Signed Builder Bid

A Signed Builder Bid is a cryptographically signed message within the MEV-Boost ecosystem where a block builder commits to constructing and delivering a specific block to a validator. It contains the block's header, the validator's fee (the value), and a signature proving the bid's authenticity. This bid is submitted to a relay, which acts as a trusted intermediary to verify the bid's validity and forward the highest-paying bid to the validator for inclusion in the blockchain. The signature, typically using the builder's Ethereum private key, ensures the bid cannot be repudiated and that the promised block payload will be delivered if the bid is accepted.

The core components of a Signed Builder Bid are defined by the MEV-Boost specification. The key data includes the header (the proposed block header, excluding transactions), the value (the amount of ether the builder pays to the validator for proposing this block), the pubkey of the builder, and the signature itself. The value is the critical economic lever, representing the builder's profit-maximizing calculation after extracting Maximal Extractable Value (MEV) from the transaction order within the block's body. Validators are economically incentivized to select the bid with the highest value, as this directly increases their staking rewards.

The signing process creates a binding commitment. When a validator accepts a Signed Builder Bid by signing the provided block header (creating a Signed Blind Beacon Block), the builder is obligated to reveal the full block body (transactions) to the relay. If the builder fails to deliver the corresponding payload, the validator can propose an empty block, and the builder's signature serves as cryptographic proof of their fault, potentially leading to slashing or reputation loss within the builder market. This mechanism enforces fairness and reliability in the proposer-builder separation (PBS) model.

Signed Builder Bids are fundamental to enabling a competitive, specialized market for block production. They allow validators to outsource the complex and resource-intensive work of MEV extraction and transaction ordering to professional builders, while simply selecting the most profitable bid. This separation enhances network efficiency and can lead to more equitable MEV distribution. The entire flow—bid creation, relay auction, and commitment—occurs within the few seconds between Ethereum slots, requiring highly optimized infrastructure from builders and relays alike.

A signed builder bid is a structured message containing a proposed block's contents, the builder's payment to the validator (the bid value), and a digital signature. The bid is submitted to a relay, which acts as a trusted intermediary. The signature, created using the builder's private key, serves two critical functions: it cryptographically proves the bid's origin and authenticity, and it commits the builder to the terms, preventing them from reneging after the validator selects their block. This creates a cryptoeconomic guarantee for the validator.

The bid's structure is defined by the BuilderBid schema, which includes the execution_payload_header (a summary of the transactions), the value (the bid in wei), the builder's public address, and the signature itself. Relays validate this structure and the signature before forwarding bids to validators. The validator, or proposer, then selects the bid offering the highest payment, as this maximizes their profit from MEV (Maximal Extractable Value). The chosen signed bid is then included in a signed blinded beacon block proposal.

Upon winning, the builder must reveal the full block contents corresponding to the header they committed to. The relay verifies this match before forwarding the complete block to the validator for inclusion in the blockchain. If the builder fails to reveal a valid block, they forfeit any bond and lose the opportunity, while the validator can fall back to a local block build. This commit-reveal scheme with financial stakes ensures the system's liveness and fairness.

This mechanism is foundational to MEV-Boost, the most widely used PBS implementation in practice. By enabling specialized builders to compete in an open market, it decentralizes block production expertise and allows validators to capture value from complex transaction ordering without needing to be MEV experts themselves. The signed bid is the enforceable contract that makes this market possible.

A Signed Builder Bid is a cryptographically signed message from a block builder containing their proposed block payload and the fee (the bid) they are willing to pay a validator (the proposer) to include it. This bid is submitted to a relay, which acts as a trusted intermediary to prevent frontrunning and ensure fair auctions. The signature, typically using the builder's Ethereum address, provides non-repudiation, guaranteeing the builder is committed to delivering the specified block if their bid wins the auction. This signed offer is the fundamental unit of exchange in PBS's decentralized marketplace for block space.

The bid structure contains several critical components: the execution payload header (a summary of the proposed transactions and state), the builder's fee (in wei, payable to the proposer), the builder's public address, and a signature over the entire bid. The relay validates this signature and the bid's contents before forwarding eligible bids to the proposer. By signing, the builder commits to having the full block data ready for delivery upon request, a process enforced by the relay. This mechanism prevents builders from submitting unrealistic or fraudulent bids they cannot fulfill.

Within the transaction flow, the signed bid enables the block auction. Proposers, upon being selected to propose a block, receive a list of signed bids from the relay. They select the bid offering the highest fee, which maximizes their profit. The chosen bid's signature is then included in the Beacon Block as part of the execution payload header, providing a verifiable link between the builder's commitment and the final proposed block. This clear separation of building and proposing roles is designed to reduce centralization pressures and MEV-related risks in Ethereum's consensus layer.

The security and liveness of the flow depend entirely on the validity of the Signed Builder Bid. If a builder fails to deliver the full block data corresponding to their signed bid after winning the auction, they are slashed—losing their staked collateral—and their bid is discarded. This economic penalty ensures builders are incentivized to act honestly. The relay's role in attesting to the bid's validity and availability is crucial, making relay trustworthiness a critical component of the PBS ecosystem's integrity.