Developing TAC Proxy

This process might be a little complicated, so please reach out to us on Discord if you have any questions.

A TAC Proxy is a Solidity contract that receives cross-chain messages and tokens bridged from the TON blockchain. When a user on TON initiates a message (and potentially sends tokens), the TAC infrastructure delivers the tokens and data to your EVM-based contract’s function. The proxy contract then processes that data—often by calling another Dapp contract or by performing some bridging logic—and optionally sends tokens back to TON using the same TAC infrastructure.

The guide below shows how to:

Install the necessary dependencies.
Write a simple proxy contract (both non-upgradeable and upgradeable).
Implement your custom logic in a proxy function that adheres to TAC’s required function signature.
Encode your arguments properly on the frontend.
Test your proxy contract locally using Hardhat and the tac-l2-ccl testing utilities.

Installation

Install the tac-l2-ccl package in your Solidity contract repository. This package includes core functionalities for cross-chain messaging and local test SDKs:
```
npm install --save tac-l2-ccl@latest
```

In most cases, you will be using Hardhat for development. Make sure you have a typical Hardhat setup and the recommended testing libraries. For instance, your package.json may include:

{
  "devDependencies": {
    "@nomicfoundation/hardhat-toolbox": "^5.0.0",
    "hardhat": "^2.22.5",
    "ethers": "^6.13.2",
    "chai": "^4.3.7",
    "ts-node": "^10.9.2",
    "typescript": "^5.6.3",
    "tac-l2-ccl": "^latest",
    "...": "..."
  }
}

If you cannot deploy your Dapp contracts directly for local testing, consider forking another network where the necessary contracts are already deployed. This can simplify local development and testing.

Creating the Proxy Contract

In your contracts folder, create a new .sol file (e.g. MyProxy.sol). Below are two variations:

Non-upgradeable Contract

For a simple, non-upgradeable contract, you can extend TacProxyV1:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.28;

import { IERC20 } from "@openzeppelin/contracts/token/ERC20/IERC20.sol";

import { TacProxyV1 } from "tac-l2-ccl/contracts/proxies/TacProxyV1.sol";
import { IDappContract } from "./IDappContract.sol"; // your Dapp interface (if needed)
import { TokenAmount, OutMessageV1, TacHeaderV1 } from "tac-l2-ccl/contracts/L2/Structs.sol";

contract MyProxy is TacProxyV1 {
    IDappContract public dappContract;

    constructor(address _dappContract, address _crossChainLayer)
        TacProxyV1(_crossChainLayer)
    {
        dappContract = IDappContract(_dappContract);
    }

    // Add your proxy functions here
    function myProxyFunction(bytes calldata tacHeader, bytes calldata arguments)
        external
        _onlyCrossChainLayer
    {
        // Implementation here
    }
}

Key Points

We pass _crossChainLayer (the CrossChainLayer contract address) to TacProxyV1’s constructor.
_onlyCrossChainLayer is a security modifier inherited from TacProxyV1. It ensures that only the recognized cross-chain layer can call this function.
IDappContract is just an example interface for some external logic contract you may want to call.

Upgradeable Contract

If you need to upgrade your contract over time, use OpenZeppelin’s upgradeable libraries and the TacProxyV1Upgradeable contract:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.28;

import { Initializable } from "@openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol";
import { UUPSUpgradeable } from "@openzeppelin/contracts-upgradeable/proxy/utils/UUPSUpgradeable.sol";
import { OwnableUpgradeable } from "@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol";

import { TacProxyV1Upgradeable } from "tac-l2-ccl/contracts/proxies/TacProxyV1Upgradeable.sol";
import { TokenAmount, OutMessageV1, TacHeaderV1 } from "tac-l2-ccl/contracts/L2/Structs.sol";

contract MyProxyUpgradeable is
    Initializable,
    OwnableUpgradeable,
    UUPSUpgradeable,
    TacProxyV1Upgradeable
{
    function initialize(address owner, address crossChainLayer) public initializer {
        __UUPSUpgradeable_init();
        __Ownable_init(owner);
        __TacProxyV1Upgradeable_init(crossChainLayer);
    }

    function _authorizeUpgrade(address) internal override onlyOwner {}

    // Add your proxy functions here
    function myProxyFunction(bytes calldata tacHeader, bytes calldata arguments)
        external
        _onlyCrossChainLayer
    {
        // Implementation here
    }
}

Key Points

Inherits from Initializable, OwnableUpgradeable, UUPSUpgradeable, and TacProxyV1Upgradeable.
Has an initialize function in place of a constructor.
_authorizeUpgrade ensures only the owner can perform contract upgrades.

Defining and Implementing Proxy Functions

Every proxy function that TAC calls must have the signature:

function <function_name>(bytes calldata, bytes calldata) external;

You can name the function as you wish (e.g. myProxyFunction, invokeWithCallback, etc.), but it must accept two bytes arguments:

The first is always the encoded TAC header.
The second is always the encoded arguments that you define.

Example Implementation

Below is an extended example of how you might implement myProxyFunction in a non-upgradeable contract. The logic is the same for an upgradeable contract.

function myProxyFunction(bytes calldata tacHeader, bytes calldata arguments)
    external
    _onlyCrossChainLayer
{
    // 1. Decode the custom arguments
    MyProxyFunctionArguments memory args = abi.decode(arguments, (MyProxyFunctionArguments));

    // 2. Approve tokens to your Dapp contract for some action
    IERC20(args.tokenFrom).approve(address(dappContract), args.amount);
    // 3. Call the Dapp contract
    uint256 tokenToAmount = dappContract.doSomething(
        args.tokenFrom,
        args.tokenTo,
        args.amount
    );

    // 4. Prepare tokens to send back to TON, if desired
    TokenAmount[] memory tokensToBridge = new TokenAmount[](1);
    tokensToBridge[0] = TokenAmount(args.tokenTo, tokenToAmount);

    // 5. Approve the CrossChainLayer to pull them
    IERC20(tokensToBridge[0].l2Address).approve(
        _getCrossChainLayerAddress(),
        tokensToBridge[0].amount
    );

    // 6. Decode the TAC header
    TacHeaderV1 memory header = _decodeTacHeader(tacHeader);

    // 7. Form an OutMessage
    //    This is how you instruct TAC to deliver tokens or data back to TON
    OutMessageV1 memory outMsg = OutMessageV1({
        shardsKey: header.shardsKey,
        tvmTarget: header.tvmCaller, // the original user (TON wallet)
        tvmPayload: "",
        toBridge: tokensToBridge
    });

    // 8. Send message back through CrossChainLayer
    _sendMessageV1(outMsg);
}

Important Notes

function myProxyFunction(bytes calldata, bytes calldata) external _onlyCrossChainLayer ensures only the TAC infrastructure can call this function.
_decodeTacHeader(...) is inherited from TacProxyV1 (or TacProxyV1Upgradeable); it transforms the raw bytes into TacHeaderV1 data.
You typically decode the second argument (arguments) using abi.decode(...) with a struct you define.

How the Cross-Chain Call Works

When a user on TON sends a message via your Dapp (e.g., using the @tonappchain/sdk on the frontend), the CrossChainLayer contract on EVM receives bridged tokens and data from the TON side. Then:

Tokens (if any) are automatically transferred from the CrossChainLayer contract to your proxy contract before the function call.
The CrossChainLayer calls myProxyFunction(tacHeader, arguments) on your proxy contract.
- tacHeader is the encoded TacHeaderV1 struct containing data like shardsKey, operationId, the user’s tvmCaller address on TON, etc.
- arguments is a bytes array containing data you defined in the Dapp’s frontend (encoded via @tonappchain/sdk or manually using ethers.AbiCoder).
Your proxy function processes the tokens, calls external contracts if necessary, and optionally prepares an OutMessageV1 with tokens to be sent back to TON.
The _sendMessageV1(...) function sends everything back to the CrossChainLayer so that tokens (and an optional message) can be bridged to TON.

Encoding Arguments on the Frontend

You typically define a struct that represents the arguments your proxy function expects. For example:

struct MyProxyFunctionArguments {
    address tokenFrom;
    address tokenTo;
    uint256 amount;
}

You then encode these fields in your frontend code using ethers.js (or another library).

Basic Example

import { ethers } from "ethers";
const abiCoder = ethers.AbiCoder.defaultAbiCoder();

const myProxyFunctionArguments = abiCoder.encode(
  ["address", "address", "uint256"],
  [tokenFromAddress, tokenToAddress, tokenFromAmount]
);

Complex Example with Nested Structures

struct AnyExtraInfo {
    address feeCollector;
    uint256 feeRate;
}

struct MyProxyFunctionArguments {
    AnyExtraInfo extraInfo;
    address tokenFrom;
    address tokenTo;
    uint256 amount;
}

Encoding:

const extraInfo = [feeCollectorAddress, feeRate];
const myProxyFunctionArguments = abiCoder.encode(
  ["tuple(address,uint256)", "address", "address", "uint256"],
  [extraInfo, tokenFromAddress, tokenToAddress, tokenAmount]
);

Complex Example with Dynamic Arrays

struct MyProxyFunctionArguments {
    address[] path;
    uint256 amount;
}

Encoding:

const path = [tokenFromAddress, tokenToAddress];
const myProxyFunctionArguments = abiCoder.encode(
  ["tuple(address[],uint256)"],
  [[path, tokenFromAmount]]
);

Specifying the Function Name in `@tonappchain/sdk`

When using the @tonappchain/sdk to create messages for bridging, you must provide:

target: the address of your Proxy contract.
method_name: the complete function signature, e.g. "myProxyFunction(bytes,bytes)".
arguments: the ABI-encoded arguments (second parameter in your proxy function).

Example:

const myProxyFunctionName = "myProxyFunction(bytes,bytes)";

const userMessage = {
  target: MyProxyContractAddress,
  method_name: myProxyFunctionName,
  arguments: myProxyFunctionArguments // from the previous encoding step
  gasLimit: the parameter that will be passed to the TAC side. The executor must allocate at least gasLimit gas for executing the transaction on the TAC side. If this parameter is not specified, it will be calculated using the `simulateEVMMessage` method(preferred).
};

Testing the Proxy Contract

Example Minimal Proxy for Testing

In many cases, you want a stripped-down contract to test basic cross-chain behavior. Below is a minimal TestProxy that:

Inherits TacProxyV1.
Has a single function invokeWithCallback(...).
Emits an event for logging.
Demonstrates bridging tokens back to TON.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.28;

import { IERC20 } from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import { OutMessageV1, TacHeaderV1, TokenAmount } from "tac-l2-ccl/contracts/L2/Structs.sol";
import { TacProxyV1 } from "tac-l2-ccl/contracts/proxies/TacProxyV1.sol";

contract TestProxy is TacProxyV1 {
    event InvokeWithCallback(
        uint64 shardsKey,
        uint256 timestamp,
        bytes32 operationId,
        string tvmCaller,
        bytes extraData,
        TokenAmount[] receivedTokens
    );

    constructor(address _crossChainLayer) TacProxyV1(_crossChainLayer) {}

    function invokeWithCallback(bytes calldata tacHeader, bytes calldata arguments)
        external
        _onlyCrossChainLayer
    {
        // 1. Decode the header
        TacHeaderV1 memory header = _decodeTacHeader(tacHeader);

        // 2. Decode the array of TokenAmount structs
        TokenAmount[] memory receivedTokens = abi.decode(arguments, (TokenAmount[]));

        // Optional: Here you could call an external Dapp contract with these tokens

        // 3. Log an event for testing
        emit InvokeWithCallback(
            header.shardsKey,
            header.timestamp,
            header.operationId,
            header.tvmCaller,
            header.extraData,
            receivedTokens
        );

        // 4. Approve and forward the tokens back via the cross-chain layer
        for (uint i = 0; i < receivedTokens.length; i++) {
            IERC20(receivedTokens[i].l2Address).approve(
                _getCrossChainLayerAddress(),
                receivedTokens[i].amount
            );
        }

        // 5. Create and send an OutMessage
        _sendMessageV1(
            OutMessageV1({
                shardsKey: header.shardsKey,
                tvmTarget: header.tvmCaller,
                tvmPayload: "",
                toBridge: receivedTokens
            })
        );
    }
}

Also define the TestToken contract.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.28;

import { ERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";

contract TestToken is ERC20 {
    constructor(string memory _name, string memory _symbol) ERC20(_name, _symbol) {}

    function mint(address _to, uint256 _amount) external {
        _mint(_to, _amount);
    }
}

Test Setup Example (Hardhat + tac-l2-ccl)

Create a test file such as TestProxy.spec.ts under your test directory. Below is a basic example test:

import hre, { ethers } from "hardhat";
import { Signer } from "ethers";
import { expect } from "chai";

// The following items come from 'tac-l2-ccl' to help test cross-chain logic locally.
import {
  deploy,
  TacLocalTestSdk,
  JettonInfo,
  TokenMintInfo,
  TokenUnlockInfo,
} from "tac-l2-ccl";

// Types for your compiled contracts
import { TestProxy, TestToken } from "../typechain-types";
import { InvokeWithCallbackEvent } from "../typechain-types/contracts/TestProxy";

describe("TestProxy with tac-l2-ccl", () => {
  let admin: Signer;
  let testSdk: TacLocalTestSdk;
  let proxyContract: TestProxy;
  let existedToken: TestToken;

  before(async () => {
    [admin] = await ethers.getSigners();

    // 1. Initialize local test SDK
    testSdk = new TacLocalTestSdk();
    const crossChainLayerAddress = testSdk.create(ethers.provider);

    // 2. Deploy a sample ERC20 token
    existedToken = await deploy<TestToken>(
      admin,
      hre.artifacts.readArtifactSync("TestToken"),
      ["TestToken", "TTK"],
      undefined,
      false
    );

    // 3. Deploy the proxy contract
    proxyContract = await deploy<TestProxy>(
      admin,
      hre.artifacts.readArtifactSync("TestProxy"),
      [crossChainLayerAddress],
      undefined,
      false
    );
  });

  it("Should correctly handle invokeWithCallback", async () => {
    // Prepare call parameters
    const shardsKey = 1n;
    const operationId = ethers.encodeBytes32String("operationId");
    const extraData = "0x"; // untrusted data from the executor
    const timestamp = BigInt(Math.floor(Date.now() / 1000));
    const tvmWalletCaller = "TVMCallerAddress";

    // Example bridging: create a Jetton and specify how many tokens to mint
    const jettonInfo: JettonInfo = {
      tvmAddress: "JettonMinterAddress",
      name: "TestJetton",
      symbol: "TJT",
      decimals: 9n,
    };

    const tokenMintInfo: TokenMintInfo = {
      info: jettonInfo,
      mintAmount: 10n ** 9n,
    };

    // Also handle an existing EVM token to simulate bridging
    const tokenUnlockInfo: TokenUnlockInfo = {
      evmAddress: await existedToken.getAddress(),
      unlockAmount: 10n ** 18n,
    };

    // Lock existedToken in the cross-chain layer to emulate bridging from EVM
    await existedToken.mint(
      testSdk.getCrossChainLayerAddress(),
      tokenUnlockInfo.unlockAmount
    );

    // You can define a native TAC amount to bridge to your proxy,
    // but you must first lock this amount on the CrossChainLayer contract
    // use the testSdk.lockNativeTacOnCrossChainLayer(nativeTacAmount) function
    const tacAmountToBridge = 0n;

    // Determine the EVM address of the bridged Jetton (for minted jettons)
    const bridgedJettonAddress = testSdk.getEVMJettonAddress(
      jettonInfo.tvmAddress
    );

    // Prepare the method call
    const target = await proxyContract.getAddress();
    const methodName = "invokeWithCallback(bytes,bytes)";

    // Our 'arguments' is an array of TokenAmount: (address, uint256)[]
    const receivedTokens = [
      [bridgedJettonAddress, tokenMintInfo.mintAmount],
      [tokenUnlockInfo.evmAddress, tokenUnlockInfo.unlockAmount],
    ];

    const encodedArguments = ethers.AbiCoder.defaultAbiCoder().encode(
      ["tuple(address,uint256)[]"],
      [receivedTokens]
    );

    // 4. Use testSdk to simulate a cross-chain message
    const { receipt, deployedTokens, outMessages } = await testSdk.sendMessage(
      shardsKey,
      target,
      methodName,
      encodedArguments,
      tvmWalletCaller,
      [tokenMintInfo], // which jettons to mint
      [tokenUnlockInfo], // which EVM tokens to unlock
      tacAmountToBridge,
      extraData,
      operationId,
      timestamp,
      0, // gasLimit - if 0 - simulate and fill inside sendMessage
      false // force send (if simulation failed)
    );

    // 5. Assertions
    expect(receipt.status).to.equal(1);

    // - Check if the Jetton was deployed
    expect(deployedTokens.length).to.equal(1);
    expect(deployedTokens[0].evmAddress).to.equal(bridgedJettonAddress);

    // - Check the outMessages array
    expect(outMessages.length).to.equal(1);
    const outMessage = outMessages[0];
    expect(outMessage.shardsKey).to.equal(shardsKey);
    expect(outMessage.operationId).to.equal(operationId);
    expect(outMessage.callerAddress).to.equal(await proxyContract.getAddress());
    expect(outMessage.targetAddress).to.equal(tvmWalletCaller);

    // - The returned tokens should be burned or locked as bridging back to TON
    expect(outMessage.tokensBurned.length).to.equal(1);
    expect(outMessage.tokensBurned[0].evmAddress).to.equal(
      bridgedJettonAddress
    );
    expect(outMessage.tokensBurned[0].amount).to.equal(
      tokenMintInfo.mintAmount
    );

    expect(outMessage.tokensLocked.length).to.equal(1);
    expect(outMessage.tokensLocked[0].evmAddress).to.equal(
      tokenUnlockInfo.evmAddress
    );
    expect(outMessage.tokensLocked[0].amount).to.equal(
      tokenUnlockInfo.unlockAmount
    );

    // - Confirm the event was emitted
    let eventFound = false;
    receipt.logs.forEach((log) => {
      const parsed = proxyContract.interface.parseLog(log);
      if (parsed && parsed.name === "InvokeWithCallback") {
        eventFound = true;
        const typedEvent =
          parsed as unknown as InvokeWithCallbackEvent.LogDescription;
        expect(typedEvent.args.shardsKey).to.equal(shardsKey);
        expect(typedEvent.args.timestamp).to.equal(timestamp);
        expect(typedEvent.args.operationId).to.equal(operationId);
        expect(typedEvent.args.tvmCaller).to.equal(tvmWalletCaller);
        expect(typedEvent.args.extraData).to.equal(extraData);
        expect(typedEvent.args.receivedTokens.length).to.equal(2);
        expect(typedEvent.args.receivedTokens[0].l2Address).to.equal(
          bridgedJettonAddress
        );
        expect(typedEvent.args.receivedTokens[1].l2Address).to.equal(
          tokenUnlockInfo.evmAddress
        );
      }
    });
    expect(eventFound).to.be.true;
  });
});

Test Flow:

Initialization
- Create a local cross-chain environment (TacLocalTestSdk).
- Deploy a test token (TestToken).
- Deploy your TestProxy.
Bridging Simulation
- Mint or lock tokens on the cross-chain layer.
- Create the parameters (shardsKey, operationId, etc.).
Invoke Proxy
- Use the testSdk.sendMessage(...) to simulate a cross-chain call to your proxy’s function.
Verification
- Confirm the transaction succeeded.
- Inspect the deployedTokens (if you minted new tokens).
- Inspect the outMessages for tokens returning to TON.
- Check emitted events for correct data.

Running the Tests

Inside your project directory, simply run:

npx hardhat test

Hardhat will compile all contracts and run the test suite. The tac-l2-ccl local test SDK helps emulate bridging logic, ensuring your proxy behaves as expected in a cross-chain scenario.

Conclusion

By following these steps, you can develop, deploy, and test a TAC Proxy contract that handles cross-chain messages and tokens from TON. Key points include:

Inheritance from TacProxyV1 or TacProxyV1Upgradeable to gain built-in cross-chain functionality and security modifiers.
Function Signatures must match functionName(bytes, bytes) external.
Decoding the TAC header and your custom arguments to implement your Dapp’s logic.
**Use _sendMessageV1(...)

Get Started

Smart Contracts

TAC SDK (Frontend)

Partner Integrations

Developing TAC Proxy

Installation

Creating the Proxy Contract

Non-upgradeable Contract

Upgradeable Contract

Defining and Implementing Proxy Functions

Example Implementation

How the Cross-Chain Call Works

Encoding Arguments on the Frontend

Basic Example

Complex Example with Nested Structures

Complex Example with Dynamic Arrays

Specifying the Function Name in `@tonappchain/sdk`

Testing the Proxy Contract

Example Minimal Proxy for Testing

Test Setup Example (Hardhat + tac-l2-ccl)

Running the Tests

Conclusion

Get Started

Smart Contracts

TAC SDK (Frontend)

Partner Integrations

​Installation

​Creating the Proxy Contract

​Non-upgradeable Contract

​Upgradeable Contract

​Defining and Implementing Proxy Functions

​Example Implementation

​How the Cross-Chain Call Works

​Encoding Arguments on the Frontend

​Basic Example

​Complex Example with Nested Structures

​Complex Example with Dynamic Arrays

​Specifying the Function Name in @tonappchain/sdk

​Testing the Proxy Contract

​Example Minimal Proxy for Testing

​Test Setup Example (Hardhat + tac-l2-ccl)

​Running the Tests

​Conclusion

Installation

Creating the Proxy Contract

Non-upgradeable Contract

Upgradeable Contract

Defining and Implementing Proxy Functions

Example Implementation

How the Cross-Chain Call Works

Encoding Arguments on the Frontend

Basic Example

Complex Example with Nested Structures

Complex Example with Dynamic Arrays

Specifying the Function Name in `@tonappchain/sdk`

Testing the Proxy Contract

Example Minimal Proxy for Testing

Test Setup Example (Hardhat + tac-l2-ccl)

Running the Tests

Conclusion