Vitalik Buterin is turning his attention to who gets to decide what transactions goes into a block. He laid out a series of ideas aimed at preventing block building, the process of assembling transactions before they’re finalized onchain, from becoming too centralized. While Ethereum’s upcoming “Glamsterdam” upgrade will formalize proposer-builder separation, which will allow validators to outsource block construction to a competitive market, Buterin argues that simply creating a marketplace of builders doesn’t solve everything. If a small number of builders dominate, they could still censor transactions or extract outsized profits from users.
One proposal, known as FOCIL, would act as a kind of anti-censorship backstop. Under the design, a small group of randomly selected participants would each choose transactions that must be included in the next block. If those transactions are missing, the block would be rejected. The idea is that even if a single hostile builder controlled the entire market, they couldn’t permanently exclude specific users. Another focus of his post is so-called “toxic MEV,” where traders exploit visibility into pending transactions to front-run or “sandwich” users’ trades.
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