What is a Block Builder?
A block builder is a specialized entity that constructs Ethereum blocks optimized for maximum value. Builders collect transaction bundles from searchers, combine them with regular transactions, and compete to produce the most profitable block. The winning block is selected by the validator (proposer).
Block builders emerged from Proposer-Builder Separation (PBS) — a design that separates block construction from block proposal to make MEV extraction more efficient and fair.
The PBS Pipeline
Searchers → Builders → Relay → Validator (Proposer)
- Searchers find MEV opportunities and submit bundles to builders
- Builders combine bundles into complete blocks, optimizing for total value
- Relay (Flashbots) delivers builder blocks to validators without revealing contents
- Validator selects the most profitable block and proposes it to the network
Why Builders Exist
Before PBS (pre-Merge), validators/miners had to:
- Build blocks themselves
- Evaluate MEV opportunities
- Compete with specialized searchers
This created an unfair advantage for sophisticated miners. PBS separates concerns:
- Builders focus on optimizing block contents (MEV extraction)
- Validators focus on consensus (proposing and attesting)
Builder Competition
Builders compete in a sealed-bid auction:
- Each builder constructs what they believe is the most valuable block
- They submit their bid (payment to the validator) through the relay
- The relay selects the highest bid
- The validator proposes the winning block
This means validators get the maximum possible MEV revenue without needing to be sophisticated MEV operators themselves.
Centralization Concerns
The builder market has become concentrated — a small number of builders produce most blocks:
- Titan Builder, Rsync Builder, Builder0x69, Beaverbuild — these entities build a large percentage of Ethereum blocks
- Centralization in building creates censorship risks (builders can choose to exclude transactions)
- This is an active area of research — solutions like SUAVE aim to further decentralize block building
Frequently Asked Questions
Q: How many block builders are there? A: There are roughly 10-20 active builders, but the top 3-4 produce the majority of blocks. This concentration is a concern for censorship resistance.
Q: Can builders censor transactions? A: Yes. If a builder chooses not to include certain transactions (e.g., sanctioned addresses), those transactions won’t be in the block. This is why block building centralization is a concern. Solutions like inclusion lists are being developed to address this.
Q: How much do builders pay validators? A: Builders pass most of their MEV revenue to validators (often 90%+) to win the block competition. The validator receives the builder’s bid plus the standard block reward.