engine-core is the open source Rust service powering thirdweb’s transactions API. Unlike your local web3 client, engine maximizes transaction throughput from a single EOA while providing strong exactly-once delivery guarantees.

This requires some creative approaches.

The Core Problem with Optimistic Nonces

Here’s the key difference: engine sends transactions with optimistically incrementing nonces without waiting for previous transaction responses.

We also can’t trust RPC error responses. “Nonce too low” doesn’t mean your transaction isn’t in the mempool. Blockchain infrastructure lies constantly.

So engine tracks every transaction it’s ever sent at each nonce, continuously reconciles against chain state, and handles nonce races when transactions get replaced.

This works great. Until flashblocks.

The Flashblocks Problem

Engine fetches transaction count to see if the EOA moved (with “pending” blocktag for flashblocks). But we noticed 50ms+ delays between when pending count increases vs when eth_getTransactionReceipt actually returns the receipt.

This breaks a core engine assumption.

When txCount = n but no receipt exists for nonce n-1, engine assumes an external transaction used that nonce. So it moves our “submitted” transactions back to pending to retry.

Duplicate transactions. Even though our transaction did get included, the receipt just wasn’t available yet.

The Solution

Engine now fetches both “pending” and “latest” transaction counts:

• Pending count: trusted for early confirmation
• Latest count: required before considering transactions “nonce replaced”

Native flashblocks understanding throughout the system.

EIP-7702 Made This Even Spicier

Base chains reject multiple inflight transactions for delegated accounts.

EIP-7702 doesn’t care about flashblocks. Even if nonce n is preconfirmed, you can’t send nonce n+1 until actual confirmation.

What started as a trivial change ended up needing native rebuilding across layers.

The Curse of Infrastructure

This is the curse of managing reliable infrastructure. Being prepared for and handling dozens of edge cases in the most random places.

What would you rather build: new features for your web3 game, or boring infrastructure that needs to overcome hundreds of these gotchas?

If infrastructure sounds fun to you, we’re hiring. But if you’re sane and want to focus on your business, leave the boring parts to us.

engine is free and open source: github.com/thirdweb-dev/engine-core