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):

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

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: SKILL.md
• Legacy bridge-only integration: SKILL.md
• Embedded crosschain actions: SKILL.md
• Deposit tracking: SKILL.md
• Chains and tokens reference: SKILL.md
• Security checklist: SKILL.md
• Resources and links: SKILL.md