[CONSTITUTIONAL] AIP: ArbOS Version 40 Callisto

Type: Constitutional AIP

Abstract

This AIP proposes to upgrade Arbitrum One and Arbitrum Nova to ArbOS 40 “Callisto.” Callisto adds support for relevant Ethereum Execution Layer (EL) changes from Ethereum’s recent Pectra network upgrade (took place on May 7, 2025 at epoch 364032) and includes a minor fix related to Arbitrum Stylus. Among the EL changes introduced are a way for Externally Owned Accounts (EOAs) to execute smart contract code directly from their own addresses (via EIP-7702) and new precompiles to efficiently perform operations over the BLS12-381 elliptic curve, including BLS signature verification (via EIP-2537).

The ArbOS 40 “Callisto” upgrade is being proposed for Arbitrum One and Arbitrum Nova, and therefore, this specific proposal only impacts these two chains, as they are directly governed by the Arbitrum DAO. On a high level, an ArbOS upgrade can be interpreted as Arbitrum’s equivalent of a hard fork. More can be read about the subject in the overview of ArbOS releases. Please note that ArbOS version 40 “Callisto” builds upon ArbOS version 32 (previously adopted by the Arbitrum DAO); the version number is incremented to 40 (instead of 33) due to technical details that allow for better Orbit chain customizability, as explained here.

Changes that will be included in ArbOS 40 “Callisto”:

EIP-7702: Set EOA account code

EIP-7702 introduces a new transaction type that allows Externally Owned Accounts (EOAs) to set executable code at their own address, effectively adding account-abstraction functionality to EOAs. This enables features such as delegation, batching, transaction sponsorship, and sub-key permission control. ArbOS 40 implements full support for this new transaction type, bringing Arbitrum chains to feature parity with Ethereum’s Pectra account abstraction changes.

EIP-2537: Precompile for BLS12-381 curve operations

This EIP introduces a set of precompile contracts for performing cryptographic operations on the BLS12-381 curve, improving efficiency and enabling secure on-chain BLS signature verification. ArbOS 40 integrates this precompile, enabling smart contracts to use modern cryptographic primitives in line with Ethereum’s upgraded capabilities.

EIP-2935: Serve historical block hashes from state

EIP-2935 proposes storing an extended window of recent block hashes in the Ethereum state. ArbOS 40 adapts this to store approximately 27 hours' worth of L2 block hashes, allowing for more accessible and trustless historical data retrieval.

Minor Stylus VM Caching Fix (#2998)

ArbOS 40 includes a fix to the Stylus VM’s handling of non-existent contracts in cache lookups. The fix is designed to make sure calls to account_code or account_code_size reflect the current state, even for addresses where a contract did not previously exist. This change is backward-compatible and does not increment the Stylus version.

Pectra Changes Not Included in ArbOS 40

The following EIPs from Ethereum’s Pectra fork are not included:

• Consensus Layer EIPs (EIP-6110, EIP-7002, EIP-7251, EIP-7549, EIP-7691): These pertain to the Ethereum Beacon Chain and are not applicable to Arbitrum’s architecture.
• EIP-7623: Increase Calldata Cost: Although included in the Nitro codebase, support is disabled by default. ArbOwner precompiles allow optional enablement, but it is not activated in ArbOS 40.
• EIP-7685: General Purpose Execution Layer Requests: Not relevant due to Arbitrum’s lack of a separate consensus layer.
• EIP-7840: Add Blob Schedule to EL Config: Arbitrum does not currently implement its own blob schedule or native blob transactions.

Implementation Details

All of the above changes are implemented via a new ArbOS release and will be shipped via two separate upgrades: a Nitro node version upgrade and an update (to use the new WASM module root) within the on-chain rollup contracts for Arbitrum One and Arbitrum Nova. This proposal payload will contain the on-chain changes, including updating the WASM module root and scheduling the ArbOS Upgrade, and assumes that the Nitro node software for all node operators and validators for the network(s) are updated to a version that supports ArbOS 40 Callisto.

An independent, third-party audit by Trail of Bits has already been completed. The results from that audit have been published and can be found here.

Note that should this vote pass, the proposal payload here will trigger the activation of ArbOS 40 Callisto at epoch timestamp: 1750176000 which is Tuesday, June 17, 2025 4:00:00 PM (GMT) after the last step in the implementation process for Constitutional AIPs, as outlined here. This is so that large, complex upgrades (like ArbOS upgrades) can be done so when members from the Arbitrum Foundation, Offchain Labs, ArbitrumDAO Security Council, are available to assist if necessary.

Verifying the ArbOS 40 Code Difference

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Implementation Steps

1. [Completed] Forum Proposal and Snapshot Vote: Initial proposal was posted on the Arbitrum Forum. The temperature check vote on Snapshot received strong community support and was passed here.
2. [Completed] Engineering Work: Features are implemented in the Nitro codebase, including support for EIP7702, EIP2537, and EIP2935, and the Stylus caching fix. The engineering work also included smart contract code changes to the Rollup contracts to support ArbOS 40 (e.g. nitro-contracts 3.1.0)
3. [Completed] Security Audit: Trail of Bits completed a successful audit; results were published, and can be found here: https://docs.arbitrum.io/audit-reports.
4. [Completed] Release Candidate: Nitro v3.6.0 was released and is now available on Dockerhub as offchainlabs/nitro-node:v3.6.0-fc07dd2. The release notes can be found here and this version includes support for ArbOS 40 Callisto. This is not the final release for mainnet but is instead the first release that adds support for ArbOS 40 Callisto. The release notes can be found here: https://github.com/OffchainLabs/nitro/releases/tag/v3.6.0Should this vote pass, the formal and recommended Nitro node version will be communicated to the community.
5. [Completed] Arbitrum Sepolia Testnet Deployment: ArbOS 40 Callisto has been activated on Arbitrum Sepolia on May 5, 2025.
6. [This post] On-chain Vote: A formal on-chain vote on Tally will be proposed and voted on by the ArbitrumDAO determine whether ArbOS 40 Callisto can be activated on Arbitrum One and Nova.
7. [Next] Upgrade Execution: If the above on-chain vote is passed, the ArbOS 40 Callisto upgrade will be enacted following DAO governance procedures and activation timelines.

https://tally.xyz/gov/arbitrum/proposal/13108804573775967668959825241666341617107666532012387058509418598838035461528