Isthmus Upgrade

Table of Contents

• Overview
• Specifications
  • Holocene Highlights
  • Isthmus Highlights
    • Ethereum Prague's EIPs
    • Optimism-Specific Improvements
• Upgrade Timelines

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
• Optimism's Isthmus Upgrade Specifications

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: Allows an EOA to function as a smart-contract wallet for a single transaction.
• EIP-2537: Adds operations on BLS12-381 curve as a precompile.
• EIP-2935: Introduces a system contract that stores the last 8,191 block hashes.
• EIP-7623: Increases the calldata gas cost, making calldata-intensive transactions more expensive.
• 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 is scheduled for activation on the 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 is set to activate for each network, the accompanying UTC date and time, and the approximate L2 block height.