Greg Markou cautions anyone who assumes Ethereum upgrades are smooth simply because they passed a testnet: often, they aren’t.
Sprinter co-founder and CEO Markou says network upgrades are difficult to test and can unexpectedly break many applications. He speaks from experience, having been there the last time it happened.
Markou made his remarks during a public discussion among developers about the realities of implementing protocol changes on the world’s busiest smart contract platform. His main point was straightforward: testnets are useful, but they cannot account for every edge case that might affect applications on the live network.
A London hard fork war story
Markou’s firsthand experience comes from a specific incident. He maintained web3.js during the EIP-1559 upgrade, which was released as part of the London hard fork in August 2021.
For those unfamiliar with it, web3.js is a JavaScript library that allows apps to communicate with the Ethereum blockchain. It acts as a translator between a website and the network.
A hard fork is a change to Ethereum’s core rules that the network adopts as a whole. Every node must follow the new rules, so software built on the old assumptions can suddenly fall out of step.
Markou said an update associated with that upgrade had unforeseen consequences. Nearly every application that depended on web3.js was affected.
The issue was an undetected invariant involving MetaMask. An invariant is a condition developers expect to remain true. If that assumption quietly stops holding, everything built on it can become unstable.
Markou described Ethereum as an “absolute monolithic beast.”
It’s a vivid description, and not a fond one. His point is that the system has become so large and interconnected that it is genuinely difficult to predict how a single change will ripple through it.
Why testnets may not provide the safety net people expect
Testnets are practice versions of a blockchain where developers can test changes without risking real money. They are a dress rehearsal before opening night.
Markou recognized the benefits of testnets. His more specific argument is that they don’t cover every edge case that can affect live applications.
On mainnet, apps interact with wallets, libraries, and other contracts in combinations that no single test environment can fully reproduce. The MetaMask invariant that affected web3.js users in 2021 illustrates precisely this kind of blind spot.
Who Markou is and what Sprinter is building
Markou previously co-founded ChainSafe Systems, the firm that maintained web3.js. Sprinter was launched out of ChainSafe, and Markou now leads it as CEO.
The company raised $5.2 million in a seed round in November 2025. It focuses on cross-chain credit and infrastructure, with a pitch it calls “solving-as-a-service.”
Sprinter also intends to launch a native governance token called SPRINT.
The discussion has not led to any new upgrades or protocol changes. It was a conversation about lessons learned, not an announcement.
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