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The Circle Forwarding Service is a service for CCTP that simplifies integration by removing the need for you to run multichain infrastructure. This can improve user experience for crosschain transfers by ensuring reliability and eliminating the need to handle destination chain gas fees.

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How it works

A CCTP transfer without the Forwarding Service is a three-step process:
  1. Create a transaction to burn USDC on the source chain and wait for Circle to sign an attestation.
  2. Request an attestation from the Circle API.
  3. Create a transaction to mint USDC on the destination chain.
This process requires you to have a wallet that can sign transactions on the source and destination chains, and native tokens for paying the transaction gas fee on both chains. You use the Forwarding Service by including a forward request in the hook data of the burn transaction on the source chain. Circle validates the hook data, signs the attestation, and broadcasts the mint transaction on the destination chain for you, removing the need for you to handle the transaction on the destination chain. For a full example of how to use the Forwarding Service, see Transfer USDC with the Forwarding Service.

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Hook format

The hook data for Forwarding Service begins with the reserved magic bytes cctp-forward followed by versioning and payload fields. You can append your own custom hook data after Circle’s reserved space. Forwarding Service doesn’t support forwarding to wrapper contracts (for example, when destinationCaller is set).
BytesTypeData
0-23bytes24cctp-forward
24-27uint32Version, set to 0
28-31uint32Length of additional Circle hook data, set to 0
32-51anyDeveloper-defined hook data
If no additional integrator hook data is required, a static hex string can be used for the forwarding hook data: 
// Includes magic bytes ("cctp-forward") + hook version (0) + empty data length (0)
const forwardHookData =
  "0x636374702d666f72776172640000000000000000000000000000000000000000";

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Solana mintRecipient

When the destination blockchain is Solana, the mintRecipient parameter in depositForBurnWithHook must be the recipient’s USDC Associated Token Account (ATA) address, not the recipient’s wallet address. Unlike EVM destinations where mintRecipient is the wallet address, Solana requires the address of the SPL token account that will hold the minted USDC. You can derive the ATA address from the recipient’s wallet address and the USDC mint address using the getAssociatedTokenAddressSync function from the @solana/spl-token library: 
import { getAssociatedTokenAddressSync } from "@solana/spl-token";
import { PublicKey } from "@solana/web3.js";

const recipientWallet = new PublicKey("RecipientWalletAddress");
const USDC_MINT = new PublicKey("4zMMC9srt5Ri5X14GAgXhaHii3GnPAEERYPJgZJDncDU"); // Solana devnet

const recipientAta = getAssociatedTokenAddressSync(USDC_MINT, recipientWallet);
const mintRecipient = `0x${Buffer.from(recipientAta.toBytes()).toString("hex")}`;

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Solana hook data for ATA creation

If the recipient does not have an existing USDC ATA, you can request the Forwarding Service to create it by encoding additional fields in the hook data. The extended hook data format is:
BytesTypeData
0-23bytes24cctp-forward
24-27uint32Version, set to 0
28-31uint32Length of additional Circle hook data, set to 33
32uint81 (request ATA creation)
33-64bytes32Recipient wallet address (ATA owner)
65+anyDeveloper-defined hook data
When using this format, the mintRecipient must be the ATA derived from the wallet address in bytes 33-64 and the USDC mint. The Forwarding Service validates that these values are consistent before creating the account. The following example shows how to construct the extended hook data for Solana with ATA creation: 
import { PublicKey } from "@solana/web3.js";

// Magic bytes "cctp-forward" padded to 24 bytes
const magicBytes = Buffer.alloc(24);
magicBytes.write("cctp-forward", "utf-8");

// Version (uint32, big-endian) = 0
const version = Buffer.alloc(4);

// Length of additional Circle hook data (uint32, big-endian) = 33
const length = Buffer.alloc(4);
length.writeUInt32BE(33);

// ATA creation flag = 1
const ataFlag = Buffer.from([1]);

// Recipient wallet address (32 bytes)
const recipientWallet = new PublicKey("RecipientWalletAddress");
const walletBytes = Buffer.from(recipientWallet.toBytes());

const forwardHookData =
  "0x" +
  Buffer.concat([magicBytes, version, length, ataFlag, walletBytes]).toString(
    "hex",
  );
If the recipient already has a USDC ATA and no ATA creation is needed, use the same static hook data as EVM: 
// Includes magic bytes ("cctp-forward") + hook version (0) + empty data length (0)
const forwardHookData =
  "0x636374702d666f72776172640000000000000000000000000000000000000000";

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Fees and execution

The Forwarding Service charges a fee for each transfer, in addition to the CCTP protocol fee. The Forwarding Service fee charged is to cover gas costs on the destination chain and a small service fee. The Forwarding Service prioritizes fast execution and quotes gas dynamically. If gas used is less than gas needed for execution, the remainder is spent as an additional priority fee where they are supported. On all chains, a higher fee provides a safety buffer for successful transaction delivery on the destination chain. Circle does not refund for excess gas and does not keep the excess gas, except in cases where excess priority fees are rejected. The depositForBurnWithHook transaction includes a maxFee parameter. When using the Forwarding Service, this parameter should be set to a value that is large enough to cover the CCTP protocol fee and the Forwarding Service fee. Because the gas budget for the destination chain comes from a USDC fee on the source chain, choosing a lower maxFee results in a lower priority fee on the destination chain. A higher maxFee results in a higher priority fee on the destination chain and can result in faster confirmation. The Forwarding Service charges a service fee for each transfer:
Destination chainService fee (USDC)
All chains$0.20
If the maxFee parameter is insufficient to cover the both Fast Transfer protocol fee and the Forwarding Service fee, CCTP will prioritize forwarding execution over Fast Transfer. This means that the transfer will execute as a Standard Transfer with the Forwarding Service.

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Solana fees

When forwarding to Solana, the Forwarding Service fee includes both a gas component and a rent component. Rent covers the cost of creating onchain accounts required by each transfer. The fee estimate API returns forwardFee values that already include rent, so no additional calculation is needed on your part. If the recipient does not already have a USDC Associated Token Account (ATA) on Solana, the transfer will fail. To have the Forwarding Service create the ATA, you must:
  1. Pass includeRecipientSetup=true when calling the fee estimate API so the returned forwardFee covers the ATA creation cost.
  2. Encode the ATA creation fields in the hook data of the burn transaction.
GET /v2/burn/USDC/fees/{sourceDomain}/{destDomain}?forward=true&includeRecipientSetup=true
For full details on this endpoint, see the GET /v2/burn/USDC/fees API reference.
includeRecipientSetup only applies when the destination blockchain is Solana. It has no effect for EVM destination blockchains.

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Supported blockchains

For a full list of supported blockchains, see CCTP Supported Blockchains.