Priority Blockspace for Humans on World Chain: Mainnet-Ready with Alchemy

Last year, World introduced Priority Blockspace for Humans (PBH), a novel way for World ID holders to receive prioritized transaction inclusion in World Chain blocks. PBH is designed to counter the proliferation of bots onchain by reserving guaranteed blockspace for real, verified humans.

According to L2Beat, World Chain has consistently been in the top 10 in total value secured (at over $450M) and top 5 of user operations per second (typically greater than 25 UOPS). With its flagship application, World App, there are few infrastructure projects in the crypto space that have so many real humans moving real value. It’s imperative that those real humans have a reliable way to access World Chain blockspace, a resource that could become one of the cornerstones of the human-centric internet.

Since its inception, Tools for Humanity (the primary contributor to World), Flashbots, and our team have worked closely to bring PBH to life. This week, PBH officially launches on World Chain mainnet. As the Rollup-as-a-Service (RaaS) provider for World Chain and the ERC-4337 bundler powering World App, we've played a central role in building, testing, and scaling the infrastructure that powers PBH.

This post dives into how PBH works and the work that went into making it real.

PBH Basics: The High-Level Flow

When a user downloads World App, they are given a World ID. They can then choose to verify their World ID at an Orb, which is an advanced camera that takes pictures of a user’s face and eyes. These are encrypted and stored only on a user’s phone. Permanently encrypted codes are generated by the photos, which are then stored across secure databases. The private portion of  World ID, stored on the user’s device, is used to prove membership in the onchain identity data set. .

Users can use World ID to prove they’re a verified human by confirming its existence in the onchain merkle tree. This is done using zero-knowledge proofs with private inputs such that the verifier doesn’t learn anything about the user other than that they are previously verified.

Tools for Humanity designed a custom block builder that can detect and prioritize onchain user operations carrying a proof of human bound to that operation.

This zero-knowledge proof of human bound to a transaction is built with:

• A private identity and a private Merkle proof proving the existence of that identity in the World ID set.

• A public signal hash binding the proof to the specific transaction.

• A public nullifier used to rate-limit proof usage and prevent abuse.

Here’s the flow:

1. The user generates a World ID proof bound to their transaction and submits it alongside the transaction.

2. The block builder checks:

   • The validity of the proof on its public inputs: current Merkle root of the ID set, the operation hash as the signal, and the nullifier.

   • That the nullifier hasn’t exceeded usage limits.

3. If checks pass, the builder includes the operation at the top of the block.

RaaS Reliability with PBH

Our RaaS team runs industry leading sequencer infrastructure, designed for high availability and low latency. In extending support for PBH at the sequencer level, our goal was to maintain the level of reliability our users expect.

To make this possible, we integrated Rollup-Boost, a system developed by Flashbots and Tools for Humanity. It enables external block production while preserving the safety and liveness guarantees of a standard op-geth-based sequencer. In this setup, blocks are built externally by the World Builder using novel PBH ordering and then validated locally by op-geth. Rollup-Boost provides a fallback path, allowing the sequencer to switch from the custom World Builder to the standard one if needed.

This architecture introduces a new tradeoff: Rollup-Boost becomes a potential single point of failure.

Our High-Availability Sequencer Stack

To address single points of failure in the op-stack, we use an op-conductor-based high-availability sequencer architecture. This setup allows the sequencer to:

• Run three replicas of each component (World Builder, Rollup-Boost, and the op-stack pair: op-node and op-geth).

• Use Raft-based leader election and automatic failover when a replica becomes unhealthy.

• Continue building blocks using op-geth if all World Builders are unavailable.

Additional routing layers, including proxies, relays, and load balancers, isolate the sequencer from external traffic while maintaining low latency. The introduction of Rollup-Boost doubled the number of block builders in the system, and PBH complicated the transaction validity checks. Tools for Humanity (TFH) developed a custom transaction proxy for World Builder traffic. We worked closely with TFH to design and integrate this proxy while maintaining high-availability for PBH validations and non-PBH traffic.

The result of all this infrastructure is that PBH runs on the most customizable high-availability sequencer stack in the industry, giving the World team the freedom to iterate on its builder logic with 99.999% availability.

Making Mainnet Launch a Success Together

Ahead of the mainnet launch, our team and Tools for Humanity collaborated on a series of simulations to test this infrastructure under stress. Over three days, 40+ hours of meetings, and 100+ hours of engineering time, the teams halted nodes, dropped peers, forced failovers, and documented anomalies.

All bugs and gaps in observability were addressed, resulting in an infrastructure stack that both teams are confident can handle production workloads with the reliability users expect from a leading L2. As a result of learnings from these simulations we:

• Hardened the automatic sequencer failover process removing undesired failovers and tuning health checks.

• Right sized all of the block-building hardware to achieve maximum throughput.

• Aligned transaction pool settings across all of the block builders in the system, World and op-geth, to improve transaction propagation through the system.

Priority Blockspace for Humans + ERC-4337

World App uses ERC-4337 user operations (UOs) to sponsor gas for its users. It accounts for nearly all UOs on World Chain, which consistently processes over 35 million UOs per month and represents two-thirds of all ERC-4337 activity across all chains.

It is essential that PBH integrate fully with ERC-4337, enabling developers to submit user operations with PBH proofs for their end users with top-of-block inclusion.

We maintain the leading ERC-4337 bundler infrastructure, powered internally by Rundler, which processes over 75% of all mined user operations across tracked chains. We collaborated with Tools for Humanity to design a PBH integration that:

1. Allows the bundler to detect PBH-eligible user operations and submit PBH-only bundles for top-of-block inclusion.

2. Preserves full ERC-4337 compatibility, maintaining its core permissionless design.

3. Avoids unnecessary bundler logic, reducing maintenance overhead.

How It Works

PBH user operations include the proof in the signature field. Using this, the bundler must:

• Detect valid PBH UOs.

• Build bundles that exclusively contain valid PBH UOs.

A naive solution would hardcode PBH-specific logic into Rundler, but this is brittle and hard to maintain. Instead, the teams used ERC-4337’s signature aggregator feature, a lightly used and often misunderstood mechanism.

In this design:

• PBH UOs return a specific PBH aggregator address from their validation function.

• Rundler detects this and calls the aggregator’s validation method to check the proof.

• Bundles are built per aggregator, ensuring PBH UOs are grouped together. Proofs are stripped from each UO and concatenated together.

• Aggregated proof data is passed onchain to the aggregator for verification. The aggregator verifies each proof before user operations are able to execute.

This approach:

• Requires no changes to Rundler’s core logic and is compatible with other aggregators, including BLS.

• Works with standard bundler APIs.

• Encapsulates PBH support behind a standard interface that any bundler could adopt.

One Roadblock: EntryPoint v0.7 and Stateful Aggregators

PBH uses nullifiers to rate-limit usage. To enforce this, the aggregator must track state. But EntryPoint v0.7 only allows view calls to aggregators, making stateful logic impossible.

To work around this, a custom PBHEntrypoint was deployed to wrap v0.7 and track nullifiers onchain.

The PBHEntrypoint coordinates with the PBHSignatureAggregator to ensure that the bundler calls through the PBH entry point prior to executing the bundle:

• Before calling into the ERC-4337 entry point, PBHEntrypoint stores a hash of all user ops in transient storage.

• The signature aggregator later verifies this hash when it is called by the ERC-4337 entry point, reverting if it doesn't match.

Thanks to feedback from TFH and our team, the view restriction has been removed in EntryPoint v0.8, paving the way for cleaner implementations.

Next Block Inclusion

Priority blockspace only matters if it’s fast. Leading up to PBH mainnet, our bundler team made several performance improvements to ensure next-block inclusion:

1. Scaled EOA relays and native gas balances.

2. Optimized bundling latency.

3. Improved relay assignment to reduce mempool queuing.

Results on World Chain show results that are directly comparable with raw transactions, showing that the bundler adds negligible overhead.

• P50 time-to-mine: < 3s

• P95 time-to-mine: < 4s

Offchain Validity and Future Optimizations

PBH proofs are large and can be expensive to verify onchain. To reduce gas costs, the PBH system supports both onchain and offchain proof verification. In offchain mode, the World Builder verifies the proofs offchain before granting top-of-block priority. They are always included onchain so that any observer can check that the World Builder is ordering correctly.

This improves efficiency and leaves room for potential future optimizations to reduce calldata footprint or transaction size. Our team, along with Tools for Humanity, is also exploring sequencer/network enhancements, including:

• Using Flashblocks for L2 pre-confirmations. This could move the P50 time to mine from 3s to 500ms

• Modifying fee markets to ensure fees stay low for end users while bots are still appropriately charged to combat spam.

On top of these proof optimizations, our teams are also exploring sequencer/network enhancements, including:

• Using Flashblocks for L2 pre-confirmations. This could move the P50 time to mine from 3s to 500ms

• Modifying fee markets to ensure fees stay low for end users while bots are still appropriately charged to combat spam.

Final Thoughts

PBH redefines how blockspace is allocated by putting humans first. But it’s more than a protocol upgrade; it’s an infrastructure challenge.

Our commitment to best-in-class sequencer reliability and Rollup-Boost integration to bundler optimizations and ERC-4337 compatibility made it possible to bring PBH from idea to mainnet.

With PBH live, World Chain is now the first L2 where being human actually matters.

We’re here to help you achieve milestones onchain just like World has.

• Start building with smart wallets and rollups

• Reach out to our team anytime for integration questions, pricing at scale, and more.