Round-Trip with Auto Back-Leg

A single Push contract that dispatches an outbound to BNB Testnet and automatically receives an inbound callback when the destination chain finishes. One user signature on Push, two coordinated transactions across two chains.

This is a loaded pattern example where the operational knobs that the basics gloss over (gasLimit floor, destination CEA prefunding, the 6-arg executeUniversalTx dispatch path, refund accumulation) all become concrete here.

For the conceptual background, see Contract-Initiated Multichain Execution (especially the Round-Trip Wire Format and Operational Knobs sections).

What this example shows

Aspect	Details
Direction	Push -> BNB -> Push, all from one user signature on Push.
Trigger	EOA calls `kickOff(...)`. The contract dispatches outbound; TSS auto-fires the inbound callback when the BNB tx settles.
Wire format	The destination CEA's outer multicall self-calls `sendUniversalTxToUEA` on the CEA itself. The CEA wraps the inner payload and submits it through BNB's gateway.
Inbound dispatch path	The 6-arg `executeUniversalTx(string, bytes, bytes, uint256, address, bytes32)`. For Push-native contracts, TSS dispatches this signature; the 2-arg `executeUniversalTx(UniversalPayload, bytes)` overload in the codebase is reserved for UEA proxy accounts and is not invoked here.
Verified on	Donut Testnet. End-to-end run with `outboundCount` and `callbacks` both advancing.

The wire format

This is the part most newcomers get wrong. The destination CEA's outer multicall must include a step that self-calls sendUniversalTxToUEA on the CEA itself. The CEA wraps and routes the inner payload through its gateway internally; TSS reads the gateway event and fires the inbound back to Push. The full layered structure:

Layer 4 - Outer multicall (what TSS submits to the destination CEA)
  ┌────────────────────────────────────────────────────────────────┐
  │ 0x2cc2842d || abi.encode(Multicall[] {                         │
  │   { to: destCEA, value: 0, data: <Layer 3> }                   │
  │ })                                                             │
  └────────────────────────────────────────────────────────────────┘
                              │
                              ▼
Layer 3 - CEA self-call to sendUniversalTxToUEA
  ┌────────────────────────────────────────────────────────────────┐
  │ sendUniversalTxToUEA(token=0, amount=0, <Layer 2>, refundTo)   │
  └────────────────────────────────────────────────────────────────┘
                              │
                              ▼
Layer 2 - Encoded UniversalPayload (vType = 1, inbound)
  ┌────────────────────────────────────────────────────────────────┐
  │ abi.encode(                                                    │
  │   address(0), uint256(0), <Layer 1>,                           │
  │   gasLimit, maxFeePerGas, 0, ueaNonce + 1, deadline, vType: 1  │
  │ )                                                              │
  └────────────────────────────────────────────────────────────────┘
                              │
                              ▼
Layer 1 - Inner multicall (what the Push UEA executes after the back-leg)
  ┌────────────────────────────────────────────────────────────────┐
  │ 0x2cc2842d || abi.encode(Multicall[] { /* no-op or your call */ })│
  └────────────────────────────────────────────────────────────────┘

The destination CEA receives Layer 4, executes the multicall step, which invokes Layer 3 (the CEA self-call), which submits Layer 2 to the BNB gateway. TSS picks up the gateway event and delivers Layer 1 to the original Push contract via the 6-arg executeUniversalTx.

The four-layer encoding looks like a lot. In practice you write it once, encapsulate it in a helper function, and call that helper from every kickoff path. The runner in this example shows the helper.

Solidity Code

Round-Trip Dispatcher

RoundtripDispatcher.sol

Open in Remix

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.26;

struct UniversalOutboundTxRequest {
    bytes recipient;
    address token;
    uint256 amount;
    uint256 gasLimit;
    uint256 gasPrice;        // 0 = per-chain default from UniversalCore (new in SDK v6)
    uint256 maxPCForGas;     // 0 = no cap on PC the AMM may spend on the gas swap (new in SDK v6)
    bytes payload;
    address revertRecipient;
}

interface IUniversalGatewayPC {
    function sendUniversalTxOutbound(UniversalOutboundTxRequest calldata req) external payable;
}

contract RoundtripDispatcher {
    address public immutable ugpc;
    address public immutable universalExecutorModule;

    bytes4 internal constant UEA_MULTICALL_SELECTOR = 0x2cc2842d;

    uint256 public outboundCount;
    uint256 public callbacks;

    mapping(bytes32 => bool) public seenTxIds;
    bytes32 public lastTxId;

    event OutboundKicked(uint256 outboundCount, bytes payload);
    event Callback(uint256 sequence, bytes32 indexed txId, string sourceChainNamespace, bytes ceaAddress, address prc20, uint256 amount);

    error NotUniversalExecutor();
    error TxAlreadyExecuted();
    error InsufficientPC(uint256 required, uint256 available);

    struct Multicall { address to; uint256 value; bytes data; }

    constructor(address _ugpc, address _module) {
        ugpc = _ugpc;
        universalExecutorModule = _module;
    }

    function fund() external payable {}

    /// @notice Trigger the round-trip: dispatch outbound to BNB; TSS auto-fires the inbound callback.
    /// @param destinationCEAAddr This contract's CEA on the destination chain.
    /// @param tokenForRouting    PRC-20 on Push that selects the destination chain (e.g. pBNB).
    /// @param protocolFeePc      PC the contract forwards to UGPC for the outbound fee.
    /// @param ueaNonce           Current Push UEA nonce. 0 for fresh.
    function kickOff(
        address destinationCEAAddr,
        address tokenForRouting,
        uint256 protocolFeePc,
        uint256 ueaNonce
    ) external {
        if (address(this).balance < protocolFeePc) {
            revert InsufficientPC(protocolFeePc, address(this).balance);
        }

        // Layer 1: inner multicall (executes on Push UEA after the back-leg).
        Multicall[] memory innerCalls = new Multicall[](1);
        innerCalls[0] = Multicall({to: address(this), value: 0, data: ""});
        bytes memory innerMulticallData = abi.encodePacked(UEA_MULTICALL_SELECTOR, abi.encode(innerCalls));

        // Layer 2: encoded UniversalPayload struct (vType = 1, inbound).
        bytes memory inboundUniversalPayload = abi.encode(
            address(0), uint256(0), innerMulticallData,
            uint256(1e7), uint256(1e10), uint256(0),
            ueaNonce + 1, uint256(9999999999), uint8(1)
        );

        // Layer 3: CEA self-call to sendUniversalTxToUEA.
        bytes memory ceaSelfCallData = abi.encodeWithSelector(
            bytes4(keccak256("sendUniversalTxToUEA(address,uint256,bytes,address)")),
            address(0), uint256(0), inboundUniversalPayload, address(this)
        );

        // Layer 4: outer multicall delivered to the destination CEA.
        Multicall[] memory outerCalls = new Multicall[](1);
        outerCalls[0] = Multicall({to: destinationCEAAddr, value: 0, data: ceaSelfCallData});
        bytes memory outerMulticallData = abi.encodePacked(UEA_MULTICALL_SELECTOR, abi.encode(outerCalls));

        IUniversalGatewayPC(ugpc).sendUniversalTxOutbound{value: protocolFeePc}(
            UniversalOutboundTxRequest({
                recipient: abi.encodePacked(destinationCEAAddr),
                token: tokenForRouting,
                amount: 0,
                gasLimit: 2_000_000,        // CRITICAL: see What can go wrong below
                gasPrice: 0,                // per-chain default from UniversalCore
                maxPCForGas: 0,             // no cap on PC for the gas swap
                payload: outerMulticallData,
                revertRecipient: address(this)
            })
        );

        outboundCount += 1;
        emit OutboundKicked(outboundCount, outerMulticallData);
    }

    /// @notice Back-leg handler. TSS calls this via UNIVERSAL_EXECUTOR_MODULE
    /// when the destination CEA finishes its multicall. For Push-native
    /// contracts, this 6-arg signature is the path TSS dispatches to; the
    /// 2-arg `executeUniversalTx(UniversalPayload, bytes)` overload in the
    /// codebase is reserved for UEA proxy accounts and is not invoked here.
    function executeUniversalTx(
        string calldata sourceChainNamespace,
        bytes calldata ceaAddress,
        bytes calldata, /* payload */
        uint256 amount,
        address prc20,
        bytes32 txId
    ) external payable {
        if (msg.sender != universalExecutorModule) revert NotUniversalExecutor();
        if (seenTxIds[txId]) revert TxAlreadyExecuted();
        seenTxIds[txId] = true;
        callbacks += 1;
        lastTxId = txId;
        emit Callback(callbacks, txId, sourceChainNamespace, ceaAddress, prc20, amount);
    }

    receive() external payable {}
}

kickOff is the only entrypoint a caller needs. executeUniversalTx is the back-leg handler TSS invokes when the destination CEA's multicall completes. After running the example end to end on Donut, outboundCount and callbacks both advance to 1.

Required configuration

Two knobs determine whether the back-leg lands. Wrong values cause TSS to silently drop the relay.

Knob	Value	Why
`gasLimit` on the UGPC outbound	`2_000_000`	UGPC's auto-floor for `gasLimit = 0` is 500k. Below ~1.5M the destination tx runs out of gas during the nested gateway call and TSS does not retry. UGPC charges only for actual gas used and refunds the surplus, so over-provisioning is essentially free.
Push contract `$PC` balance	covers `protocolFee + inbound execution fee`	Inbound execution on Push pays gas in `$PC`, charged to the dispatching contract. UGPC refunds surplus into `address(this)` via `receive()`.

The destination CEA does not need pre-funding. When TSS submits the destination tx it forwards the converted gas value to the CEA as msg.value, so the CEA has the native balance it needs for the nested gateway.sendUniversalTxFromCEA(...) call during the duration of that tx.

Source Code

Run

The runner deploys the contract, funds it with PC if needed, derives the destination CEA, fires kickOff(), and watches both outboundCount and callbacks advance.

git clone https://github.com/pushchain/push-chain-examples.git
cd push-chain-examples/core-sdk-functions/contract-initiated-roundtrip-execution

forge build
npm install
cp .env.sample .env
# Edit .env: set PUSH_PRIVATE_KEY (Push native wallet with at least 12 PC).
npm start

Prerequisites

Foundry and Node.js v18+.
A Push native wallet on Donut Testnet with at least 12 PC (deploy + 8 PC funded into the contract + protocol fees + headroom). Push faucet.

The first run deploys RoundtripDispatcher, funds it with 8 PC, fires kickOff, and watches the cascade unfold. After ~30 to 90 seconds you should see the outbound on BNB and the inbound callback land on Push.

What can go wrong

Symptom	Cause	Fix
Push tx succeeds but no BNB tx fires	`gasLimit` was 0 or under the auto-floor (~500k)	Pass `gasLimit: 2_000_000`. The example hardcodes this; do not lower it.
Both legs land but the back-leg never reaches `executeUniversalTx`	The outer multicall is missing the destination CEA's self-call to `sendUniversalTxToUEA`	Use the layered encoding above. Plain multicalls without that self-call step do not trigger a back-leg.
Inbound callback never lands on `executeUniversalTx`	Implementing the 2-arg `executeUniversalTx(UniversalPayload, bytes)` overload as your primary handler	TSS dispatches only the 6-arg overload `(string, bytes, bytes, uint256, address, bytes32)` to Push-native contracts. The 2-arg signature is reserved for UEA proxies. Implement only the 6-arg version.
Solana destination outbound reverts with `STF`	`msg.value` to UGPC under-sized the on-chain $PC -> pSOL Uniswap V3 swap	Off-chain compute the right value via the Solana sizing snippet and store it on the contract via a setter. Never use a flat `balance/2`.
EOA wallet drains over many runs even though the contract is funded	UGPC routes its surplus refund to `address(this)`, not back to the EOA that called `kickOff()`	Plan a `withdraw()` path or treasury sweep. Refunds accumulate on the contract over time.

Contract-Initiated Multichain Execution → The conceptual reference for everything related to contract-initiated execution.
Other Basic Examples → Inbound to Push Chain and Outbound from Push Chain.
Advanced Patterns → Harder variants: FIFO state machine, deposit-and-execute, recipient bridge, three-chain cascade.
How CEA Works → The identity model that makes the round-trip back-leg land on the same Push contract.

Start building

Quick Start

Learn with tutorials

Explore Push Chain

Join Discord

Explore Examples

Round-Trip with Auto Back-Leg

What this example shows

The wire format

Solidity Code

Required configuration

Source Code

Run

Prerequisites

What can go wrong

Start building

Quick Start

Learn with tutorials

Explore Push Chain

Join Discord

Explore Examples

What this example shows​

The wire format​

Solidity Code​

Required configuration​

Source Code​

Run​

Prerequisites​

What can go wrong​

Related​

What this example shows

The wire format

Solidity Code

Required configuration

Source Code

Run

Prerequisites

What can go wrong

Related