Across Protocol Development Skill

What this Skill is for

Use this Skill when the user asks for:

• Crosschain bridge or swap integration (any-to-any token transfers)

• Wallet or dApp integration with Across APIs

• Embedded crosschain actions (bridge + mint/stake/deposit in one tx)

• Deposit tracking and status monitoring

• Fee quoting, transfer limits, or route discovery

• Integrator fee collection setup

• On-chain intent construction (ERC-7683 / SpokePool deposits)

• Relayer operation or configuration

• Security review of crosschain integration code

Route type abbreviations

The Swap API classifies routes using a shorthand based on whether each token is directly bridgeable (B) or requires a swap to/from a bridgeable token (A = Any):

Abbreviation Meaning Example

B2B Bridgeable → Bridgeable USDC on Arbitrum → USDC on Base (no swaps, bridge only)

A2B Any → Bridgeable WBTC on Arbitrum → USDC on Base (origin swap + bridge)

B2A Bridgeable → Any USDC on Arbitrum → WBTC on Base (bridge + destination swap)

A2A Any → Any WBTC on Arbitrum → DAI on Base (origin swap + bridge + destination swap)

These abbreviations appear in crossSwapType responses and in refund behavior defaults.

Default stack decisions (opinionated)

1. Swap API first (recommended for most integrators)

• Use GET /swap/approval for all crosschain swaps. It returns executable calldata to sign and submit.

• Use POST /swap/approval when you need embedded destination actions (mint, stake, deposit).

• The Swap API handles origin swaps, bridging, and destination swaps in a single call.

• Supports appFee

• appFeeRecipient for integrator fee collection.

2. Suggested-fees API: legacy only

• Use /suggested-fees only when you control your own swap infrastructure and just need bridge fee quotes.

• This path requires you to assemble transactions yourself (construct depositV3 calls manually).

• It does not handle origin or destination swaps.

3. Trade type selection

• exactInput (default): user specifies how much to send. Best for "swap X tokens" flows.

• minOutput : user specifies minimum to receive. Best for simple swaps without post-bridge actions.

• exactOutput : user needs a precise amount on destination. Best for multi-step flows (for example, exact ERC-20 amount for a mint).

4. Slippage

• Default to slippage=auto .

• Numeric slippage (0 to 1) is split across origin and destination swaps when both swaps exist.

• If only one swap happens, the full slippage is applied to that leg.

5. Refund behavior

• B2B or A2B routes (no destination swap): refunds default to origin chain.

• B2A or A2A routes (destination swap involved): refunds default to destination chain.

• Override with refundOnOrigin=true/false when needed.

• Refund recipient priority: refundAddress

recipient depositor .

Operating procedure (how to execute tasks)

1. Classify the task

• UI or wallet integration

• Backend or script

• Embedded actions

• Tracking or monitoring

• On-chain or ERC-7683

• Relayer operation

2. Pick the right integration path

Task Use

Crosschain swap (any token to any token) Swap API GET /swap/approval

Bridge + destination action (mint, stake) Swap API POST /swap/approval with actions body

Programmatic TypeScript integration App SDK createAcrossClient()

Bridge-only with custom swap routing /suggested-fees

• manual depositV3

Direct on-chain intent (ERC-7683) AcrossOriginSettler.open() on supported chains

Track a deposit GET /deposit/status with depositTxnRef or originChainId

• depositId

3. Implement with Across-specific correctness

Always be explicit about:

• Integrator ID (2-byte hex string)

• Token addresses: must match the specific chain (use wrapped addresses for native tokens)

• Amount units: always in smallest unit (wei for ETH, 1e6 for USDC, 1e18 for WETH)

• Chain IDs: use exact numeric chain IDs, not chain names

• Approval transactions: check approvalTxns in Swap API responses

• Do not cache /swap/approval and /suggested-fees responses

4. Test on testnet, ship on mainnet

• Testnet base URL: https://testnet.across.to/api

• Use small amounts (around $10) on testnet. Testnet fills take about 1 minute (vs around 2 seconds on mainnet).

• Testnet relayers are manually funded. Do not test with large amounts.

• Switch to mainnet (https://app.across.to/api ) once integration logic is verified.

5. Deliverable expectations

When implementing changes, provide:

• Exact files changed with diffs

• Commands to install, build, and test

• A Risk Notes section for anything touching signing, fees, slippage, approvals, refunds, or crosschain messages

Progressive disclosure (read when needed)

• Swap API deep-dive: swap-api.md

• Legacy bridge-only integration: suggested-fees-api.md

• Embedded crosschain actions: embedded-actions.md

• Deposit tracking: deposit-tracking.md

• Chains and tokens reference: chains-and-tokens.md

• Security checklist: security.md

• Resources and links: resources.md