---
title: "execution/chainspec"
description: "Defines OP chain parameters, genesis state, and hardfork activation for the Base execution layer."
source: https://basehub.org/crates/execution/chainspec/
---
import { LinkCard } from '@astrojs/starlight/components';

## Overview

The `execution/chainspec` sub-crate (`reth-optimism-chainspec`) is part of the [execution crate group](/crates/execution/overview/).

It defines the OP chain spec, including genesis state, gas parameters, and hardfork activation rules consumed by the EVM and consensus layers.

## Source

<LinkCard title="View source on GitHub" href="https://github.com/base/base/tree/main/crates/execution/chainspec" />