Fusaka Devnet 5 Launch Soon

Ethereum’s developers are preparing to launch Fusaka Devnet-5, the next major testing phase for the upcoming Fusaka upgrade. Building on lessons from Devnet-4, this iteration focuses on stability, client diversity, specification alignment, and enhanced testing practices to ensure a smooth transition toward public testnets and eventual mainnet deployment.

• From Devnet-4 Lessons to Devnet-5 Readiness
• Standardising Fork-Boundary Rules
• Minority-Client Inclusion & Launch Criteria

From Devnet-4 Lessons to Devnet-5 Readiness

Fusaka Devnet-4 revealed critical issues that need resolution before the next testing phase can begin. These included blob-fee miscalculations in the Erigon client, peer discovery problems for Lighthouse and Nimbus, and inconsistencies in fork-parameter logic.

Devnet-5 aims to address all these challenges through targeted fixes, improved boot-node diversity, and updated fork-ID logic across all clients. The overarching goal is to prevent consensus-breaking forks and ensure full sync stability before proceeding to public testnets.

Standardising Fork-Boundary Rules

A key technical decision for Devnet-5 is the adoption of a unified approach to fork-boundary parameter calculations. During Devnet-4, clients differed on whether to use the parent block’s constants or the current block’s when updating the base-fee fraction, leading to network splits.

For Devnet-5, all clients will use the current block’s parameters. This rule is now codified in updated EIPs, with static test suites being developed to enforce consistent implementation across all clients and avoid similar issues in the future.

To prevent hidden configuration mismatches between clients, Devnet-5 will require the implementation of a new JSON-RPC method: eth_getChainConfig. This method will expose each client’s static fork-ID configuration, allowing developers and test coordinators to verify alignment before launch.

Minority-Client Inclusion & Launch Criteria

Devnet-5 will feature a more diverse client mix, with Grandine, Erigon, and Lodestar collectively making up 3–5% of the validator set. This inclusion is designed to surface edge-case bugs and improve network resilience.

Before launch, four readiness criteria must be met:

1. All Devnet-4 bugs resolved and verified.
2. Passing static tests for fork-boundary logic.
3. Implementation of eth_getChainConfig across all execution clients.
4. Demonstrated peer discovery and sync stability in a diverse boot-node environment.

Conclusion

[Fusaka](https://etherworld.co/2025/07/03/fusaka-devnet-3-planning-roadmap-ahead/) Devnet-5 is more than just the next step in Ethereum’s upgrade testing, i.e., it’s a consolidation phase aimed at eliminating instability, unifying specifications, and ensuring configuration transparency. With stronger testing standards and broader client participation, Devnet-5 is set to lay a solid foundation for Sepolia and Holesky testnet forks, moving Ethereum closer to a successful Fusaka mainnet launch.

If you find any issues in this blog or notice any missing information, please feel free to reach out at yash@etherworld.co for clarifications or updates.

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