TLDR
- Vitalik Buterin published a blog post outlining ways to prevent Ethereum block building from becoming too centralized
- The upcoming Glamsterdam upgrade will introduce ePBS, which separates block proposers from builders, but Buterin warns this could still lead to builder centralization
- A mechanism called FOCIL would require randomly selected participants to include certain transactions, preventing censorship even if one actor dominates block building
- “Big FOCIL” would expand this further, handling all transactions in a block and reducing builders to only MEV-relevant work
- Encrypted mempools could stop “toxic MEV” like frontrunning and sandwich attacks by hiding transactions until they are finalized
Vitalik Buterin published a blog post on Monday laying out ideas to address centralization risks in Ethereum’s block building process.
Block building is how transactions get assembled before they are confirmed on the blockchain. Buterin argues this process has quietly become one of Ethereum’s biggest pressure points.
Finally, the block building pipeline.
In Glamsterdam, Ethereum is getting ePBS, which lets proposers outsource to a free permissionless market of block builders.
This ensures that block builder centralization does not creep into staking centralization, but it leaves the…
— vitalik.eth (@VitalikButerin) March 2, 2026
The concern is that a small number of sophisticated actors tend to dominate block building. They can optimize transaction ordering to extract maximum value, which gives them an edge over smaller participants.
Ethereum’s upcoming Glamsterdam upgrade, expected in the first half of 2026, will introduce a mechanism called enshrined Proposer-Builder Separation, or ePBS. This separates the role of block proposers from block builders by outsourcing construction to an open market.
Buterin supports ePBS but warns it does not fully solve the problem. Even with an open market, a few dominant builders could still censor transactions or extract outsized profits from users.
FOCIL and Big FOCIL
To counter this, the Glamsterdam upgrade will also include FOCIL, which stands for Forward Obligatory Commitment to Inclusion Lists. It works by having 16 randomly selected participants each mandate transactions that must appear in the next block.
If those transactions are missing, the block gets rejected. Buterin says this means even if one hostile actor controlled all block building, they still could not prevent transactions from being included.
Buterin goes further by proposing “Big FOCIL,” an expanded version that would handle all transactions in a block. This would reduce the block builder’s role to only MEV-relevant transactions and state computation, effectively making block building a commodity.
Encrypted Mempools and Network Privacy
Buterin also addressed “toxic MEV,” a practice where traders exploit visibility into pending transactions to frontrun or sandwich other users’ trades.
His proposed fix is encrypting the mempool, where transactions wait before being confirmed. If a transaction is encrypted until it is included in a block, no one can exploit it in advance.
Beyond that, Buterin flagged risks at the networking layer itself. Transactions can be observed by intermediaries before they even reach a block builder.
He pointed to anonymized routing tools like Tor and Ethereum-focused mixnets like Flashnet as potential defenses. He also highlighted the Ethereum Foundation’s Kohaku privacy initiative as a relevant ongoing effort.
Buterin’s post is part of a broader series of technical reviews he has published over the past several days. These have covered topics including quantum resistance plans and execution layer changes.
Earlier this year, Buterin said he would sell ETH to fund open source development as the Ethereum Foundation entered what he described as a “period of mild austerity.”
