Fee Management

TAC’s cross-chain messaging system has different fee structures depending on message direction and type. Understanding these fees is crucial for implementing cost-effective proxy contracts.

OutMessageV1 Structure

The OutMessageV1 structure contains all the fields needed for cross-chain messages:

struct OutMessageV1 {
    uint64 shardsKey;           // Developer ID for linking messages
    string tvmTarget;           // The recipient address on TON network
    string tvmPayload;          // Custom payload (currently not supported - must be empty)
    uint256 tvmProtocolFee;     // Protocol fee you pay (0 for RoundTrip messages)
    uint256 tvmExecutorFee;     // Executor fee you pay (0 for RoundTrip messages)
    string[] tvmValidExecutors; // List of valid executors (empty array for RoundTrip)
    TokenAmount[] toBridge;     // ERC20 tokens to bridge to TON
    NFTAmount[] toBridgeNFT;    // NFTs to bridge to TON
}

Field Usage Guide

shardsKey: Developer ID. It is recommended to set it from the tacHeader. tvmTarget: The recipient address on the TON network in base64 format starting with “EQ”. tvmPayload: A custom payload to be executed on the TON side. Currently not supported — must be empty. tvmProtocolFee: The protocol fee you must pay. For roundTrip messages, this fee is already covered on the TON side, so set this field to 0. tvmExecutorFee: The fee you offer to the executor on the TON side (in TAC tokens). For roundTrip messages, the fee is already locked on TON, so set this field to 0. tvmValidExecutors: A list of executors you trust to execute the message on the TON side. For roundTrip messages, this must be an empty array; the trusted executors are already defined in the initial TON message. For direct messages, you can get trusted executors using settings.getTrustedTVMExecutors() or use an empty array for default executors. toBridge: List of ERC20 tokens you want to bridge to the TON network and transfer to tvmTarget. Learn how to compose them here. toBridgeNFT: List of NFTs you want to bridge to the TON network and transfer to tvmTarget. Learn how to compose them here.

Round-Trip Messages (TON → TAC → TON)

For messages responding back to TON users, fees are paid upfront on TON:

function respondToTON(bytes calldata tacHeader, bytes calldata arguments)
    external
    _onlyCrossChainLayer
{
    TacHeaderV1 memory header = _decodeTacHeader(tacHeader);

    // Process your logic here

    // Send result back to TON - NO FEES REQUIRED
    OutMessageV1 memory outMsg = OutMessageV1({
        shardsKey: header.shardsKey,        // Links to original operation
        tvmTarget: header.tvmCaller,        // Send back to original user
        tvmPayload: "",                     // Must be empty
        tvmProtocolFee: 0,                  // Set to 0 for RoundTrip - already paid on TON
        tvmExecutorFee: 0,                  // Set to 0 for RoundTrip - already paid on TON
        tvmValidExecutors: new string[](0), // Empty for RoundTrip - already defined on TON or you can use getTrustedTVMExecutors(),
        toBridge: tokensToSend,
        toBridgeNFT: new NFTAmount[](0)
    });

    _sendMessageV1(outMsg, 0); // 0 TACs to send back
}

You can retrieve the list of default trusted executors by calling settings.getTrustedTVMExecutors() on the Settings contract.

Key Points:

User pays all fees upfront on TON
RoundTrip response messages must set tvmProtocolFee: 0 and tvmExecutorFee: 0
Use same shardsKey from incoming header
Set tvmValidExecutors to empty array - executors already defined in original TON message

Direct TAC → TON Messages

For messages initiated directly from TAC (not in response), your contract pays the fees:

interface ICrossChainLayer {
    function getProtocolFee() external view returns (uint256);
}

function sendDirectMessage() external payable {
    // Get current protocol fee dynamically (example usage)
    uint256 protocolFee = ICrossChainLayer(_getCrossChainLayerAddress()).getProtocolFee();

    // Note: Currently there is no official library to calculate the exact executor fee
    // It is recommended to:
    // - Overestimate slightly, or
    // - Use TAC SDK for better fee estimation
    uint256 executorFee = msg.value - protocolFee; // Rough example - use tacSDK for production

    OutMessageV1 memory outMsg = OutMessageV1({
        shardsKey: generateNewShardsKey(),     // New operation
        tvmTarget: "EQUserAddress...",         // TON recipient
        tvmPayload: "",                        // Must be empty - not supported
        tvmProtocolFee: protocolFee,           // Contract pays current protocol fee
        tvmExecutorFee: executorFee,           // Contract pays executor fee
        tvmValidExecutors: new string[](0),    // Use default executors
        toBridge: tokensToSend,
        toBridgeNFT: new NFTAmount[](0)
    });

    _sendMessageV1(outMsg, 0); // 0 TACs to send back
}

Fee Estimation Challenge: Currently, there is no official library to calculate the exact executor fee, so it is recommended to:

Overestimate slightly, or
Use tacSDK for better fee estimation

Executor fees are market-determined based on network conditions and can vary significantly.

What’s Next?

Now that you understand the theory, let’s dive into the implementation:

Develop Custom Proxy

Build the core logic that processes cross-chain calls and arguments

​OutMessageV1 Structure

​Field Usage Guide

​Round-Trip Messages (TON → TAC → TON)

​Direct TAC → TON Messages

​What’s Next?