What Is Proposer-Builder Separation? (PBS Explained)

Proposer-Builder Separation (PBS) is a structural change to how blocks are created and proposed on Ethereum and other proof-of-stake blockchains. It splits the block production process into two distinct roles: the proposer, who selects a block, and the builder, who compiles the transactions. This separation prevents a single validator from controlling both selection and content, reducing risks of censorship and centralization.

Why Proposer-Builder Separation Was Introduced

Before PBS, every Ethereum validator acted as both the block proposer and the builder. This meant the validator could see every pending transaction, decide which ones to include, and reorder them for profit — a practice known as Maximal Extractable Value (MEV). Validators with more resources could run specialized software to capture MEV, such as frontrunning large trades or sandwiching users, giving them a significant advantage over smaller validators.

The result was a growing centralization pressure. Large staking pools and sophisticated operators earned consistently higher returns, while solo stakers — using only a home computer — struggled to compete. Proposer-Builder Separation was designed to level the playing field by splitting these responsibilities, ensuring that even a small validator can earn fair rewards without needing complex MEV strategies.

A Practical Example of the Old Problem

Imagine a validator that sees a massive DEX trade about to happen. The validator can insert its own buy order before the trade and a sell order right after — a “sandwich attack” — extracting value from the user. With PBS, the validator cannot see the trade contents until after it commits to a block, making such attacks impossible for the proposer. Instead, a professional builder assembles the block and bids to have it included.

How Proposer-Builder Separation Works

The PBS mechanism introduces a trustless marketplace for block space. Here’s the step-by-step flow:

1. Builders construct full blocks containing transactions, often including complex MEV extraction strategies. They create a short commitment (a block header — a cryptographic hash) and submit it along with a bid (a fee) to a relay.
2. Relays (or direct proposer connections) forward these headers and bids to proposers without revealing the underlying transactions.
3. Proposers (validators assigned to propose a slot) see only the header and the bid. They select the header with the highest bid and sign a block commitment.
4. The selected builder then reveals the full block. The proposer broadcasts the block to the network.

Because the proposer never sees the block contents until after signing, they cannot front-run or censor individual transactions. The builder, in turn, is incentivized to include high-value transactions to win the auction.

Comparison: Old Block Creation vs. PBS

Feature	Old Model (Validator as Builder)	PBS Model
Block assembly	Validator builds block	Specialist builder compiles transactions
Transaction visibility	Full visibility to proposer	Only block header visible to proposer
MEV extraction	Kept entirely by validator	Split between builder (via transactions) and proposer (via bid)
Barriers to entry	Requires complex MEV software	Solo stakers only need to pick the best bid
Censorship resistance	Proposer can exclude any transaction	Proposer cannot see or censor individual txs

Benefits of Proposer-Builder Separation for Ethereum

PBS delivers several concrete advantages that strengthen the network:

• Decentralization: Small validators no longer need to run expensive MEV software or join a centralized staking pool. They earn reliable rewards simply by choosing the highest bid.
• Censorship resistance: Since the proposer cannot inspect the block, they cannot pick and choose which transactions to include. This makes it harder for any single entity to blacklist addresses or block certain protocols.
• Fairer MEV distribution: Instead of large validators capturing all MEV, builders compete to pay the proposer. The proposer receives a steady income stream, smoothing out the variable payouts that previously gave large operators an edge.
• Security: By reducing the economic incentive for proposers to act maliciously (e.g., by reorging transactions), PBS lowers the attack surface. The builder’s bid acts as a bond — if they submit incorrect data, they risk their entire deposit.

Real-World Example: A Solo Staker’s Experience

Alice stakes 32 ETH on her home server. Without PBS, she would need to run an MEV-Boost-like system and potentially still miss profitable bundles because she lacks the low-latency infrastructure. With PBS, Alice simply connects to a relay. Each slot, she receives several block headers with bids. She picks the highest bid — often yielding a small fee — and signs. She doesn’t need to understand MEV at all. This makes solo staking viable and contributes to a more distributed validator set.

The Role of Relays and the Future of PBS

Relays act as a middle layer that ensures builders cannot cheat by submitting incomplete blocks. They verify that the builder has the correct transactions before forwarding the header to proposers. However, relays introduce their own centralization risk — if a single relay censors builders or blocks, the entire PBS pipeline is affected. To solve this, the Ethereum community is working toward enshrined PBS (ePBS), where the separation is baked directly into the protocol, removing the need for trusted relays.

A Quick Glance at Different PBS Implementations

Implementation	Description	Status
MEV-Boost (current)	Relay-mediated PBS used today	Live on mainnet
ePBS (future)	Protocol-level PBS, no relays	In research/development
Client-based PBS	Direct proposer–builder communication	Experimental

Conclusion

Proposer-Builder Separation is a key innovation that makes Ethereum more decentralized, censorship-resistant, and fair. By splitting block creation from block proposal, PBS ensures that small validators can compete on equal footing with large operators, while reducing the ability of any single entity to control transaction ordering. For beginners entering staking, understanding PBS means you can focus on running a simple validator rather than chasing complex MEV strategies.