Overview

Builder Execution Extension (BEX) is a unique Eureka Builder capability that lets protocols perform an off-chain compute against live block state and use the result to drive an on-chain transaction — all within the same block, atomically.

This solves a fundamental problem in DeFi: many on-chain actions require knowing a value (a balance, a price, a position size) that only exists at execution time. With BEX, the builder reads that value right before executing, eliminating the need for on-chain oracles or pre-committed calldata.

It also offloads gas-heavy logic to the block builder when it would be uneconomic or infeasible to run inside an Ethereum transaction.

How It Works

A BEX request targets a specific block and consists of two parts:

Request envelope

• targetBlock: the block number this request targets

Compute — a staticcall the builder runs off-chain against the current mid-block state:

• compute.to: the contract to call off-chain

• compute.calldata: the calldata for the off-chain compute

Execute — the on-chain transaction the builder signs and submits using the compute result:

• execute.to: the contract to call on-chain

• execute.selector: the function to call - 4 byte function selector, hex-encoded with 0x prefix

• execute.prefixArgs: fixed arguments prepended before the compute result

• execute.appendComputeResult: if true, the raw return data from compute is appended

Replacement fields (optional, for replaceable bundles):

• replacementUuid: a stable UUID identifying a replaceable bundle slot. Requires X-Flashbots-Signature.

• replacementNonce: a monotonically increasing nonce; higher nonce replaces lower nonce for the same (replacementUuid, signer) pair.

The builder assembles the final calldata as:

selector ++ prefixArgs ++ (computeReturnData if appendComputeResult)

The resulting transaction is signed by the builder's coinbase key and included in the block.

Use Cases

• Conditional liquidations — read a position's health factor, execute only if below threshold

• Dynamic arbitrage — read current pool reserves, compute optimal swap amount on-chain

• Oracle-free settlements — read live price data and settle a contract without a trusted oracle

• Rebalancing — read current allocations and rebalance to target weights in one transaction

Default Limits

Call bex_capabilities to query the current limits for this builder:

Limit	Default	Description
maxCalldataBytes	24,576 bytes (24 KiB)	Maximum size of compute.calldata or execute.prefixArgs
maxComputeGas	10,000,000	Maximum gas for the off-chain compute staticcall
maxExecuteGas	500,000	Maximum gas for the on-chain execute transaction

Requests expire after 60 seconds if their target block hasn't been built.

Authentication

BEX requires the X-Flashbots-Signature header when:

• An allowlist is configured (production) — your signing address must be whitelisted

• Using replacementUuid — the same signer must submit all replacements and cancellations

Contact the Eureka team to get your address added to the allowlist.

API Methods

Method	Description	Response
bex_sendBundle	Submit a BEX compute+execute request	{ bundleHash }
bex_cancelBundle	Cancel a replaceable BEX bundle	true
bex_simulate	Validate a BEX request without storing it	{ accepted, bundleHash, issues }
bex_capabilities	Query builder limits (calldata size, gas caps)	{ maxCalldataBytes, maxComputeGas, maxExecuteGas }

All requests are sent as JSON-RPC 2.0 POST to:

https://rpc.eurekabuilder.xyz