Greg Markou has a warning for anyone who assumes Ethereum upgrades go smoothly because they pass on a testnet: they often don’t.

The co-founder and CEO of Sprinter says upgrades to the network are hard to test and can break a large number of applications without warning. He speaks from direct experience, having been in the room the last time it happened.

His comments came during a public discussion among developers about the realities of shipping protocol changes on the world’s busiest smart contract platform. His central point was blunt: testnets help, but they cannot account for every edge case that might trip up applications running on the live network.

A London Hard Fork War Story

Markou’s credibility on the subject rests on a specific incident. He was the maintainer of web3.js during the EIP-1559 upgrade, which shipped as part of the London hard fork in August 2021. EIP-1559 overhauled Ethereum’s transaction fee market, replacing the old first-price auction model with an algorithmic base fee and optional tips—with the base fee burned, permanently removed from circulation, on every transaction.

For the uninitiated, web3.js is a JavaScript library that lets applications communicate with the Ethereum blockchain—it is one of the most widely used JavaScript libraries in the Ethereum ecosystem. Think of it as the translator sitting between a website and the network itself.

A hard fork, meanwhile, is a change to Ethereum’s core rules that the network adopts as a whole. Every node has to follow the new rules, which means any software built on old assumptions can suddenly find itself out of step.

According to Markou, an update tied to that upgrade had consequences nobody saw coming. Nearly every application that relied on web3.js was affected. That is the structural risk of shared infrastructure: because so many applications depend on the same core libraries, a defect at that layer does not stay contained to one product—it can surface across a wide swath of the ecosystem at once.

The culprit was an invariant involving MetaMask—one of the most widely used Ethereum wallets—that went undetected. An invariant is a condition that developers assume will always hold true. When one of those assumptions quietly stops being true, everything built on top of it can wobble.

Markou described Ethereum as an “absolute monolithic beast.”

It is a vivid description, and not an affectionate one. The point is that the system has grown so large and so interconnected that predicting how a single change ripples outward is genuinely difficult, even for teams that have shipped upgrades before.

Why Testnets Aren’t the Safety Net People Assume

Testnets are practice versions of a blockchain where developers can trial changes without risking real money. They are, in effect, the dress rehearsal before opening night.

Markou acknowledged that testnets are beneficial. His argument is narrower: they fail to cover all the edge cases that can hit live applications.

On mainnet, apps interact with wallets, libraries, and other contracts in combinations that no single test environment fully reproduces. The MetaMask invariant that tripped up web3.js users in 2021 is a case study in exactly that kind of blind spot. And Ethereum continues to ship hard forks on a recurring schedule, which keeps the question of how changes get tested a standing one for teams building against the network.

Who Markou Is, and What Sprinter Is Building

Markou previously co-founded ChainSafe Systems, the Toronto-based firm that maintained web3.js. Sprinter launched out of ChainSafe, and he now leads it as CEO.

The company raised $5.2 million in a seed round in November 2025. Its focus is cross-chain credit and infrastructure, with a pitch it calls “solving-as-a-service.”

Sprinter also plans a native governance token called SPRINT.

No new upgrades or protocol changes have been introduced as a result of the discussion. This was a conversation about lessons learned, not an announcement.

For Markou, the exchange served as a reminder that protocol upgrades carry risks that standard testing may not catch.

Disclosure: This article was edited by Vivian Nguyen.

Source: CryptoBriefing