OP Stack support

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.

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 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 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:

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:

      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.