How do I choose a good block builder?

There are a few criteria to look for in a block builder:

  • Are they committed to fair and unbiased execution?
    • A good builder will not front-run, sandwich or censor bundles, or otherwise engage in activities that abuse privileged data access.
  • Do they connect to a trusted relay?
    • Keep in mind that the relay can also see raw transactions, which gives them the ability to front-run, censor, etc.
  • Does their relay connect to enough validators?
    • The more validators a relay connects to, the more slots will generally be available for builders connected to that relay. When you’re targeting a specific block/slot, it’s imperative that you send your transactions to a builder which is connected to the validator responsible for proposing a block in that slot. More validators ⇒ better inclusion rates.
    • Note: Any validator can use mev-boost to connect to the Flashbots relay and other relays.
    • It’s also worth considering how much collective stake the validators connected to a relay have. Generally speaking, if more than one block is proposed to the network (unusual but possible), the block with the most collective stake attesting to it will be included. This scenario is explained in greater detail in the Ethereum docs.

Also note that block builders have the freedom to specialize. You may find that one builder is more or less friendly to your strategy than others. Builders are competing with each other, so they are all incentivized to include your bundles in their blocks, but you may find that some builders will prioritize certain strategies over others regardless of potential profits. Builders might also censor certain bundles due to local regulations or corporate strategies and policies. There are a lot of variables in play here, so I recommend trying a few trusted builders and seeing how your mileage varies first-hand.

Flashbots will run a builder that follows the same principles we’ve adhered to in PoW Ethereum: democratized access to MEV, fair & reliable execution, and privacy.