simulation in Hardhat

Note: This document was authored using MDX

  Source: https://github.com/NomicFoundation/hardhat-website/tree/main/src/content/docs/docs/reference/op-stack-support.mdx

When you set `chainType` to `"op"`, Hardhat adjusts its simulation to match OP Stack execution semantics. Only standard OP Stack behavior is simulated, so any chain-specific behavior that falls outside the OP Stack specification is not supported.

To learn more about Chain Types in general, see the [Multichain support explanation](/docs/explanations/multichain-support).

## What the `op` Chain Type changes

Here's what changes compared to the `l1` Chain Type:

- **Predeploy contracts**: OP Stack predeploy contracts are automatically deployed at genesis. The `L1Block` and `GasPriceOracle` predeploys are fully functional, while other predeploys are present at their expected addresses but will revert if called.
- **Hardfork schedule**: Instead of Ethereum hardforks like `cancun` or `prague`, the `op` Chain Type uses its own set of OP Stack hardforks. See the [configuration reference](/docs/reference/configuration#network-configuration) for the full list.
- **Transaction receipts**: Receipts for non-deposit transactions include OP Stack L1 fee fields like `l1GasUsed`, `l1Fee`, `l1GasPrice`, `l1FeeScalar`, and `l1BlobBaseFee`.

## Supported chains

Hardhat includes built-in chain-specific parameters for many OP Stack chains, including OP Mainnet, Base, and their testnets. These parameters are sourced from the [Optimism Superchain Registry](https://github.com/ethereum-optimism/superchain-registry) and include chain IDs, hardfork activation block numbers, and EIP-1559 base fee configuration.

When you configure a network with `chainType: "op"` and the right `chainId`, the correct parameters are applied automatically.

OP Mainnet and Base are actively maintained, while other chains have default parameters that may not be fully up-to-date.

Here's an example of forking OP Mainnet:

```ts
// hardhat.config.ts
// ... imports ...

export default defineConfig({
  // ... other config ...
  networks: {
    // ... other networks ...
    opMainnetFork: {
      type: "edr-simulated",
      chainType: "op",
      chainId: 10,
      forking: {
        url: configVariable("OP_MAINNET_RPC_URL"),
      },
    },
  },
});
```

To fork Base instead, just change the `chainId` and RPC URL:

```ts del={2-3} ins={4-5}
      chainType: "op",
      chainId: 10,
      forking: { url: configVariable("OP_MAINNET_RPC_URL") },
      chainId: 8453,
      forking: { url: configVariable("BASE_MAINNET_RPC_URL") },
```

### Base fee accuracy on local chains

Since the Holocene hardfork, OP Stack chains define their EIP-1559 base fee parameters dynamically, and these are expressed as block-number activations. On forked chains this works as expected, since they start at the forked block number. On local (non-forked) chains, the genesis block starts at block `0`, so some of these activations may not take effect as expected.

{/* NOTE: If Hardhat exposes EDR's `baseFeeConfig` option in the future, update the paragraph above to mention that users can override base fee parameters manually. */}