Proposer-builder separation - PBS - was introduced to Ethereum’s ecosystem through MEV-Boost shortly after the Merge, and it solved a real problem. Validators were being pressured to run complex MEV extraction software themselves, which created centralisation risk at the consensus layer. By splitting the role of building a block from the role of proposing it, PBS let validators stay simple and outsource the lucrative, complicated work of block construction to specialised builders.
That logic holds. What’s less discussed is what it produced downstream.
The Builder Market Is Narrow
In practice, the builder market that emerged under MEV-Boost has concentrated quickly. A small number of builders - entities that aggregate transactions, optimise ordering, and capture MEV - have come to dominate block production on Ethereum. Data from mevboost.pics and independent researchers has shown that at various points, the top three or four builders have been responsible for the majority of Ethereum blocks. These builders run sophisticated arbitrage pipelines, maintain private order flow arrangements with wallets and applications, and operate infrastructure that individual validators cannot match.
This wasn’t an accident or a bug. It’s the predictable result of a competitive market where the ability to extract marginal value from block space compounds over time. Capital and latency advantages are self-reinforcing.
What Enshrined PBS Is Supposed to Fix
Ethereum’s roadmap includes enshrinement of PBS at the protocol level - removing the reliance on MEV-Boost, a piece of middleware that validators opt into rather than a native consensus feature. Enshrined PBS (ePBS) would formalise the builder-proposer split in the protocol itself, potentially with attester-proposer separation layered on top.
The argument for enshrinement is that the current setup creates trust assumptions. Validators using MEV-Boost relays have to trust that the relay is honest about block contents and won’t front-run or censor. Enshrinement would reduce that trust surface.
But enshrinement doesn’t dissolve builder concentration. It makes the existing structure more permanent and legible, not more competitive.
The Part That Gets Skipped
The conversation around PBS tends to stay at the mechanism design level - who proposes, who builds, how commitments are verified. What gets less attention is that whoever controls consistent access to private order flow effectively controls what gets included in blocks before the open mempool ever enters the picture.
That’s not a solvable problem through consensus layer changes alone. It’s a market structure question, and Ethereum doesn’t have a clean answer to it yet.