Across Developer DocumentationWhat is Across V4?
ZK-powered settlement using Succinct SP1 for permissionless chain expansion.
Across V4 integrates zero-knowledge proofs with Succinct's SP1 proving network to decouple settlement from chain-specific verification logic. This enables permissionless expansion to new chains without custom bridge adapters or audits for each new deployment.
V4 only changes the settlement layer (Phase 3 of the intent lifecycle). The user experience and relayer network remain the same — users still get ~2 second fills.
Why V4?
In V3, adding a new chain requires deploying chain-specific adapters for the canonical bridge between Ethereum and that chain. Each adapter needs custom code and an audit. V4 replaces this with a universal verification mechanism: ZK proofs of Ethereum state.
| V3 Settlement | V4 Settlement | |
|---|---|---|
| New chain support | Custom adapter + audit | Deploy UniversalSpokePool (no custom code) |
| Verification | Optimistic Oracle + canonical bridges | ZK proofs of Ethereum state |
| Trust model | Tied to canonical bridge of each chain | Universal — proves Ethereum state directly |
V4 Architecture
On Ethereum L1
- UMA's Optimistic Oracle — Verifies repayment bundles (unchanged from V3)
- HubPoolStore — Stores message hashes in storage slots and emits
StoredCallDataevents, anchoring settlement data to L1
Off-Chain Infrastructure
- Finalizer — Monitors
StoredCallDataevents from HubPoolStore. After Ethereum reaches finality (12–15 minutes), initiates ZK proof generation - ZK API — Stateful service managing proof lifecycle through four statuses:
WaitingForFinality,Generating,Success,Errored
On Destination Chain
- SP1Helios — A ZK light client contract that verifies proofs and stores proven L1 state. Built on Succinct's SP1 proving system
- UniversalSpokePool — Executes verified messages. Calls
relayRootBundleto process settlement data andexecuteRelayerRefundLeaf()to process relayer repayments
Settlement Flow
Bundle Finalized on L1
After the optimistic challenge period passes, the confirmed bundle is written to the HubPoolStore contract. A StoredCallData event is emitted containing the message hash.
Await Ethereum Finality
The Finalizer waits for Ethereum to finalize the block containing the StoredCallData event (12–15 minutes). This ensures the proof is based on irreversible state.
ZK Proof Generation
The Finalizer submits a proof request to the ZK API, which:
- Gathers proof inputs: finalized beacon headers, sync committee signatures (512 validators), and a Merkle proof of the HubPoolStore storage slot
- Runs a pre-flight check on a local RISC-V virtual machine
- Sends the validated job to Succinct's SP1 proving network
Proof Submission
Once the proof is generated, the Finalizer submits two transactions on the destination chain:
SP1Helios.update()— Provides the ZK proof, updating the light client's view of Ethereum stateUniversalSpokePool.executeMessage()— Executes the verified settlement message
Relayer Refunds Processed
With the message verified, the UniversalSpokePool processes relayRootBundle and executes executeRelayerRefundLeaf() to repay relayers on the destination chain.
Benefits
For users — More chains supported, more routes available, faster token availability on new networks.
For builders — A single integration that automatically works on new chains as they're added. No per-chain audits needed.
For relayers — Repayments on origin chains with flexible rebalancing. New chains mean new fill opportunities.
For chains — Near-frictionless integration with Across. New chains can be added permissionlessly once a UniversalSpokePool is deployed.
Production Status
Across V4 is live in production on BNB Smart Chain, the first chain using ZK settlement. More chains will follow as the system is proven.