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Ethereum Builder API Specification


The Builder API is an interface for consensus layer clients to source blocks built by external entities.

In this repository is the API specification along with specifications for actors in this ecosystem broken out by fork.


Block building is a specialized activity that requires high fixed costs to be an efficient validator. This creates an advantage for staking pools as they can effectively distribute the cost across many validators.

Proposer-builder separation (PBS) fixes this by splitting the roles of a validator into block proposing and block building. However, PBS requires modifications to the Beacon chain and will therefore not be possible at the time of the merge.

The Builder API is a temporary solution that requires higher trust assumptions than PBS, but can be fully implemented without modification to the base protocol. This is done by providing the proposer with a "blind" execution layer header to incorporate into a block and a "value" amount which will be transferred to the proposer once they create a block with the aforementioned header. Once the proposer signs a block with the header, they are bound to the choice (or risk being slashed due to equivocation). That allows the builder to reveal the blinded transactions without the possibility of the proposer tampering with them.

This design is based on the original proposal for trusted external builders: "MEV-Boost: Merge ready Flashbots Architecture".

Builder software

Users will typically connect their CL clients to builders with builder multiplexers. Please see their respective repositories for more information:


The API specification is checked for lint errors before merging pull requests.

To run the linter locally, install it with:

npm install -g @redocly/cli

and then run it:

redocly lint builder-oapi.yaml

Render API Specification

To render spec in browser, you will simply need an HTTP server to load the index.html file in root of the repo.

For example:

python -m http.server 8080

The spec will render at http://localhost:8080.


Local changes will be observable if "dev" is selected in the "Select a definition" drop-down in the web UI.

It may be necessary to tick the "Disable Cache" box in their browser's developer tools to see changes after modifying the source.


  1. Create and push tag
    • Make sure info.version in builder-oapi.yaml file is updated before tagging.
    • CI will create a github release and upload bundled spec file
  2. Add release entrypoint in index.html

In SwaggerUIBundle configuration (inside index.html file), add another entry in urls field. Entry should be in following format (replace <tag> with real tag name from step 1.):

{ url: "./releases/<tag>/builder-oapi.json", name: "<tag>" },