Ethereum upgrades pose testing challenges, says Sprinter co-founder
Ethereum upgrades often break applications despite passing testnet validation, warns Sprinter co-founder Greg Markou.
Greg Markou has a warning for anyone who assumes Ethereum upgrades go smoothly because they passed 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.
London hard fork incident
Markou's credibility on this topic comes from 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. For the uninitiated, web3.js is a JavaScript library that lets apps communicate with the Ethereum blockchain.
A hard fork is a change to Ethereum's core rules that the network adopts as a whole. According to Markou, an update tied to that upgrade had consequences nobody saw coming. Nearly every application that relied on web3.js was affected.
The culprit was an invariant involving MetaMask 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.
Testnet limitations
Markou described Ethereum as an 'absolute monolithic beast,' emphasizing how the system has grown so large and interconnected that predicting how a single change ripples outward is genuinely difficult.
Testnets are practice versions of a blockchain where developers can try changes without risking real money. Markou acknowledged that testnets are beneficial but argued 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.
Sprinter's focus
Markou previously co-founded ChainSafe Systems, the 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.
Sprinter's focus is cross-chain credit and infrastructure, with a pitch it calls 'solving-as-a-service.' The company also plans a native governance token called SPRINT. No new upgrades or protocol changes have been introduced as a result of this discussion.
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