Ethereum upgrades are difficult to test and can break numerous applications without warning, even when testnets pass successfully. Greg Markou, co-founder and CEO of Sprinter, experienced this during the August 2021 London hard fork while maintaining the web3.js library. An invariant involving MetaMask that developers assumed would always hold was not detected before deployment and affected nearly every application relying on web3.js. Testnets remain useful, but they do not reproduce every possible interaction between wallets, libraries, and contracts on the mainnet. Sprinter raised $5.2 million in seed funding in November 2025 and focuses on cross-chain credit and infrastructure, with a planned native governance token called SPRINT.
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