Across Protocol Documentation

Across Protocol is a crosschain interoperability protocol powering fast, low-cost token transfers across 23+ chains. Three settlement mechanisms: Intents (relayer network), CCTP (USDC), and OFT (USDT). API base URL: https://app.across.to/api

Introduction

• Actors in the System: The key participants that make crosschain transfers work — users, relayers, LPs, dataworkers, and the UMA oracle.
• API Keys & Integrator ID: How to get an API key and integrator ID for production use of the Across APIs.
• Audits: Security audits of the Across Protocol and UMA smart contracts.
• Bug Bounty: Report vulnerabilities and earn rewards through the Across Protocol bug bounty program.
• Crosschain Live: Weekly livestream and podcast exploring crosschain interoperability with builders, founders, and community members from across Web3.
• Deposit Addresses: Generate a counterfactual deposit address that users can send funds to in order to initiate a crosschain transfer.
• Direct Route Linking: Link users directly to a pre-filled bridge route on the Across UI.
• Technical FAQ: Frequently asked questions about integrating Across Protocol.
• Key Features: What makes Across the fastest, cheapest, and most secure crosschain interoperability protocol.
• Fees in the System: How LP fees, relayer fees, gas fees, and capital fees are calculated in Across.
• Working with Hypercore: Bridge assets to Hyperliquid's sub-second settlement layer via Across.
• Welcome to Across: Crosschain interoperability with ~2 second fills across 23+ chains.
• Refunds: How refunds work when a crosschain transfer expires without being filled.
• Across for Stablecoins: Native USDC, USDT0, and USDH transfers using CCTP V2, OFT, and sponsored settlement mechanisms.
• Introduction to Swap API: The unified entry point for all crosschain operations on Across.
• Tracking Deposits: Monitor crosschain transfer status using the deposit tracking API.
• Why Across: Why Across is the fastest, cheapest, and most secure crosschain interoperability protocol.
• Deposit ETH into Aave: Swap to ETH crosschain and deposit directly into Aave's lending pool.
• Add Liquidity to Across HubPool: Swap to ETH and add liquidity to the Across HubPool in one crosschain transaction.
• Embedded Crosschain Actions: Execute custom on-chain operations on the destination chain immediately after a crosschain swap.
• Simple Native ETH Transfer: Send native ETH to an address after a crosschain swap.
• Handling Nested Parameters: How to encode tuples and struct parameters in embedded action function signatures.
• Transfer ERC-20 Tokens After Swap: Transfer swapped ERC-20 tokens to a different address on the destination chain.
• Relayer Nomination: How exclusive relayer nomination works and how to participate.
• Running a Relayer: Set up and operate your own Across V3 relay infrastructure.
• Disputing Root Bundles: How to challenge invalid bundle proposals — manually or with automated tooling.
• Security Model and Verification: How Across secures crosschain transfers through optimistic verification, bonds, and UMA's oracle.
• Validating Root Bundles: How root bundles are validated and what the verification process checks.

Guides

• Guides: Concepts, developer guides, and migration references for Across Protocol.
• Crosschain Deposit into Aave: Bridge USDC from Arbitrum, swap to ETH on Ethereum, and deposit into Aave's lending pool — in a single transaction.
• Developer Guides: Hands-on, step-by-step integration guides for building with Across Protocol.
• Integrate Across Swap API: Build a complete crosschain USDC bridge from scratch — discover chains, look up tokens, get a quote, and execute with viem.
• What is Across V4?: ZK-powered settlement using Succinct SP1 for permissionless chain expansion.
• What are Crosschain Intents?: How intent-based bridging differs from traditional bridges and why it matters.
• ERC-7683 in Production: Submit crosschain intents using the ERC-7683 standard via AcrossOriginSettler.
• Concepts: Core concepts behind Across Protocol's crosschain architecture.
• Intent Lifecycle in Across: The three phases every crosschain intent goes through — initiation, fill, and settlement.
• Intent Architecture in Across: The three-layer architecture — RFQ, relayer network, and settlement — that powers crosschain intents.
• BNB Smart Chain Migration Guide: Enable BNB Smart Chain (Chain ID 56) bridging for USDC, USDT, and ETH.
• Migration Guides: Step-by-step migration references for upgrading your Across integration.
• Solana Migration Guide: Prepare for USDC bridging on Solana — API changes, deposit flow, and relayer requirements.
• Migration from V2 to V3: Upgrade from Across V2 to V3 — new events, functions, fee structure, and contract changes.
• Direct Route Linking: Link users directly to a pre-filled bridge route on Across using URL query parameters.
• User Guides: Guides for using the Across Protocol frontend and tools.
• CCTP Migration for API Users: API endpoint changes, new token addresses, and deprecated parameters for CCTP migration.
• Migration to CCTP: Transition from Bridged USDC to Native USDC via Circle's Crosschain Transfer Protocol.
• CCTP Migration for Relayers: Relayer changes for handling Native USDC routes after CCTP migration.
• Breaking Changes for API Users: Required parameter changes for deposit transactions after the non-EVM migration.
• Non-EVM and Prefills Migration: Support non-EVM chains and deterministic relay hashes for prefills.
• Breaking Changes for Indexers: Updated SpokePool events for non-EVM address support — indexers must update or face system failures.
• Breaking Changes for Relayers: Updated events and fill functions for relayers supporting non-EVM chains.
• Testnet Environment: Sepolia testnet deployment for testing the non-EVM migration changes.

AI Agents

• Agent Workflows: End-to-end examples showing real AI agent sessions performing crosschain tasks with the Across Protocol.
• AGENTS.md: A ready-to-copy AGENTS.md template that gives coding agents full context on your Across integration.
• AI Agents: Give your AI agent crosschain superpowers with a single command.
• Machine-Readable Docs: Feed Across documentation directly to AI agents and RAG pipelines via llms.txt endpoints.
• MCP Server: Give any MCP-compatible AI client full access to Across Protocol documentation, chain data, and live bridge fees.
• Prompt Library: Curated prompts for AI agents working with the Across Protocol — organized by task category.

Tools

• AI Transaction Builder: Describe your crosschain transfer in natural language and get a ready-to-use Across API call.
• Chain Checker: Check if Across supports your chain.
• Tools: Tools and utilities for working with Across Protocol.
• Get API Key & Integrator ID: Register your project to get an Across API key and integrator ID — everything you need to start building with the Across API.
• Status Tracker: Track the status of your Across deposit by transaction hash or deposit ID.
• Token Checker: Check if Across supports your token across all chains.
• Transaction Builder: Build crosschain swap + action requests for the Across API.

API Reference

• API Reference: Complete API reference for the Across Protocol.
• Retrieve available routes for transfers: Returns available routes based on specified parameters.
• Get all details for a single deposit: Returns a single deposit matching the provided query parameters. Searches across all deposit types: Across intents (V3FundsDeposited), CCTP (DepositForBurn), OFT (OFTSent).

At least one of the following parameter combinations is required:

1. depositTxnRef
2. depositId AND originChainId
3. relayDataHash

• Get all deposits for a given depositor: Returns all deposits for a given depositor.

This endpoint loads data queried by an indexing service that polls relevant events on a 10-second cadence. Users should therefore expect an average latency of 1 to 15 seconds after submitting a deposit to see the status changed in this endpoint. This delay comes from the time it takes for the internal indexing to include the deposit transaction.

• Retrieve current transfer limits of the system: Returns transfer limits for inputToken, outputToken, originChainId, and destinationChainId.

• Retrieve suggested fee quote for a deposit.: Returns suggested fees based on inputToken, outputToken, originChainId, destinationChainId, and amount. Also includes transfer limits of the system and data used to compute the fees.

• Track the lifecycle of a deposit: Returns the fill status of a deposit along with a corresponding fill transaction hash if filled.

This endpoint loads data queried by an indexing service that polls relevant events on a 10-second cadence. Users should therefore expect an average latency of 1 to 15 seconds after submitting a deposit to see the status changed in this endpoint. This delay comes from the time it takes for the internal indexing to include the deposit transaction.

• Get swap approval data: Returns data required to execute a crosschain swap. If the input token requires approval, approvalTxns will be included in the response.

• Build embedded crosschain swap actions with the Swap API: Use the /swap/approval API to build embedded crosschain actions (e.g., mint, deposit) that execute on the destination chain immediately after a swap. Define an action with its target, functionSignature, and value. For contract calls, provide an args[] array where each parameter can be static or dynamically populated with a token balance by setting populateDynamically: true and specifying the balanceSourceToken. In this example we will transfer the entire balance of a newly swapped token, call the ERC20 transfer function with a static recipient address and a dynamic amount argument flagged for population.

• Get supported chains for swap operations: Returns a list of all supported blockchains that support crosschain swaps on Across Protocol.

• Generate a counterfactual deposit address: Returns a counterfactual deposit address that the user can send funds to in order to initiate a crosschain transfer. This address is deterministically computed based on the swap parameters. The user sends the input token to this address on the origin chain, and Across handles the crosschain transfer to the recipient on the destination chain.

The response includes the deposit address, token details, and an optional swap transaction that must be executed before sending funds (e.g. for token approvals or wrapping). Supported origin chains (USDC): Ethereum, Optimism, Unichain, Polygon, Base, Arbitrum, World Chain, Linea, Ink, Monad. Supported destination chains: HyperEVM (USDH, USDC), HyperCore (USDC-SPOT, USDH-SPOT).

• Get supported sources for swap operations: Returns a list of all supported sources that support crosschain swaps on Across Protocol.
• Get supported tokens for swap operations: Returns a list of all supported tokens that support crosschain swaps on Across Protocol.

Optional

• Chains & Contracts: Deployed contract addresses for all supported Across chains.