Bombora

Overview.

Bombora builds Ethereum mainnet blocks. Under the proposer-builder separation (PBS) model that Ethereum uses today, validators (proposers) outsource block construction to specialised operators (builders) who optimise for transaction inclusion, ordering, and MEV extraction.

Builders submit bids to relays; the relay forwards the highest-bid header to the proposer; the proposer signs. Bombora is one of these builders.

Why we built it.

The Ethereum PBS ecosystem has historically been dominated by a small number of builders. We built Bombora for two reasons. First, builder diversity matters for the same reason client diversity matters: concentration creates centralisation pressure on the most performance-critical part of the network.

Second, operating a builder is useful for the protocol-layer security work we do. The MEV supply chain spans searchers, builders, relays, and proposers. It is one of the most complex areas of the modern Ethereum stack, and operating inside it gives us perspective that desk research alone cannot.

Status.

Bombora launched on Ethereum mainnet in May 2026. Current share sits around one to one-and-a-half percent of mainnet blocks. That is small relative to the largest builders, but useful for ongoing operational learning.

How it informs audits.

The PBS supply chain shows up in audits whenever protocol security depends on transaction ordering, MEV resistance, or assumptions about searcher behaviour. When we audit DeFi, Layer 2, or protocol-layer systems, the engineers reviewing the work have operated on the builder side of the supply chain.

The boundaries between protocol design, builder behaviour, and relay trust are sharper to us than they are to firms that read about it secondhand.