Bombora

Overview.

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

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

Why we built it.

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

Second, running a builder helps the protocol-layer security work we do. The MEV supply chain spans searchers, builders, relays, and proposers. It is one of the hardest parts of the modern Ethereum stack, and operating inside it gives us a view 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 next 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.