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

# Isthmus Upgrade

## Overview

The Isthmus upgrade for the Celo network adopts features from the **Holocene** and **Isthmus** upgrades of Optimism, incorporating important improvements.

This upgrade aligns Celo architecture closely with the Optimism ecosystem, bringing established and tested improvements to our network.

## Specifications

The Celo Isthmus upgrade brings in all consensus and execution changes from Optimism’s Holocene and Isthmus hardforks. For full technical details, see the Optimism specs linked below.

* [Optimism's Holocene Upgrade Specifications](https://specs.optimism.io/protocol/holocene/overview.html)
* [Optimism's Isthmus Upgrade Specifications](https://specs.optimism.io/protocol/isthmus/overview.html)

### Holocene Highlights

* Upgraded derivation pipeline – A stricter, simpler derivation pipeline enhances the Fault Proof System's worst-case behaviour and compatibility.
* EIP-1559 configurability - The `SystemConfig` L1 contract lets operators adjust the elasticity and denominator parameters, so the gas target and gas limit can be tuned independently.

### Isthmus Highlights

Isthmus incorporates the execution-layer EIPs from Ethereum’s Prague upgrade, along with several Optimism-specific enhancements.

#### Ethereum Prague's EIPs

* [EIP-7702](https://eips.ethereum.org/EIPS/eip-7702): Allows an EOA to function as a smart-contract wallet for a single transaction.
* [EIP-2537](https://eips.ethereum.org/EIPS/eip-2537): Adds operations on BLS12-381 curve as a precompile.
* [EIP-2935](https://eips.ethereum.org/EIPS/eip-2935): Introduces a system contract that stores the last 8,191 block hashes.
* [EIP-7623](https://eips.ethereum.org/EIPS/eip-7623): Increases the calldata gas cost, making calldata-intensive transactions more expensive.
* [EIP-7685](https://eips.ethereum.org/EIPS/eip-7685): Extends the block header with a 32-byte `requests_hash` commitment for conveying execution-layer requests to the consensus layer. **In Celo, this field is always `sha256("")`.**

> **Note:** Although **EIP-6110**, **EIP-7002**, and **EIP-7251** exist in the execution-layer codebase, they remain **disabled** in the Isthmus upgrade.

#### Optimism-Specific Improvements

* Updated `withdrawalsRoot`: The `withdrawalsRoot` in block header points to the storage root of `L2ToL1MessagePasser`, simplifying proof generation.
* Operator Fee: A flexible surcharge that can be tuned to cover chain-specific costs such as Alt-DA storage, ZK proving, or custom gas-token overhead. **In Celo, as with the L1 data fee, the Operator Fee is set to zero by setting `operatorFeeScalar` and `operatorFeeConstant` in `SystemConfig` contract to 0.**

## Upgrade Timelines

The Isthmus upgrade was activated on Mainnet, Alfajores, and Baklava, with the activation process for each network occurring independently.

The table below shows the UNIX timestamp at which the Isthmus upgrade activated for each network, the accompanying UTC date and time, and the approximate L2 block height.

| Network   | Unix Timestamp |     Date & Time (UTC)    |  Block Height |
| --------- | :------------: | :----------------------: | ------------: |
| Mainnet   |   1752073200   | Wed Jul 09 2025 15:00:00 | \~ 40,172,440 |
| Alfajores |   1750863600   | Wed Jun 25 2025 15:00:00 | \~ 49,908,280 |
| Baklava   |   1749654000   | Wed Jun 11 2025 15:00:00 | \~ 37,881,140 |