A Deep Dive Into the Ethereum Fusaka Upgrade

Introduction

The Ethereum network is on the cusp of another monumental evolution with the forthcoming Fusaka hard fork, a significant network upgrade scheduled for late 2025. Positioned as the largest bundled set of improvements since The Merge, Fusaka represents a critical step in Ethereum's ongoing scalability roadmap. Unlike upgrades that introduce new user-facing features, Fusaka is described as a "behind the curtain" enhancement, meticulously designed to tune and optimize Ethereum's core engine.

Its primary objectives are to dramatically increase Layer 2 data capacity, reduce transaction costs for users, and bolster overall network scalability, node resilience, and operational efficiency. This report will provide an exhaustive analysis of the Fusaka upgrade, covering its core narrative and strategic importance, its complex technical architecture, recent pivotal developments within its testing environment, and the potential fallout and wide-ranging impact it is expected to have across the entire Ethereum ecosystem.

The Story and Strategic Context of Fusaka

At its core, the Ethereum Fusaka upgrade is a foundational infrastructural overhaul. Its focus is not on altering the user experience directly but on reinforcing the protocol's underlying mechanics to support future growth and demand. This deliberate focus on infrastructure distinguishes it sharply from the preceding Pectra upgrade, which introduced more visible changes to the network, such as account abstraction. The core development team has engineered Fusaka to be fully compatible with all existing decentralized applications (dApps) and smart contracts, a crucial decision that ensures a seamless transition for the vibrant developer community by eliminating the risk of breaking changes.

Fusaka's strategic importance is underscored by its position within Ethereum's ambitious 2025 roadmap, which has adopted an accelerated six-month upgrade cadence to push innovation forward more rapidly. The upgrade is a direct successor to the principles introduced in the Dencun upgrade, specifically building upon the "blob transaction" framework to further expand the network's data throughput capabilities.

This timeline is strategically chosen to culminate just before the Devconnect conference in Buenos Aires, which runs from November 17-22, 2025. This timing allows the global Ethereum community to convene and discuss the implications of the freshly deployed upgrade. Furthermore, Fusaka is not an end in itself; it lays the essential groundwork for future ambitious proposals, including a potential reduction in block time being considered for the 2026 "Glamsterdam" hard fork.

A Technical Deep Dive into Fusaka's Architecture

The Fusaka upgrade is a sophisticated package of between 11 and 12 core EIPs, each designed to refine a specific aspect of the Ethereum protocol. The precise number varies slightly between sources, with one noting that more complex proposals, such as EIP-7907, were intentionally deferred to future upgrades to keep Fusaka focused, manageable, and thoroughly testable within the tight schedule. The upgrade's technical power is concentrated in two groundbreaking components: PeerDAS and Verkle Trees.

Core Technical Components

PeerDAS (Peer-to-Peer Data Availability Sampling) (EIP-7594)

This is unequivocally the centerpiece of the Fusaka upgrade. PeerDAS revolutionizes how data availability is verified on the network. Historically, validators needed to download and verify entire blocks of data, a process that is both bandwidth-intensive and a bottleneck to scalability. PeerDAS introduces a new paradigm where validators need only download small, random samples of the data blobs to cryptographically verify that the entire dataset is available. This method is analogous to quality control in a factory, where sampling a small percentage of products can reliably determine the quality of the entire batch. By implementing this, PeerDAS achieves enormous savings in bandwidth and storage for validators while maintaining robust security. Crucially, it helps to spread the "data storage work" more evenly across all network validators, democratizing the validation process and preventing bottlenecks.

Verkle Trees

The second major innovation is the introduction of Verkle Trees as a new data structure for managing Ethereum's state. This advanced cryptographic structure significantly reduces the size of the proofs required to verify information about the network's state. Smaller proof sizes have profound implications, most notably making it substantially easier and less resource-intensive to run light clients. This development is expected to foster better mobile adoption of Ethereum applications, as users could run a secure, self-verifying node directly on their smartphones.

Key Ethereum Improvement Proposals (EIPs) in Fusaka

Beyond the headline features, Fusaka includes a suite of EIPs that collectively enhance the network's performance and security:

• EIP-7935 (Gas Limit Increase): This proposal outlines a plan to substantially increase the block gas limit, the maximum amount of computational work allowed in a single block. The limit is planned to start at approximately 45 million and scale progressively towards a target of 150 million, a change that will enable significantly more transactions to be included in each block.
• EIP-7825 (Spam Resistance Checks): Considered a headline item, this EIP introduces robust checks to prevent malicious transaction spam. By mitigating these types of attacks, it ensures that individual nodes remain stable and responsive even under periods of high demand or hostile activity.
• EIP-7951 (secp256r1 Precompile): This EIP bridges Ethereum closer to traditional Web2 security standards. It adds native support for the P-256 elliptic curve, a cryptographic standard widely used in hardware security modules, smartphones, and many existing digital wallets and security systems, thereby enhancing interoperability and security.
• EIP-7939 (CLZ Opcode): A more technical addition, this EIP adds a "count leading zeros" instruction to the Ethereum Virtual Machine (EVM). This simple but powerful operation is highly useful for optimizing cryptographic functions and other bit-level manipulations within smart contracts.
• Other Supporting EIPs: The upgrade is rounded out by several other targeted improvements, including EIP-7883, which adjusts gas costs for certain cryptographic operations; EIP-7823, which strengthens the network's resilience to denial-of-service attacks; and EIP-7892, which improves the protocol's framework to make future upgrades smoother.

Recent Developments: The End of an Era for the Holesky Testnet

A major development unfolding in parallel with the Fusaka upgrade is the planned shutdown of Holesky, Ethereum's largest public testnet. After two years of service, this once-essential piece of infrastructure is being retired.

Holesky was launched in 2023 with the specific purpose of stress-testing Ethereum's proof-of-stake consensus mechanism at a massive scale. It quickly became the largest public testnet, providing a crucial platform where thousands of validators could trial major network upgrades before their deployment on the mainnet. Both the Dencun and Pectra upgrades, which delivered lower transaction costs and improved validator efficiency respectively, were first put through their paces on Holesky.

However, the testnet began to show its age and limitations. After the Pectra upgrade was activated in early 2025, Holesky started to suffer from severe "inactivity leaks". This phenomenon occurs when a large number of validators go offline simultaneously, which in turn created a massive backlog for other validators attempting to exit the system. The result was months-long queues, rendering the testnet impractical for developers who needed to test the full validator lifecycle—from entry to exit—with fast feedback loops. For them, Holesky had transformed from a useful tool into a significant "roadblock".

In response to these challenges, the core development team has orchestrated a transition to a new, more robust testing environment. The shutdown of Holesky is scheduled for two weeks after the Fusaka upgrade is finalized on the mainnet, at which point client and infrastructure teams will officially cease their support. A fresh slate of testnets has been established to take its place, each with a specialized role:

• Hoodi: Launched in March 2025, Hoodi is a "clean-slate" testnet built specifically to sidestep Holesky's problems. It has taken over as the primary environment for validator and staking provider testing.
• Sepolia: This long-standing testnet continues to serve as the main staging ground for dApp and smart contract developers, providing a stable environment for application-level testing.
• Ephemery: This testnet serves a niche but important function, offering quick-reset validator cycles every 28 days for specific testing scenarios.

Potential Fallout and Ecosystem Impact

The effects of the Fusaka upgrade will permeate every layer of the Ethereum ecosystem, though the impact will be felt differently by various stakeholders.

• For Users: For the average user, the changes from Fusaka will not be immediately obvious or tangible. However, the downstream effects will be significant. Users should experience steadier and more predictable gas fees and a smoother overall transaction flow, especially during periods of high network congestion. The ultimate goal of these deep-level improvements is to make the Ethereum network more accessible, reliable, and affordable for everyday use cases.
• For Layer 2 Networks: Fusaka is poised to be a transformative event for Layer 2 rollups. By dramatically increasing the data throughput available on the main chain, the upgrade could unlock unprecedented scalability for L2s. This could allow these networks to process tens of thousands of transactions per second, leading to a substantial reduction in their operational costs and, consequently, the fees passed on to their users.
• For Developers: The upgrade has been described as a "quiet powerhouse" for developers. The higher gas ceilings introduced by EIP-7935 will allow for the creation of more complex and feature-rich applications, as more computational work can be done within a single block. The most critical assurance for developers is the guarantee of full backward compatibility, which means that existing dApps, client software, and smart contracts will continue to function without any required modifications.
• For Validators and Network Health: The upgrade introduces a critical trade-off for node operators. On one hand, PeerDAS is designed to relieve the workload on individual nodes by reducing bandwidth and storage demands for data verification. However, this benefit is counterbalanced by the increased block gas limit, which will inevitably increase the storage and bandwidth requirements for validators who must process larger blocks. This could place a significant strain on smaller, independent node operators, potentially nudging the network toward a greater reliance on industrial-scale, centralized staking providers who can better afford the requisite hardware. This tension between scalability and decentralization will be a key dynamic to watch in the post-Fusaka era.