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USDT0 on Stable is both the native gas token and an ERC-20 token. This dual-role model affects balance behavior, contract design, and event handling.
  • For background on why and how USDT0 operates this way, see USDT as Gas.
  • To experience USDT0’s dual-role behavior through actual transfers, see Deploy Smart Contract.

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Balance reconciliation

USDT0 uses 18 decimals as the native asset and 6 decimals as an ERC-20 token. Native transfers and ERC-20 transfers operate on the same underlying balance, but the 12-digit precision gap means the system must reconcile fractional amounts when a transfer involves sub-integer precision. 
before
  0.000001 USDT0 (ERC-20) + 0.000000000000000000 USDT0 (internal)
  // address(account).balance = 0.000001000000000000
  // USDT0.balanceOf(account) = 0.000001

if transfer 0.0000001 USDT0 to another account

after
  0.000000 USDT0 (ERC-20) + 0.000000900000000000 USDT0 (internal)
  // address(account).balance = 0.000000900000000000
  // USDT0.balanceOf(account) = 0.000000
This can cause address(account).balance and USDT0.balanceOf(account) to differ by up to 0.000001 USDT0.

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Event handling

Each reconciliation transfer emits an additional Transfer event. A single logical USDT0 transfer can produce up to two extra Transfer events depending on how the sender’s and receiver’s fractional balances are affected:
  • Sender adjustment: If the sender’s fractional balance is insufficient, 0.000001 USDT0 is moved from the sender to the reserve address. This emits an extra Transfer event.
  • Receiver adjustment: If the receiver’s fractional balance overflows, 0.000001 USDT0 is moved from the reserve address to the receiver. This emits an extra Transfer event.
  • Both adjustments: If both conditions occur in the same transfer, the reserve is bypassed. The sender transfers 0.000001 USDT0 directly to the receiver as part of the main transfer. No extra event is emitted.
These auxiliary events involve the reserve address 0x6D11e1A6BdCC974ebE1cA73CC2c1Ea3fDE624370. Indexers and off-chain services that track USDT0 balances by replaying Transfer events must filter or account for transfers to and from this address.

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Contract design requirements

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Native balance mutability

On Ethereum, a contract’s native balance typically changes only as a result of contract execution. On Stable, a contract’s native USDT0 balance may also change due to ERC-20 allowance-based operations, including transferFrom and permit. These operations can reduce a contract’s native balance without invoking any contract code. As a result, the following assumption is invalid on Stable:
A contract’s native balance can only decrease if the contract is called.

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Do not mirror native balance

On Ethereum, it is common to track deposits with an internal variable. On Stable, this is unsafe because ERC-20 transferFrom can drain the native balance externally. 
// UNSAFE on Stable
uint256 public deposited;

function deposit() external payable {
    deposited += msg.value;
}

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Always check real balance before transfers

All native value transfers must verify solvency using address(this).balance just before transfer, not internal accounting variables: 
// SAFE
function withdraw() external {
    uint256 amount = credit[msg.sender];
    credit[msg.sender] = 0;
    require(address(this).balance >= amount, "insufficient balance");
    payable(msg.sender).call{value: amount}("");
}

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State progression must be balance-independent

Protocol logic that depends on progression, milestones, or completion conditions must track these explicitly using non-balance state variables, such as counters or epochs. Native balances should be used only for solvency verification at the moment of payout.

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No zero-address transfers

On Stable, both native and ERC-20 transfers to address(0) revert. 
// REVERT on Stable
payable(address(0)).call{value: amount}("")
USDT0.transfer(address(0), amount);
Any contract logic that sends native USDT0 should validate the recipient and explicitly reject address(0) before the transfer call: 
// SAFE
require(recipient != address(0), "zero address recipient");
payable(recipient).call{value: amount}("");
If a contract uses zero-address transfers as a burn mechanism, it must be redesigned. Use explicit sink contracts if irreversible loss semantics are required.

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EXTCODEHASH behavior

On Ethereum, the EXTCODEHASH opcode returns:
  • Zero hash (0x0000...): if an address has never been used (nonce=0, balance=0, no code).
  • Empty hash (0xc5d2…a470, the Keccak-256 hash of empty code): if an address exists but has no code.
On Ethereum, once an address transitions from zero hash to empty hash, it cannot return to zero hash. On Stable, because USDT0 supports permit()-based approvals, an address can create approvals without sending a transaction. Combined with transferFrom(), this allows native balance changes without nonce increment, potentially allowing EXTCODEHASH to oscillate between zero hash and empty hash. 
// UNSAFE on Stable
function isUnusedAddress(address addr) public view returns (bool) {
    bytes32 codeHash;
    assembly {
        codeHash := extcodehash(addr)
    }
    return codeHash == bytes32(0);
}
Use explicit tracking instead: 
// SAFE
contract SafeAddressTracker {
    mapping(address => bool) public hasBeenUsed;

    function markAsUsed(address addr) internal {
        hasBeenUsed[addr] = true;
    }

    function isUnused(address addr) public view returns (bool) {
        return !hasBeenUsed[addr];
    }
}

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Testing requirements

Test suites for Stable deployments should include:
  • Allowance-based drain scenarios (approve + transferFrom)
  • Solvency enforcement using real native balance
  • Address usage logic without reliance on EXTCODEHASH
  • Explicit failure cases for zero-address transfers

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Migration checklist

When porting contracts from Ethereum to Stable:
  • Remove internal native balance mirrors
  • Replace all solvency checks with address(this).balance
  • Remove all native or ERC-20 transfers to address(0)
  • Audit all USDT0 approvals
  • Add tests covering permit and allowance-based flows
  • Verify off-chain indexers handle auxiliary Transfer events from fractional balance reconciliation

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Key takeaways

Correct contract design on Stable requires:
  • Treating USDT0 as a dual-role asset
  • Enforcing solvency against real balances
  • Avoiding allowance-based drain paths
  • Eliminating reliance on Ethereum-specific balance and address assumptions
Off-chain services and indexers should:
  • Account for auxiliary Transfer events from fractional balance reconciliation
  • Use direct balance queries instead of event-based balance reconstruction
See also:
Last modified on April 16, 2026