Understanding how messages flow between TON and TAC EVM is crucial for building effective proxy contracts. This guide explains the data structures and message patterns.

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RoundTrip Messages

If a TON → TAC call is reverted on the EVM side after tokens have been bridged, a rollback transaction is created to send the bridged assets back to the originating TON wallet.To guarantee this behavior, the call must be treated (and paid) as a RoundTrip. The SDK sets messages to RoundTrip by default, so on revert (e.g., slippage), funds will be returned automatically.
RoundTrip messages follow this flow:
  1. TON user sends transaction with assets and function call
  2. TAC proxy receives the call and processes it
  3. TAC proxy sends result assets back to the same TON user
With RoundTrip messages, the user pays all fees upfront on TON, so your proxy sets fees to 0 when responding.
If you expect the TAC-side transaction may revert and assets need to be bridged back to TON, you must pay fees as for a RoundTrip message.
By default, the SDK treats all messages as RoundTrip, so even if a revert occurs (e.g., due to slippage), the funds will be returned to the original TON user because the RoundTrip fee was already paid.

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Message Processing

When a TON user interacts with your EVM contract:
  1. User submits a transaction on TON with target contract, function name, and arguments
  2. Assets (if any) are bridged to TAC and transferred to your proxy contract
  3. CrossChainLayer calls your proxy function with TAC header and arguments
  4. Your contract can optionally send assets back to TON using _sendMessageV1()

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TAC Header Structure

Every cross-chain call includes a TAC header with verified information about the original TON transaction: 
struct TacHeaderV1 {
    uint64 shardsKey;      // Developer/operation identifier
    uint256 timestamp;     // Block timestamp from TON
    bytes32 operationId;   // Unique operation ID
    string tvmCaller;      // TON user's address (base64, starts with EQ)
    bytes extraData;       // Additional data (currently unused)
}

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Header Field Details

shardsKey: Links related cross-chain operations together. Use this in your response messages to maintain the connection. timestamp: The block timestamp from the original TON transaction. Useful for time-based logic or debugging. operationId: Unique identifier for this specific cross-chain operation. Use for logging and tracking. tvmCaller: The TON user’s wallet address. !!! Important !!! It is always base64 mainnet bounceable format and starts with “EQ”. This is your authenticated user identity - treat it like msg.sender in regular Ethereum contracts. extraData: For now it’s always a zero-bytes array and not used.

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Decoding the Header

function processMessage(bytes calldata tacHeader, bytes calldata arguments)
    external
    _onlyCrossChainLayer
{
    // Decode header using inherited function
    TacHeaderV1 memory header = _decodeTacHeader(tacHeader);

    // Access user information
    string memory tonUser = header.tvmCaller;
    uint256 operationTime = header.timestamp;
    bytes32 opId = header.operationId;

    // Use header data in your logic
    require(block.timestamp - operationTime < 3600, "Operation too old");

    emit MessageProcessed(tonUser, opId, block.timestamp);
}

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Asset Handling in Messages

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Token Assets

Tokens are automatically transferred to your contract before your function is called: 
function handleTokens(bytes calldata tacHeader, bytes calldata arguments)
    external
    _onlyCrossChainLayer
{
    // Decode the tokens you're expecting
    TokenAmount[] memory expectedTokens = abi.decode(arguments, (TokenAmount[]));

    // Tokens are already in your contract balance
    for (uint i = 0; i < expectedTokens.length; i++) {
        uint256 balance = IERC20(expectedTokens[i].evmAddress).balanceOf(address(this));
        require(balance >= expectedTokens[i].amount, "Expected tokens not received");

        // Process each token
        processToken(expectedTokens[i].evmAddress, expectedTokens[i].amount);
    }
}

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What’s Next?

Now that you understand the message flow, learn how to implement the core proxy function logic:

Implementing Proxy Functions

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

Fee Management

Understand how to handle protocol fees and executor payments