Overview

Eureka Builder accepts standard Flashbots-compatible orderflow — bundles and raw transactions — via a JSON-RPC 2.0 API. All submissions are kept private and never forwarded to the public mempool.

How It Works

1. The searcher sends transactions and submits them via one of the order flow endpoints.

2. The builder places incoming orders into a private pool, isolated from the public mempool.

3. At each slot, the builder simulates all pending orders and selects the optimal combination to maximize block value.

4. The winning block is submitted to connected relays. Included orders are settled on-chain.

Use Cases

• Atomic bundles — group multiple transactions that must succeed or fail together

• Bundle replacement — update a bundle's contents up until the target block is built

• Private transactions — submit transactions that are never exposed to the public mempool

• Backrunning — pair your transaction with a target transaction in the same bundle

• Refund bundles — receive a percentage of the builder's profit back via refundPercent

Authentication

Certain endpoints require a request signature in the X-Flashbots-Signature HTTP header:

X-Flashbots-Signature: <public_address>:<EIP-191 signature of the request body>

Sign the keccak256 hash of the JSON-encoded request body using EIP-191. Most Flashbots-compatible SDKs handle this automatically.

SDKs & Libraries

Any Flashbots-compatible SDK works out of the box:

• ethers.js — @flashbots/ethers-provider-bundle

• viem — native JSON-RPC support

• Rust — ethers-flashbots

• Python — flashbots-py

Error Handling

Errors follow the standard JSON-RPC 2.0 format:

{
  "jsonrpc": "2.0",
  "error": { "code": -32602, "message": "Invalid params" },
  "id": 1
}

Code	Meaning
-32700	Parse error
-32600	Invalid request
-32601	Method not found
-32602	Invalid params
-32603	Internal error

Rate Limits

API calls are rate-limited per IP and per signing key. If you hit limits, you will receive an HTTP 429 response. Contact the Eureka team to request higher limits for production integrations.

API Methods

Method	Description
eth_sendBundle	Submit an atomic bundle of transactions
eth_cancelBundle	Cancel a previously submitted bundle
eth_sendRawTransaction	Submit a single raw transaction (private mempool)

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

https://rpc.eurekabuilder.xyz