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 builder	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
0x81babeec8c9f2bb9c329fd8a3b176032fe0ab5f3b92a3f44d4575a231c7bd9c31d10b6328ef68ed1e8c02a3dbc8e80f9
0x81beef03aafd3dd33ffd7deb337407142c80fea2690e5b3190cfc01bde5753f28982a7857c96172a75a234cb7bcb994f
0xa1dead1e65f0a0eee7b5170223f20c8f0cbf122eac3324d61afbdb33a8885ff8cab2ef514ac2c7698ae0d6289ef27fc
0xa1defa73d675983a6972e8686360022c1ebc73395067dd1908f7ac76a526a19ac75e4f03ccab6788c54fdb81ff84fc1b
0x81babad2d5fd9413c942f49bfd86bc1dca5b02ff4cd065a10c7ab05713e63883056e6a87777e236424574aa25bbe3e99
0xa35e2b13ef528efbed8d2f709c0eb9eceb1225ed0605a653ba923588b0150c94772a9ba1c809d048e321f6b73d905c60

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.org* - Tracking MEV-Boost relays and block builders. A quick hack by Anish. Design inspired by file.app.

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.