---
tags: eip-4844,ethereum
---

# Builder Updates for Blobs (EIP-4844)

Builder API is an interface that enables validators to delegate block building duties. The set of endpoints are exposed by an external block builder (e.g. mev-boost), and consumed by CL clients when proposing a block. EIP-4844 introduces additional changes to beacon block body, execution payload and "blob" data, hence changes will be required on both the Beacon API & builder API to support this.

## Block Proposal - Builder flow

### End-to-end block proposer flow


### Beacon API & Builder API changes

sequenceDiagram
    participant validator
    participant consensus
    participant mev_boost
    Title: Block Proposal with Builder API (EIP-4844)
    validator->>consensus: GET v2/validator/blinded_blocks/{slot}
    consensus->>mev_boost: GET v1/builder/header/{slot}/{parent_hash}/{pubkey}
    mev_boost-->>consensus: GET v1/builder/header/{slot}/{parent_hash}/{pubkey} response
    Note left of mev_boost: add `blinded_blobs_bundle` to response (`SignedBuilderBid`)
    consensus-->>validator: GET v2/validator/blinded_blocks/{slot} response
    Note left of consensus: add `blinded_block_sidecars` to response
    Note over validator: include `blob_kzg_commitments` in block and sign the header
    Note over validator: sign each `blinded_blob_sidecar` individually
    validator->>consensus: POST beacon/blinded_blocks
    consensus->>mev_boost: POST /eth/v2/builder/blinded_blocks
    Note right of consensus: sends `SignedBlindedBlockContents` (contains `signed_blinded_blobs`) 
    mev_boost-->>consensus: POST /eth/v2/builder/blinded_blocks response
    Note left of mev_boost: returns revealed payload and full blobs
    Note over consensus: construct block and broadcast block and blobs

## Links

- [**consensus-spec#3244**](https://github.com/ethereum/consensus-specs/pull/3244)
- [**beacon-APIs#302**](https://github.com/ethereum/beacon-APIs/pull/302)
- [**builder-specs#61**](https://github.com/ethereum/builder-specs/pull/61)
- [Flashbots tracking issue](https://github.com/flashbots/mev-boost/issues/392)