Block Builders

Builder Fundamentals

What is a Builder?

Block builders are highly specialized actors who construct blocks from transaction orderflow (public transactions, bundles, private transactions, etc).

The Role of Builders

Builders run algorithms and simulations (e.g. First Come First Serve, First Price Auctions, etc.) to order bundles and TXs in a block template (technically: execution payload) that maximizes profit. Builders then bid for and buy the validators’ blockspace, facilitated by one or more relays, so their execution payloads are proposed to the blockchain.

How do builders pay block proposers?

Flashbots proposed a standardized specification for how payments are made from builders to block proposers through the following process:

1. The builder sets their own address as the feeRecipient of the payload block header they are constructing.
2. The builder includes a transaction which pays ETH to the proposer’s feeRecipient address at the end of their proposed block.

Determining the value of blocks

A standard method for determining block value is crucial for multiple components of the MEV-Boost ecosystem; including relay monitoring, validator accounting, builder payments, block explorers, payment proofs, and MEV hiding.

Various methods for defining block value were considered by members of the community. It was determined that block level scoring was the most simple and intuitive method for scoring block value.

Block level scoring

Block level scoring looks at the difference in the balance of the fee recipient account before and after the block execution.

Note that a “block score” is not meant to be a formal definition of realized extractable value, since this is a difficult metric to quantify. For example, a Layer 2 transfer to a validator’ fee recipient address could be considered extractable value, but falls outside the scope of a block score calculation.

Constructing a payment proof for this scoring method requires a Merkle Proof of the fee recipient balance in block (n - 1), and a Merkle Proof of the fee recipient balance in block n. Payment proofs have not yet been put into production. Active discussion about payment proof implementation is still on-going. For more details or to participate in the discussion around payment proofs and block-level scoring, please check out to the block scoring forum thread.

External Builders

External builders can submit blocks to Mainnet, Goerli and Sepolia Flashbots relays. The table below outlines Builder API methods available on each network.

Relay Block Submission Endpoints by Network

		Mainnet	Goerli	Sepolia
getValidators	GET Request - Returns an array of validator registrations with assigned duties in the current and next epoch	Mainnet	Goerli	Sepolia
submitBlock	POST Request - submits a block to the relay	Mainnet	Goerli	Sepolia

• See also the Relay API documentation - Block Builder API for more details on the API and payloads.
• The example Flashbots builder implementation is a good external builder reference, and is currently used in production by several builders.

Rate-limits

Submissions to all relays are currently rate-limited to 600 submissions / 5m / IP, which translates to in average 2 submissions / sec /IP.

Flashbots Builders

All Flashbots builders pay block proposers from the flashbots-builder.eth ENS address. Each Flashbots builder uses a different public key (builder_pubkey) for relay identification and analytics purposes.

The various builder_pubkeys used to identify Flashbots builders to relays are listed below:

Builder Public Key
0xa01a00479f1fa442a8ebadb352be69091d07b0c0a733fae9166dae1b83179e326a968717da175c7363cd5a13e8580e8d
0xa01b00a4ab433cbb0a0801cff3815722d56e1980caad7ed156900563e6670cdf6280535dae331f358c647c4bf4558a85
0xa02a0054ea4ba422c88baccfdb1f43b2c805f01d1475335ea6647f69032da847a41c0e23796c6bed39b0ee11ab9772c6
0xa02b009596e741d5f61d18b900cbd03bbcdb9c0f16b1981928d13b57fcb48d4ddce21a96c523bf84425b3a4e6e6b3f14
0xa03a000b0e3d1dc008f6075a1b1af24e6890bd674c26235ce95ac06e86f2bd3ccf4391df461b9e5d3ca654ef6b9e1ceb
0xa03b00ddb78b2c111450a5417a8c368c40f1f140cdf97d95b7fa9565467e0bbbe27877d08e01c69b4e5b02b144e6a265
0xa08a00b8d1521ddc7e51717f9e1ed77266108008acec8cb58aa492ed0a17cc4c55330cfb1871d4471a7451d3f7c89192
0xa08b00cedceeb18c97d723f9338ead7d660fffc9050e487a5219e334e08e3d15faf4d8b51b0daf0e792f5f27a8c54da0
0x800a002dd9e1afc77af8ae909cf7f8169b413a92cfd43caa56ac749024774d9817a806dae49f4bd5af0661b054595ea4
0x800b00c6e03a92b910cfe928fe6d5bff63eb326af308a6224c512a82c6fdeae92e4d3a39e7b8dbfc572af5d2411cb26c
0x801a00923e9949a7c510f565d92d282bcc79d79da6d57c98972891553443877ba5905b8bdf8145e23a06dac45b9a4d69
0x801b00e0e3a828ef58174652bc74cc6695fee2c7035a935b739c59cdd958c69564668ca5334dc51a85421eba77f9acc2

Additional Links & References

• MEV-Boost Geth Builder - an example builder implementation
• Relay API documentation - Block Builder API
• Block Builder Self-Help Group: https://collective.flashbots.net/c/builders/14
• Github issue about becoming block builder: https://github.com/flashbots/mev-boost/issues/145.
• Mevboost.pics - Tracking MEV-Boost relays and block builders, by Toni Wahrstätter.
• Relayscan.io - Up-to-date stats on the MEV-Boost ecosystem, by Chris Hager.

Note: Flashbots does not control and cannot verify the data coming from external people and organizations. Please direct questions or issues directly to the creators of external data sources.